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Rpt (SWPPP) STC LD20220109 Civil Engineers ● Structural Engineers ● Landscape Architects ● Community Planners ● Land Surveyors Construction Stormwater Pollution Prevention Plan PREPARED FOR: Purvis/Evans c/o Scott Kaul Tacoma, WA 98403 PROJECT: The Summit at Thompson Creek Yelm, WA 2220471.11 PREPARED BY: Michael C. Hager, PE Project Engineer REVIEWED BY: Scott T. Kaul, PE, LEED AP Associate Principal DATE: May 2024 Construction Stormwater Pollution Prevention Plan PREPARED FOR: Purvis/Evans c/o Scott Kaul Tacoma, WA 98403 PROJECT: The Summit at Thompson Creek Yelm, WA 2220471.11 PREPARED BY: Michael C. Hager, PE Project Engineer REVIEWED BY: Scott T. Kaul, PE, LEED AP Associate Principal DATE: May 2024 I hereby state that this Construction Stormwater Pollution Prevention Plan for The Summit at Thompson Creek project has been prepared by me or under my supervision and meets the standard of care and expertise that is usual and customary in this community for professional engineers. I understand that City of Yelm does not and will not assume liability for the sufficiency, suitability, or performance of drainage facilities prepared by me. 05/08/2024 Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 Table of Contents Section Page 1.0 Introduction ............................................................................................................................... 1 2.0 Project Description ................................................................................................................... 2 3.0 Existing Site Conditions ........................................................................................................... 2 4.0 Adjacent Areas and Drainage ................................................................................................... 2 5.0 Critical Areas ............................................................................................................................. 3 6.0 Soils ........................................................................................................................................... 3 6.1 Potential Erosion Problems ............................................................................................. 3 7.0 Construction Stormwater Pollution Prevention Elements ...................................................... 3 7.1 Mark Clearing Limits ....................................................................................................... 4 7.2 Establish Construction Access ........................................................................................ 4 7.3 Control Flow Rates ......................................................................................................... 4 7.4 Install Sediment Controls ................................................................................................ 4 7.5 Stabilize Soils ................................................................................................................. 4 7.6 Protect Slopes ................................................................................................................ 4 7.7 Protect Drain Inlets ......................................................................................................... 5 7.8 Stabilize Channels and Outlets ....................................................................................... 5 7.9 Control Pollutants ........................................................................................................... 5 7.10 Control Dewatering ......................................................................................................... 7 7.11 Maintain BMPs ............................................................................................................... 7 7.12 Manage the Project ........................................................................................................ 7 7.13 Protect Low Impact Development BMPs ......................................................................... 8 8.0 Construction Sequence and Phasing ...................................................................................... 8 8.1 Construction Sequence .................................................................................................. 8 8.2 Construction Phasing...................................................................................................... 9 9.0 Construction Schedule ............................................................................................................. 9 10.0 Financial/Ownership Responsibilities...................................................................................... 9 11.0 Certified Erosion and Sediment Control Lead (CESCL) .......................................................... 9 Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 Exhibits Exhibit 1 NRCS Soils Map Exhibit 2 TESC Calculations Exhibit 3 Inspection Logs Exhibit 4 Selected Best Management Practices (BMPs) Exhibit 5 Geotechnical Engineering Report South Sound Geotechnical Consulting, Inc. June 16, 2022 Geotechnical Seasonal Groundwater Monitoring (Winter 2022-2023) South Sound Geotechnical Consulting, Inc. May 12, 2023 Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 1 2220471.11 1.0 Introduction In 1972, Congress passed the Federal Water Pollution Control Act (FWPCA), also known as the Clean Water Act (CWA), to restore and maintain the quality of the nation's waterways. The ultimate goal was to ensure that rivers and streams were fishable, swimmable, and drinkable. In 1987, the Water Quality Act (WQA) added provisions to the CWA that allowed the Environmental Protection Agency (EPA) to govern stormwater discharges from construction sites. The National Pollutant Discharge Elimination System (NPDES) General Permit includes provisions for development of a Stormwater Pollution Prevention Plan (SWPPP) to maximize the potential benefits of pollution prevention and sediment and erosion control measures at construction sites. The proposed project will disturb more than 1 acre of area, and therefore is required to obtain an NPDES General Permit for Stormwater Associated with Construction Activities. The most recent Department of Ecology (DOE) Stormwater Management Manual for Western Washington (SWMMWW), as adopted by the City of Yelm, requires a Construction Stormwater Pollution Prevention Plan (CSWPPP) for projects that add or replace more than 2,000 square feet of impervious surface. The proposed project will exceed this threshold; therefore, a CSWPPP is required. Development, implementation, and maintenance of the CSWPPP will provide the selected General Contractor with the framework for reducing soil erosion and minimizing pollutants in stormwater during construction. The CSWPPP will: • Define the characteristics of the site and the type of construction that will occur. • Describe the practices that will be implemented to control erosion and the release of pollutants in stormwater. • Create an implementation schedule to ensure that the practices described in this CSWPPP are in fact implemented, and to evaluate the plan's effectiveness in reducing erosion, sediment, and pollutant levels in stormwater discharged from the site. • Describe the final stabilization/termination design to minimize erosion and prevent stormwater impacts after construction is complete. This CSWPPP: • Identifies the Certified Erosion and Sedimentation Control Lead (CESCL) with a description of this person's duties. • Identifies the Stormwater Pollution Prevention Team (SWPP Team) that will assist in implementation of the CSWPPP during construction. • Describes the existing site conditions, including existing land use, soil types at the site, and the location of surface waters that are located on or next to the site. • Identifies the body or bodies of water that will receive runoff from the construction site, including the ultimate body of water that receives the stormwater. • Identifies the drainage areas and potential stormwater contaminants. • Describes the stormwater management controls and various Best Management Practices (BMPs) necessary to reduce erosion, sediment, and pollutants in stormwater discharge. Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2 2220471.11 • Describes the facility monitoring plan and how controls will be coordinated with construction activities. • Describes the implementation schedule and provisions for amendment of the plan. 2.0 Project Description This CSWPPP accompanies the civil engineering plans submitted for a site development permit for the proposed project, The Summit at Thompson Creek. The 28.86-acre site is located in Section 24, Township 17 North, Range 01 East, W. M. The Thurston County tax parcel numbers associated with the project are 21724230100, 21723140000, and 21723140102. The project includes the addition of 101 residential lots for single-family homes, a new roadway and sidewalks, sewer, water services, and stormwater facilities to treat and dispose of the project's stormwater. The proposed roadway features and utilities will be extended from the new extension of Berry Valley Road SE, to include two new cul-de-sac roads, a road to be connected in the future to the west, and the extension of Jackson Street SE, connecting to utilities in the Tahoma Terra Project southwest of the site at Jackson Street SE. 3.0 Existing Site Conditions The northwestern portion of the site is densely forested with mature trees and underbrush. The eastern portion of the site consists of grass with sparse trees and includes physical improvements associated with the existing residence, including gravel driveways and outbuildings. The southern portion of the project site is primarily grass-covered with a few sporadic trees and shrubs, with greater vegetation density along the southern boundary. The southwestern and western portions of the parcel will be left outside the project boundaries and include a couple of wetlands and a forested buffer area. The development portion of the site contains moderate to steep slopes, with a ridge line running south to north. Runoff from the eastern portion of the site flows easterly toward Thompson Creek, while the portion to the west of the ridge flows to the wetlands located at the western site boundary. 4.0 Adjacent Areas and Drainage Drainage review of available GIS topographic information and topographic survey data indicates the following: An existing, undeveloped parcel is located south and southeast of this project site that is higher in elevation and will likely produce runon to the project site during rain events. The area is covered in a grassy and forested area. The western and southern basins of the project site will drain to the west into wetlands that will be preserved with this project. Soils in this area are not conducive to infiltration and, in the final project design, the runoff will remain as dispersed into existing wetlands. The eastern basin of the project site will drain to the east and toward Thompson Creek at the eastern portion of the site. The eastern portion of the site has well-draining soils that will infiltrate runoff; final project design has this area being used for infiltration. Areas used for infiltrating in the final design shall be protected from compaction throughout construction. Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 3 2220471.11 5.0 Critical Areas The eastern and southern portions of the project site will drain into a system of wetlands. A wetland delineation and stream assessment was performed by Land Services Northwest, which identified three wetlands and a Type F stream on or adjacent to the project site. The identified wetlands, stream, and associated buffers will be preserved by protected tracts within the development. Runoff from portions of the project site will be dispersed toward the wetland buffer areas to preserve the existing flow patterns. The runoff from the southern wetpond/detention pond will be discharged to the larger onsite wetland, matching the existing runoff discharge location. The western portion of the project site will drain to Thompson Creek, located at the western portion of the site. An associated wetland has also been identified with Thompson Creek. 6.0 Soils The soils identified by the Natural Resources Conservation Service (NRCS) Web Soil Survey (Exhibit 1) vary across the project site. The soils on the lower eastern portion of the site associated with Thompson Creek are identified as Nisqually loamy fine sand, 3 to 15 percent slopes (74). The soils mid-slope within the East Basin are Spanaway gravelly sandy loam, 3 to 15 percent slopes (111). This portion of the project site is conducive the infiltration. Lots within the Spanaway soil area will be provided within individual roof drain infiltration systems. The largest soil area in the central, higher elevation portion of the site is identified as McChord- Everett complex, 3 to 15 percent slopes. This soil is classified as hydrologic soil group Type A; however, soil test holes dug by South Sound Geotechnical Consulting determined that the surface soils were underlain by glacial till/drift located approximately 1 to 5 feet below ground surface. Therefore, for stormwater modelling purposes, this soil is assumed to function as Type C. The soils located on the western perimeter of the site associated with the wetland areas are identified as Steilacoom-Yelm complex, 0 to 2 percent slopes (100) and Mckenna gravelly silt loam, 0 to 5 percent slopes (65). Soil test holes dug across the proposed project site generally match the soil types identified by the NRCS soil survey. 6.1 Potential Erosion Problems No known historical erosion problems are located on the site. No known potential erosion problems will be created onsite. 7.0 Construction Stormwater Pollution Prevention Elements The purpose of this section is to describe how each of the 13 Construction Stormwater Pollution Prevention elements has been addressed and to identify the type and location of BMPs used to satisfy the required element. If an element is not applicable to the project, a reason is provided. Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 4 2220471.11 7.1 Mark Clearing Limits Prior to beginning land disturbing activities, clearing limits will be marked with high visibility plastic or metal fence (BMP C103). Significant vegetation to remain will be marked and protected by fencing. 7.2 Establish Construction Access The existing paved driveway access from Berry Valley Road SE and Jackson Street SE will serve as the site’s construction entrance (BMP C105). If sediment tracking should occur, the Contractor will be required to sweep the impacted roadways. Dump trucks hauling material to and from the site will be covered by a tarp. 7.3 Control Flow Rates Temporary Sediment Ponds (BMP C241) will be used to control flows during construction. The three separate basins will each have their own sediment ponds designed. The sediment pond in the eastern portion of the site will be able to use infiltration, but shall be placed outside of future areas proposed for use with final infiltration design. The eastern and southern basins will be located in the final design pond locations. Outlets and emergency spillways are proposed for each of the ponds and have been designed in accordance with DOE requirements. Sizing of the facilities is found in Exhibit 2. 7.4 Install Sediment Controls As part of the initial construction activities, BMPs will be installed to trap sediment onsite. The identified BMPs include a sediment pond (BMP C241) and silt fencing (BMP C233). 7.5 Stabilize Soils To protect soil from the erosive forces of raindrops, flowing water, and wind, the following BMPs will be implemented: • All disturbed areas that will remain unworked will be stabilized with temporary hydroseed (BMP C120) or mulch (BMP C121) within two days (October 1 through April 30) or seven days (May 1 through September 30). • After fertilizing, all areas that will not be impacted by construction will be seeded (BMP C120). • Topsoil stockpiles will be stabilized with plastic coverings (BMP C123). • Dust control (BMP C140) will be provided by sprinkling the site with water. • Permanent erosion control measures will include site paving and seeding of exposed soils. 7.6 Protect Slopes Slopes on the site will be protected to minimize erosion. Temporary and permanent seeding (BMP C120) will be used to reduce erosion of exposed soils on slopes. Runoff collection methods include temporary interceptor swales (BMP C200). Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 5 2220471.11 7.7 Protect Drain Inlets Proposed drain inlets shall be protected until final site stabilization. Any storm drain inlets downstream shall be protected so that surface water runoff does not enter the conveyance system without first being filtered. Inlets shall be inspected weekly, at a minimum, and daily during storm events. Storm Drain Inlet Protection (BMP C220) will be provided. 7.8 Stabilize Channels and Outlets Interceptor swales are proposed for the project to divert stormwater away from the construction area and direct it to the sediment ponds. Stabilized channels will be provided for interceptor swales descending the slopes associated with the proposed daylight lots. Outlets to the sediment ponds will be stabilized to prevent erosion, and check dams (BMP C207) will be provided. 7.9 Control Pollutants All waste materials will be collected and stored in a securely closed metal dumpster. All trash and construction debris from the site will be deposited in the dumpster. The dumpster will be emptied a minimum of once per week, and the trash will be hauled to the local landfill. No construction materials will be buried onsite. All personnel will be instructed regarding the correct procedure for waste disposal. All sanitary waste will be collected from the portable units a minimum of three times per week. Good housekeeping and spill control practices will be followed during construction to minimize stormwater contamination from petroleum products, fertilizers, and concrete. Table 1 below lists several pollutants that are commonly found on construction sites that have the potential to contaminate storm runoff. These pollutants will be present, mainly in areas of building and pavement construction. The Contractor and the CESCL will be responsible for identifying areas where these pollutants are being used and monitor runoff coming from these areas. Pollutant sources will be covered with plastic if contaminated runoff is observed from these areas. If contaminated runoff is found in the sediment trap or soils, the CESCL will direct the Contractor to remove the polluted water/soil and dispose of it in an approved area offsite. Table 1 – Potential Construction Site Stormwater Pollutants Trade Name Material Chemical/Physical Description (1) Stormwater Pollutants (1) Pesticides (insecticides, fungicides, herbicide, rodenticides) Various colored to colorless liquid, powder, pellets, or grains Chlorinated hydrocarbons, organophosphates, carbamates, arsenic Fertilizer Liquid or solid grains Nitrogen, phosphorous Plaster White granules or powder Calcium sulphate, calcium carbonate, sulfuric acid Cleaning solvents Colorless, blue, or yellow-green liquid Perchloroethylene, methylene chloride, trichloroethylene, petroleum distillates Asphalt Black solid Oil, petroleum distillates Concrete White solid Limestone, sand Glue, adhesives White or yellow liquid Polymers, epoxies Paints Various colored liquid Metal oxides, Stoddard solvent, talc, calcium carbonate, arsenic Curing compounds Creamy white liquid Naphtha Wastewater from construction equipment washing Water Soil, oil & grease, solids Wood preservatives Clear amber or dark brown liquid Stoddard solvent, petroleum distillates, arsenic, copper, chromium Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 6 2220471.11 Trade Name Material Chemical/Physical Description (1) Stormwater Pollutants (1) Hydraulic oil/fluids Brown oily petroleum hydrocarbon Mineral oil Gasoline Colorless, pale brown or pink petroleum hydrocarbon Benzene, ethyl benzene, toluene, xylene, MTBE Diesel fuel Clear, blue-green to yellow liquid Petroleum distillate, oil & grease, naphthalene, xylenes Kerosene Pale yellow liquid petroleum hydrocarbon Coal oil, petroleum distillates Antifreeze/coolant Clear green/yellow liquid Ethylene glycol, propylene glycol, heavy metals (copper, lead, zinc) Erosion Solid Particles Soil, Sediment (1) Data obtained from MSDS when available 7.9.1 Required BMPs The following BMPs or equivalent measures are required of all businesses and agencies during concrete pouring and asphalt application at temporary sites: • Employees must be educated on the pollution hazards of concrete and asphalt application and cutting. • Loose aggregate chunks and dust must be swept or shoveled and collected (not hosed down a storm drain) for recycling or proper disposal at the end of each work day, especially at work sites such as streets, driveways, parking lots, sidewalks, curbs, and gutters where rain can readily pick up the loose material and carry it to the nearest stormwater conveyance. Small amounts of excess concrete, grout, and mortar can be disposed of in the trash. • Storm drain covers or similarly effective containment devices must be placed over all nearby drains at the beginning of each day. Shovel or vacuum slurry and remove from the site. All accumulated runoff and solids must be collected and properly disposed at the end of each workday, or more often if necessary. • Exposed aggregate washing, where the top layer of unhardened concrete is hosed or scraped off to leave a rough finish, must be done with a mechanism for containment and collection of the discarded concrete slurry (such as the storm drain covers mentioned above). The easiest way to contain the wash water will be to direct the washings to a hole in the ground where the water can percolate into the ground and the solids later covered with soil. • If directed to a drain, a catch basin filter insert must be used to remove the solids. This is especially useful if the activity must proceed on rainy days. • Cleaning of concrete application and mixing equipment or concrete vehicles on the work site must be done in a designated area where the rinse water is controlled. The rinse water must either be collected for proper disposal or put into a hole in the ground where the water can percolate away and the solids later covered with soil or recovered and disposed or recycled. The use of any treatment BMP must not result in the violation of groundwater, surface water, or drinking water quality standards. Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 7 2220471.11 7.10 Control Dewatering Dewatering is not anticipated for the project. 7.11 Maintain BMPs Temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed to assure performance of their intended functions. Sediment control BMPs such as silt fencing, slope blankets, and drain inlet protection shall be inspected weekly or after a runoff-producing event. Temporary erosion and sediment control BMPs will be removed within 30 days after final site stabilization is achieved. The following inspection and maintenance practices will be used to maintain erosion and sediment controls: • Built-up sediment will be removed from silt fencing when it has reached one-third the height of the fence. • Silt fences will be inspected for depth of sediment, tears in the fabric, and attachment to the fence posts, and to ensure that fence posts are firmly in the ground. Accumulated sediment will be removed from behind the fence. • Check dams will be inspected for depth of sediment. Accumulated sediment will be removed when it reaches 6 inches in depth. • Temporary and permanent seeding will be inspected for bare spots, washouts, and healthy growth. • The Contractor CESCL (BMP C160) will provide erosion control inspection services and stormwater disposal monitoring through construction. The City Inspector will be notified of daily construction activities and scheduled meetings between the CESCL and the Contractor. The maintenance inspection report will be made after each inspection. Copies of the report forms to be completed by the CESCL are attached as Exhibit 3 of this CSWPPP. Completed forms will be provided to the City Inspector and will also be maintained onsite during the entire construction project. If construction activities or design modifications are made to the site plan that could impact stormwater, or if AHBL determines that the measures are not adequate to prevent erosion and the discharge of sediment from the site (based on turbidity measurements), this CSWPPP will be amended appropriately. The amended CSWPPP will have a description of the new activities that contribute to the increased pollutant loading and the planned source control activities. 7.12 Manage the Project The following practices will be required during construction to properly manage activities: • Comply with seasonal work limitations. • Inspect, maintain, and repair BMPs. • Identify a CESCL (BMP C160). • Maintain the CSWPPP onsite at all times, including narrative and plans. Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 8 2220471.11 7.13 Protect Low Impact Development BMPs • Proposed infiltration location will be protected from construction vehicles and equipment to the maximum extent practical. • Proposed sediment traps designed to not impact infiltration interface of future infiltration facility. • All Low Impact Development (LID) BMPs should be kept clean of sediment and equipment to the maximum extent practical. 8.0 Construction Sequence and Phasing 8.1 Construction Sequence The construction sequence is described below: 1. Stake/flag clearing and construction limits. 2. Arrange and attend a pre-construction meeting with City of Yelm. 3. Install construction entrance. 4. Provide demolition as required to install all temporary erosion control BMPs according to the TESC plan. Install inlet sediment protection in existing catch basins. 5. Provide all perimeter erosion control and site barrier fencing, including filter fabric fence, prior to start of any clearing or grading activities. 6. Excavate temporary TESC ponds (protect subgrades from compaction – used for infiltration). 7. Demolish remainder of existing site features, as designated for demolition on the plans. 8. Maintain erosion control measures as site development progresses. 9. Rough grade site and provide erosion control BMPs to stabilize the site and direct all surface runoff to the TESC plans. 10. Apply erosion control mulch and seeding, straw mulch or equal to areas that will not be brought to final grade or permanently vegetated within seven days of exposure during the dry season and two days of exposure during the wet season (October 1 through April 30). 11. Relocate erosion control measures or install new measures so that, as site conditions change, erosion and sediment control is always in accordance with the CSWPPP minimum requirements. 12. Construct storm system and miscellaneous utilities and install inlet sediment protection to new catch basins. 13. Fine grade asphalt paved areas, place curb and gutters, and pave. Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 9 2220471.11 14. After site is stabilized, complete bio-retention construction: 14.1. Scarify bio-retention subgrade (do not compact bioretention bottoms). Schedule geotechnical engineer for infiltration testing to ensure bioretention functions per design. Provide infiltration testing, to include a minimum of two tests per bioretention system. 14.2. Upon approval of infiltration testing results, provide bioretention treatment soil mix per plans. Bioretention soil mix placement and bioretention excavation shall not be conducted during wet or saturated conditions. 15. Provide landscape plantings and final bioretention stabilization per landscape design. 16. Remove remaining temporary erosion control items once site has been stabilized and upon approval from the City. 17. Perform final closeout items, as required by City and owner. 8.2 Construction Phasing No phasing is proposed. The proposed construction will be done as one project. 9.0 Construction Schedule Construction is anticipated to begin in summer 2024 and be completed in summer 2025. Based on the construction schedule, construction may be ongoing during the wet season. During construction, measures will be taken to prevent the transportation of sediment from the site to receiving waters. These measures include the use of, but are not limited to, the BMPs listed in Exhibit 4. 10.0 Financial/Ownership Responsibilities The developer is the party responsible for initiation of bonds and other financial securities. The project must comply with City of Yelm financial liability requirements. 11.0 Certified Erosion and Sediment Control Lead (CESCL) The General Contractor shall be required to provide a CESCL prior to permit issuance. The CESCL can be identified at the preconstruction meeting. Once this individual is identified, the City Inspector will be notified. The contractor will designate their CESCL here: Name: _________________________________ Address: _________________________________ Phone: _________________________________ Email: _________________________________ The CESCL is required to meet DOE certification requirements. The City Inspector will be provided with CESCL information. Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 10 2220471.11 The duties of the CESCL include: • Implementing the CSWPPP/TESC plan with the aid of the SWPP Team. • Overseeing maintenance practices identified as BMPs in the CSWPPP. • Conducting or providing for inspection and monitoring activities. • Sampling stormwater for turbidity using a turbidity meter. • Identifying other potential pollutant sources and ensuring they are added to the plan. • Identifying any deficiencies in the CSWPPP and ensuring they are corrected. • Ensuring that any changes in construction plans are addressed in the CSWPPP. To aid in the implementation of the CSWPPP, the members of the SWPP Team include the following: General Contractor, CESCL, City of Yelm Inspector, the geotechnical engineering consultant, and AHBL. The General Contractor will ensure that all housekeeping and monitoring procedures are implemented, while the CESCL will ensure the integrity of the structural BMPs. The SWPP Team will observe construction and erosion control practices and recommend revisions or additions to the CSWPPP and drawings. This analysis is based on data and records either supplied to or obtained by AHBL, Inc. These documents are referenced within the text of the analysis. The analysis has been prepared using procedures and practices within the standard accepted practices of the industry. We conclude that this project, as proposed, will not create any new problems within the existing downstream drainage system. This project will not noticeably aggravate any existing downstream problems due to either water quality or quantity. AHBL, Inc. Michael C. Hager, PE Project Engineer MCH/lsk May 2024 Q:\2022\2220471\WORDPROC\Reports\20240507 Rpt (SWPPP) 2220471.11.docx Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 Exhibit 1 NRCS Soils Map Soil Map—Thurston County Area, Washington (Summit at Thompson Creek - Yelm, WA) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 5/6/2024 Page 1 of 3 51 9 9 1 0 0 51 9 9 1 8 0 51 9 9 2 6 0 51 9 9 3 4 0 51 9 9 4 2 0 51 9 9 5 0 0 51 9 9 5 8 0 51 9 9 1 0 0 51 9 9 1 8 0 51 9 9 2 6 0 51 9 9 3 4 0 51 9 9 4 2 0 51 9 9 5 0 0 51 9 9 5 8 0 527500 527580 527660 527740 527820 527900 527980 528060 528140 528220 527500 527580 527660 527740 527820 527900 527980 528060 528140 528220 46° 56' 57'' N 12 2 ° 3 8 ' 2 0 ' ' W 46° 56' 57'' N 12 2 ° 3 7 ' 4 3 ' ' W 46° 56' 41'' N 12 2 ° 3 8 ' 2 0 ' ' W 46° 56' 41'' N 12 2 ° 3 7 ' 4 3 ' ' W N Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 150 300 600 900 Feet 0 50 100 200 300 Meters Map Scale: 1:3,560 if printed on A landscape (11" x 8.5") sheet. Soil Map may not be valid at this scale. MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Thurston County Area, Washington Survey Area Data: Version 17, Aug 29, 2023 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: May 26, 2023—Aug 14, 2023 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—Thurston County Area, Washington (Summit at Thompson Creek - Yelm, WA) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 5/6/2024 Page 2 of 3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI 65 McKenna gravelly silt loam, 0 to 5 percent slopes 3.0 9.5% 74 Nisqually loamy fine sand, 3 to 15 percent slopes 3.5 11.0% 111 Spanaway gravelly sandy loam, 3 to 15 percent slopes 2.0 6.1% 1100 Steilacoom-Yelm complex, 0 to 2 percent slopes 5.8 18.3% 2101 McChord-Everett complex, 3 to 15 percent slopes 17.6 55.1% Totals for Area of Interest 32.0 100.0% Soil Map—Thurston County Area, Washington Summit at Thompson Creek - Yelm, WA Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 5/6/2024 Page 3 of 3 Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 Exhibit 2 TESC Calculations SN Element Description Data Data Rainfall Rain State County Return Rainfall Rainfall ID Source Source Type Units Period Depth Distribution ID (years) (inches) 1 Rain Gage-01 Time Series 2yr24hr 3"Storm (15 TS) Cumulative inches Washington Thurston 2 3 SCS Type IA 24-hr SN Element Description Area Drainage Impervious Pervious Impervious Rain Gage Total Total Peak Time ID Node ID Area Area Area ID Precipitation Runoff Runoff of Curve Curve Concentration Number Number (acres) (%) (inches) (inches) (cfs)(days hh:mm:ss) 1 EAST 8.92 EPONDOUT 98.00 76.00 40.00 Rain Gage-01 2.99 1.71 3.50 0 00:05:00 2 SOUTH 10.16 SPONDOUT 98.00 76.00 40.00 Rain Gage-01 2.99 1.71 3.99 0 00:05:00 3 WEST 8.70 WPONDOUT 98.00 76.00 50.00 Rain Gage-01 2.99 1.88 3.88 0 00:05:00 SN Element Description Data Data Rainfall Rain State County Return Rainfall Rainfall ID Source Source Type Units Period Depth Distribution ID (years) (inches) 1 Rain Gage-01 Time Series 10YR24HR 4" Storm (10min TS) Cumulative inches Washington Thurston 10 4 SCS Type IA 24-hr SN Element Description Area Drainage Impervious Pervious Impervious Rain Gage Total Total Peak Time ID Node ID Area Area Area ID Precipitation Runoff Runoff of Curve Curve Concentration Number Number (acres) (%) (inches) (inches) (cfs)(days hh:mm:ss) 1 EAST 8.92 EPONDOUT 98.00 76.00 40.00 Rain Gage-01 3.98 2.54 5.40 0 00:05:00 2 SOUTH 10.16 SPONDOUT 98.00 76.00 40.00 Rain Gage-01 3.98 2.54 6.15 0 00:05:00 3 WEST 8.70 WPONDOUT 98.00 76.00 50.00 Rain Gage-01 3.98 2.74 5.78 0 00:05:00 SN Element Description Data Data Rainfall Rain State County Return Rainfall Rainfall ID Source Source Type Units Period Depth Distribution ID (years) (inches) 1 Rain Gage-01 Time Series 100yr24hr 6" Storm (15TS) Cumulative inches Washington Thurston 100 6 SCS Type IA 24-hr SN Element Description Area Drainage Impervious Pervious Impervious Rain Gage Total Total Peak Time ID Node ID Area Area Area ID Precipitation Runoff Runoff of Curve Curve Concentration Number Number (acres) (%) (inches) (inches) (cfs)(days hh:mm:ss) 1 EAST 8.92 EPONDOUT 98.00 76.00 40.00 Rain Gage-01 5.98 4.32 9.38 0 00:05:00 2 SOUTH 10.16 SPONDOUT 98.00 76.00 40.00 Rain Gage-01 5.98 4.32 10.69 0 00:05:00 3 WEST 8.70 WPONDOUT 98.00 76.00 50.00 Rain Gage-01 5.98 4.55 9.67 0 00:05:00 TESC Storm Events and Runoff Volumes 2 2215 N. 30th Street, #300 Tacoma, WA 98403 253.383.2422 TEL 253.383.2572 FAX www.ahbl.com Summit at Thompson Creek SOUTH SEDIMENT POND & DEWATERING ORIFICE SIZING 2.B AO = AS(2h)0.5 / (0.6*3600*Tg0.5) ; h =4 then AO = 0.00001*A S D = 13.54*√AO WITH A 3.5' DEPTH DEWATERING ORIFICE AREA (24HR DEWATERING TIME) A0 = AS(2h)^0.5 / (0.6*3600*24g^0.5) A0 = 12,792(7)^0.5 / 294,166 AO = 33,845 / 294,166 AO = 0.115 SF = 3.14/4*D^2 AO = 0.115 SF = 0.785*D^2 D = 0.488 FT = 5.85" INSTALL A 6" ORIFICE DIAMETER FOR THE SEDIMENT POND DEWATERING ORIFICE TESC POND SURFACE AREA 2,080 SF * Q2YR,PEAK Q 2YR = 3.99 CFS Q 10YR = 6.15 8,299 SF 12,792 2215 N. 30th Street, #300 Tacoma, WA 98403 253.383.2422 TEL 253.383.2572 FAX www.ahbl.com Summit at Thompson Creek WEST SEDIMENT POND & DEWATERING ORIFICE SIZING 2.C AO = AS(2h)0.5 / (0.6*3600*Tg0.5) ; h =4 then AO = 0.00001*A S D = 13.54*√AO WITH A 3.5' DEPTH DEWATERING ORIFICE AREA (24HR DEWATERING TIME) A0 = AS(2h)^0.5 / (0.6*3600*24g^0.5) A0 = 12,022(7)^0.5 / 294,166 AO = 31,807 / 294,166 AO = 0.108 SF = 3.14/4*D^2 AO = 0.108 SF = 0.785*D^2 D = 0.371 FT = 4.45" INSTALL A 6" ORIFICE DIAMETER FOR THE SEDIMENT POND DEWATERING ORIFICE TESC POND SURFACE AREA 2,080 SF * Q2YR,PEAK Q 2YR = 3.88 CFS Q 10YR = 5.78 8,070 SF 12,022 2215 N. 30th Street, #300 Tacoma, WA 98403 253.383.2422 TEL 253.383.2572 FAX www.ahbl.com Summit at Thompson Creek EAST SEDIMENT POND & DEWATERING ORIFICE SIZING 2.D AO = AS(2h)0.5 / (0.6*3600*Tg0.5) ; h =4 then AO = 0.00001*A S D = 13.54*√AO WITH A 3.5' DEPTH DEWATERING ORIFICE AREA (24HR DEWATERING TIME) A0 = AS(2h)^0.5 / (0.6*3600*24g^0.5) A0 = 12,792(7)^0.5 / 294,166 AO = 29,717 / 294,166 AO = 0.101 SF = 3.14/4*D^2 AO = 0.115 SF = 0.785*D^2 D = 0.359 FT = 4.30" INSTALL A 6" ORIFICE DIAMETER FOR THE SEDIMENT POND DEWATERING ORIFICE TESC POND SURFACE AREA 2,080 SF * Q2YR,PEAK Q 2YR = 3.50 CFS Q 10YR = 5.40 7,280 SF 11,232 Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 Exhibit 3 Inspection Logs Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 The Summit at Thompson Creek Plat Construction Stormwater Pollution Prevention Plan Inspection and Maintenance Report Form To be completed every 7 days and within 24 hours of a rainfall event of 0.5 inches or more Inspector: Date: Inspector's Qualifications: Days since last rainfall: Amount of last rainfall: inches Stabilization Measures Drainage Area Date Since Last Disturbance Date of Disturbance Stabilized (yes/No) Stabilized With Condition Stabilization required: To be performed by: On or before: Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 The Summit at Thompson Creek Plat Construction Stormwater Pollution Prevention Plan Inspection and Maintenance Report Form Perimeter Structural Controls: Date: Silt Fence Drainage Area Perimeter Has Silt Reached 1/3 of Fence Height? Is Fence Properly Secured? Is There Evidence of Washout or Overtopping? Maintenance required for silt fence and straw bales: To be performed by: On or before: Construction Stormwater Pollut ion Prevention Plan The Summit at Thompson Creek 2220471.11 The Summit at Thompson Creek Plat Construction Stormwater Pollution Prevention Plan Inspection and Maintenance Report Form Changes required to the pollution prevention plan: Reasons for changes: I certify under penalty of law that this document and all attachments were prepared under my direction or supervision in accordance with a system designed to assure that qualified personnel properly gathered and evaluated the information submitted . Based on my inquiry of the person or persons who manage the system, or those persons directly responsible for gathering the information, the information submitted is, to the best of my knowledge and belief, true, accurate, and complete. I am aware that there are significant penalties for submitting false information, including the possibility of fine and imprisonment for knowing violations . Signature: Date: ____________________ Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 Exhibit 4 Selected Best Management Practices (BMPs) • High Visibility Fence (BMP C103) • Stabilized Construction Access (BMP C105) • Temporary and Permanent Seeding (BMP C120) • Mulching (BMP C121) • Nets and Blankets (BMP C122) • Plastic Covering (BMP C123) • Dust Control (BMP C140) • Materials on Hand (BMP C150) • Concrete Handling (BMP C151) • Sawcutting and Surfacing Pollution Prevention (BMP C152) • Material Delivery, Storage, and Containment (BMP C153) • Concrete Washout Area (BMP C154) • Certified Erosion and Sediment Control Lead (BMP C160) • Scheduling (BMP C162) • Interceptor Dike and Swale (BMP C200) • Riprap Channel Lining (BMP C202) • Check Dams (BMP C207) • Outlet Protection (BMP C209) • Inlet Protection (BMP C220) • Silt Fence (BMP C233) • Sediment Pond (BMP C241) BMP C103: High-Visibility Fence Purpose High-visibility fencing is intended to: l Restrict clearing to approved limits. l Prevent disturbance of sensitive areas, their buffers, and other areas required to be left undis- turbed. l Limit construction traffic to designated construction entrances, exits, or internal roads. l Protect areas where marking with survey tape may not provide adequate protection. Conditions of Use To establish clearing limits plastic, fabric, or metal fence may be used: l At the boundary of sensitive areas, their buffers, and other areas required to be left uncleared. l As necessary to control vehicle access to and on the site. Design and Installation Specifications High-visibility plastic fence shall be composed of a high-density polyethylene material and shall be at least four feet in height. Posts for the fencing shall be steel or wood and placed every 6 feet on center (maximum) or as needed to ensure rigidity. The fencing shall be fastened to the post every six inches with a polyethylene tie. On long continuous lengths of fencing, a tension wire or rope shall be used as a top stringer to prevent sagging between posts. The fence color shall be high-visibility orange. The fence tensile strength shall be 360 lbs/ft using the ASTM D4595 testing method. If appropriate install fabric silt fence in accordance with BMP C233: Silt Fence to act as high-visibility fence. Silt fence shall be at least 3 feet high and must be highly visible to meet the requirements of this BMP. Metal fences shall be designed and installed according to the manufacturer's specifications. Metal fences shall be at least 3 feet high and must be highly visible. Fences shall not be wired or stapled to trees. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 274 Maintenance Standards If the fence has been damaged or visibility reduced, it shall be repaired or replaced immediately and visibility restored. BMP C105: Stabilized Construction Access Purpose Stabilized construction accesses are established to reduce the amount of sediment transported onto paved roads outside the project site by vehicles or equipment. This is done by constructing a sta- bilized pad of quarry spalls at entrances and exits for project sites. Conditions of Use Construction accesses shall be stabilized wherever traffic will be entering or leaving a construction site if paved roads or other paved areas are within 1,000 feet of the site. For residential subdivision construction sites, provide a stabilized construction access for each res- idence, rather than only at the main subdivision entrance. Stabilized surfaces shall be of sufficient length/width to provide vehicle access/parking, based on lot size and configuration. On large commercial, highway, and road projects, the designer should include enough extra mater- ials in the contract to allow for additional stabilized accesses not shown in the initial Construction SWPPP. It is difficult to determine exactly where access to these projects will take place; additional materials will enable the contractor to install them where needed. Design and Installation Specifications See Figure II-3.1: Stabilized Construction Access for details. Note: the 100’ minimum length of the access shall be reduced to the maximum practicable size when the size or configuration of the site does not allow the full length (100’). Construct stabilized construction accesses with a 12-inch thick pad of 4-inch to 8-inch quarry spalls, a 4-inch course of asphalt treated base (ATB), or use existing pavement. Do not use crushed con- crete, cement, or calcium chloride for construction access stabilization because these products raise pH levels in stormwater and concrete discharge to waters of the State is prohibited. A separation geotextile shall be placed under the spalls to prevent fine sediment from pumping up into the rock pad. The geotextile shall meet the standards listed in Table II-3.2: Stabilized Con- struction Access Geotextile Standards. Geotextile Property Required Value Grab Tensile Strength (ASTM D4751)200 psi min. Table II-3.2: Stabilized Construction Access Geotextile Standards 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 275 Geotextile Property Required Value Grab Tensile Elongation (ASTM D4632)30% max. Mullen Burst Strength (ASTM D3786-80a)400 psi min. AOS (ASTM D4751)20-45 (U.S. standard sieve size) Table II-3.2: Stabilized Construction Access Geotextile Standards (continued) l Consider early installation of the first lift of asphalt in areas that will be paved; this can be used as a stabilized access. Also consider the installation of excess concrete as a stabilized access. During large concrete pours, excess concrete is often available for this purpose. l Fencing (see BMP C103: High-Visibility Fence) shall be installed as necessary to restrict traffic to the construction access. l Whenever possible, the access shall be constructed on a firm, compacted subgrade. This can substantially increase the effectiveness of the pad and reduce the need for maintenance. l Construction accesses should avoid crossing existing sidewalks and back of walk drains if at all possible. If a construction access must cross a sidewalk or back of walk drain, the full length of the sidewalk and back of walk drain must be covered and protected from sediment leaving the site. Alternative Material Specification WSDOT has raised safety concerns about the Quarry Spall rock specified above. WSDOT observes that the 4-inch to 8-inch rock sizes can become trapped between Dually truck tires, and then released off-site at highway speeds. WSDOT has chosen to use a modified specification for the rock while continuously verifying that the Stabilized Construction Access remains effective. To remain effective, the BMP must prevent sediment from migrating off site. To date, there has been no per- formance testing to verify operation of this new specification. Jurisdictions may use the alternative specification, but must perform increased off-site inspection if they use, or allow others to use, it. Stabilized Construction Accesses may use material that meets the requirements of WSDOT's Stand- ard Specifications for Road, Bridge, and Municipal Construction Section 9-03.9(1) (WSDOT, 2016) for ballast except for the following special requirements. The grading and quality requirements are listed in Table II-3.3: Stabilized Construction Access Alternative Material Requirements. Sieve Size Percent Passing 2½″99-100 Table II-3.3: Stabilized Construction Access Alternative Material Requirements 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 276 Sieve Size Percent Passing 2″65-100 ¾″40-80 No. 4 5 max. No. 100 0-2 % Fracture 75 min. Table II-3.3: Stabilized Construction Access Alternative Material Requirements (continued) l All percentages are by weight. l The sand equivalent value and dust ratio requirements do not apply. l The fracture requirement shall be at least one fractured face and will apply the combined aggregate retained on the No. 4 sieve in accordance with FOP for AASHTO T 335. Maintenance Standards Quarry spalls shall be added if the pad is no longer in accordance with the specifications. l If the access is not preventing sediment from being tracked onto pavement, then alternative measures to keep the streets free of sediment shall be used. This may include replace- ment/cleaning of the existing quarry spalls, street sweeping, an increase in the dimensions of the access, or the installation of BMP C106: Wheel Wash. l Any sediment that is tracked onto pavement shall be removed by shoveling or street sweep- ing. The sediment collected by sweeping shall be removed or stabilized on site. The pavement shall not be cleaned by washing down the street, except when high efficiency sweeping is inef- fective and there is a threat to public safety. If it is necessary to wash the streets, the con- struction of a small sump to contain the wash water shall be considered. The sediment would then be washed into the sump where it can be controlled. l Perform street sweeping by hand or with a high efficiency sweeper. Do not use a non-high effi- ciency mechanical sweeper because this creates dust and throws soils into storm systems or conveyance ditches. l Any quarry spalls that are loosened from the pad, which end up on the roadway shall be removed immediately. l If vehicles are entering or exiting the site at points other than the construction access(es), BMP C103: High-Visibility Fence shall be installed to control traffic. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 277 l Upon project completion and site stabilization, all construction accesses intended as per- manent access for maintenance shall be permanently stabilized. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 278 Figure II-3.1: Stabilized Construction Access 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 279 Approved as Functionally Equivalent Ecology has approved products as able to meet the requirements of this BMP. The products did not pass through the Technology Assessment Protocol – Ecology (TAPE) process. Local jurisdictions may choose not to accept these products, or may require additional testing prior to consideration for local use. Products that Ecology has approved as functionally equivalent are available for review on Ecology’s website at: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-per- mittee-guidance-resources/Emerging-stormwater-treatment-technologies 2019 Stormwater Management Manual for Western Washington Volume II - Chapter 3 - Page 280 BMP C120: Temporary and Permanent Seeding Purpose Seeding reduces erosion by stabilizing exposed soils. A well-established vegetative cover is one of the most effective methods of reducing erosion. Conditions of Use Use seeding throughout the project on disturbed areas that have reached final grade or that will remain unworked for more than 30 days. The optimum seeding windows for western Washington are April 1 through June 30 and September 1 through October 1. Between July 1 and August 30 seeding requires irrigation until 75 percent grass cover is established. Between October 1 and March 30 seeding requires a cover of mulch or an erosion control blanket until 75 percent grass cover is established. Review all disturbed areas in late August to early September and complete all seeding by the end of September. Otherwise, vegetation will not establish itself enough to provide more than average pro-tection. Mulch is required at all times for seeding because it protects seeds from heat, moisture loss, and transport due to runoff. Mulch can be applied on top of the seed or simultaneously by hydroseeding. See BMP C121: Mulching for specifications. Seed and mulch all disturbed areas not otherwise vegetated at final site stabilization. Final sta- bilization means the completion of all soil disturbing activities at the site and the establishment of a permanent vegetative cover, or equivalent permanent stabilization measures (such as pavement, riprap, gabions, or geotextiles) which will prevent erosion. See BMP T5.13: Post-Construction Soil Quality and Depth. Design and Installation Specifications General l Install channels intended for vegetation before starting major earthwork and hydroseed with a Bonded Fiber Matrix. For vegetated channels that will have high flows, install erosion control blankets over the top of hydroseed. Before allowing water to flow in vegetated channels, establish 75 percent vegetation cover. If vegetated channels cannot be established by seed 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 284 before water flow; install sod in the channel bottom — over top of hydromulch and erosion con- trol blankets. l Confirm the installation of all required surface water control measures to prevent seed from washing away. l Hydroseed applications shall include a minimum of 1,500 pounds per acre of mulch with 3 per- cent tackifier. See BMP C121: Mulching for specifications. l Areas that will have seeding only and not landscaping may need compost or meal-based mulch included in the hydroseed in order to establish vegetation. Re-install native topsoil on the disturbed soil surface before application. See BMP T5.13: Post-Construction Soil Quality and Depth. l When installing seed via hydroseeding operations, only about 1/3 of the seed actually ends up in contact with the soil surface. This reduces the ability to establish a good stand of grass quickly. To overcome this, consider increasing seed quantities by up to 50 percent. l Enhance vegetation establishment by dividing the hydromulch operation into two phases: o Phase 1- Install all seed and fertilizer with 25-30 percent mulch and tackifier onto soil in the first lift. o Phase 2- Install the rest of the mulch and tackifier over the first lift. Or, enhance vegetation by: o Installing the mulch, seed, fertilizer, and tackifier in one lift. o Spread or blow straw over the top of the hydromulch at a rate of 800-1000 pounds per acre. o Hold straw in place with a standard tackifier. Both of these approaches will increase cost moderately but will greatly improve and enhance vegetative establishment. The increased cost may be offset by the reduced need for: o Irrigation. o Reapplication of mulch. o Repair of failed slope surfaces. This technique works with standard hydromulch (1,500 pounds per acre minimum) and Bon- ded Fiber Matrix/ Mechanically Bonded Fiber Matrix (BFM/MBFMs) (3,000 pounds per acre minimum). l Seed may be installed by hand if: o Temporary and covered by straw, mulch, or topsoil. o Permanent in small areas (usually less than 1 acre) and covered with mulch, topsoil, or erosion blankets. l The seed mixes listed in Table II-3.4: Temporary and Permanent Seed Mixes include 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 285 recommended mixes for both temporary and permanent seeding. l Apply these mixes, with the exception of the wet area seed mix, at a rate of 120 pounds per acre. This rate can be reduced if soil amendments or slow-release fertilizers are used. Apply the wet area seed mix at a rate of 60 pounds per acre. l Consult the local suppliers or the local conservation district for their recommendations. The appropriate mix depends on a variety of factors, including location, exposure, soil type, slope, and expected foot traffic. Alternative seed mixes approved by the local authority may be used, depending on the soil type and hydrology of the area. Common Name Latin Name % Weight % Purity % Germination Temporary Erosion Control Seed Mix A standard mix for areas requiring a temporary vegetative cover. Chewings or annual blue grass Festuca rubra var. commutata or Poa anna 40 98 90 Perennial rye Lolium perenne 50 98 90 Redtop or colonial bentgrass Agrostis alba or Agrostis tenuis 5 92 85 White dutch clover Trifolium repens 5 98 90 Landscaping Seed Mix A recommended mix for landscaping seed. Perennial rye blend Lolium perenne 70 98 90 Chewings and red fescue blend Festuca rubra var. commutata or Fes- tuca rubra 30 98 90 Low-Growing Turf Seed Mix A turf seed mix for dry situations where there is no need for watering. This mix requires very little main- tenance. Dwarf tall fescue (several varieties) Festuca arundin- acea var. 45 98 90 Dwarf perennial rye (Barclay) Lolium perenne var. barclay 30 98 90 Red fescue Festuca rubra 20 98 90 Colonial bentgrass Agrostis tenuis 5 98 90 Bioswale Seed Mix A seed mix for bioswales and other intermittently wet areas. Tall or meadow fes-Festuca arundin-75-80 98 90 Table II-3.4: Temporary and Permanent Seed Mixes 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 286 Common Name Latin Name % Weight % Purity % Germination cue acea or Festuca elatior Seaside/Creeping bentgrass Agrostis palustris 10-15 92 85 Redtop bentgrass Agrostis alba or Agrostis gigantea 5-10 90 80 Wet Area Seed Mix A low-growing, relatively non-invasive seed mix appropriate for very wet areas that are not regulated wet- lands. Consult Hydraulic Permit Authority (HPA) for seed mixes if applicable. Tall or meadow fes- cue Festuca arundin- acea or Festuca elatior 60-70 98 90 Seaside/Creeping bentgrass Agrostis palustris 10-15 98 85 Meadow foxtail Alepocurus praten- sis 10-15 90 80 Alsike clover Trifolium hybridum 1-6 98 90 Redtop bentgrass Agrostis alba 1-6 92 85 Meadow Seed Mix A recommended meadow seed mix for infrequently maintained areas or non-maintained areas where col- onization by native plants is desirable. Likely applications include rural road and utility right-of-way. Seed- ing should take place in September or very early October in order to obtain adequate establishment prior to the winter months. Consider the appropriateness of clover, a fairly invasive species, in the mix. Amending the soil can reduce the need for clover. Redtop or Oregon bentgrass Agrostis alba or Agrostis ore- gonensis 20 92 85 Red fescue Festuca rubra 70 98 90 White dutch clover Trifolium repens 10 98 90 Table II-3.4: Temporary and Permanent Seed Mixes (continued) Roughening and Rototilling l The seedbed should be firm and rough. Roughen all soil no matter what the slope. Track walk slopes before seeding if engineering purposes require compaction. Backblading or smoothing of slopes greater than 4H:1V is not allowed if they are to be seeded. l Restoration-based landscape practices require deeper incorporation than that provided by a simple single-pass rototilling treatment. Wherever practical, initially rip the subgrade to improve long-term permeability, infiltration, and water inflow qualities. At a minimum, 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 287 permanent areas shall use soil amendments to achieve organic matter and permeability per- formance defined in engineered soil/landscape systems. For systems that are deeper than 8 inches complete the rototilling process in multiple lifts, or prepare the engineered soil system per specifications and place to achieve the specified depth. Fertilizers l Conducting soil tests to determine the exact type and quantity of fertilizer is recommended. This will prevent the over-application of fertilizer. l Organic matter is the most appropriate form of fertilizer because it provides nutrients (includ- ing nitrogen, phosphorus, and potassium) in the least water-soluble form. l In general, use 10-4-6 N-P-K (nitrogen-phosphorus-potassium) fertilizer at a rate of 90 pounds per acre. Always use slow-release fertilizers because they are more efficient and have fewer environmental impacts. Do not add fertilizer to the hydromulch machine, or agit- ate, more than 20 minutes before use. Too much agitation destroys the slow-release coating. l There are numerous products available that take the place of chemical fertilizers. These include several with seaweed extracts that are beneficial to soil microbes and organisms. If 100 percent cottonseed meal is used as the mulch in hydroseed, chemical fertilizer may not be necessary. Cottonseed meal provides a good source of long-term, slow-release, available nitrogen. Bonded Fiber Matrix and Mechanically Bonded Fiber Matrix l On steep slopes use Bonded Fiber Matrix (BFM) or Mechanically Bonded Fiber Matrix (MBFM) products. Apply BFM/MBFM products at a minimum rate of 3,000 pounds per acre with approximately 10 percent tackifier. Achieve a minimum of 95 percent soil coverage during application. Numerous products are available commercially. Most products require 24-36 hours to cure before rainfall and cannot be installed on wet or saturated soils. Generally, products come in 40-50 pound bags and include all necessary ingredients except for seed and fertilizer. l Install products per manufacturer's instructions. l BFMs and MBFMs provide good alternatives to blankets in most areas requiring vegetation establishment. Advantages over blankets include: o BFM and MBFMs do not require surface preparation. o Helicopters can assist in installing BFM and MBFMs in remote areas. o On slopes steeper than 2.5H:1V, blanket installers may require ropes and harnesses for safety. o Installing BFM and MBFMs can save at least $1,000 per acre compared to blankets. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 288 Maintenance Standards Reseed any seeded areas that fail to establish at least 75 percent cover (100 percent cover for areas that receive sheet or concentrated flows). If reseeding is ineffective, use an alternate method such as sodding, mulching, nets, or blankets. l Reseed and protect by mulch any areas that experience erosion after achieving adequate cover. Reseed and protect by mulch any eroded area. l Supply seeded areas with adequate moisture, but do not water to the extent that it causes run- off. Approved as Functionally Equivalent Ecology has approved products as able to meet the requirements of this BMP. The products did not pass through the Technology Assessment Protocol – Ecology (TAPE) process. Local jurisdictions may choose not to accept these products, or may require additional testing prior to consideration for local use. Products that Ecology has approved as functionally equivalent are available for review on Ecology’s website at: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-per- mittee-guidance-resources/Emerging-stormwater-treatment-technologies BMP C121: Mulching Purpose Mulching soils provides immediate temporary protection from erosion. Mulch also enhances plant establishment by conserving moisture, holding fertilizer, seed, and topsoil in place, and moderating soil temperatures. There are a variety of mulches that can be used. This section discusses only the most common types of mulch. Conditions of Use As a temporary cover measure, mulch should be used: l For less than 30 days on disturbed areas that require cover. l At all times for seeded areas, especially during the wet season and during the hot summer months. l During the wet season on slopes steeper than 3H:1V with more than 10 feet of vertical relief. Mulch may be applied at any time of the year and must be refreshed periodically. For seeded areas, mulch may be made up of 100 percent: l cottonseed meal; l fibers made of wood, recycled cellulose, hemp, or kenaf; 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 289 l compost; l or blends of these. Tackifier shall be plant-based, such as guar or alpha plantago, or chemical-based such as poly- acrylamide or polymers. Generally, mulches come in 40-50 pound bags. Seed and fertilizer are added at time of application. Recycled cellulose may contain polychlorinated biphenyl (PCBs). Ecology recommends that products should be evaluated for PCBs prior to use. Refer to BMP C126: Polyacrylamide (PAM) for Soil Erosion Protection for conditions of use. PAM shall not be directly applied to water or allowed to enter a water body. Any mulch or tackifier product used shall be installed per the manufacturer’s instructions. Design and Installation Specifications For mulch materials, application rates, and specifications, see Table II-3.6: Mulch Standards and Guidelines. Consult with the local supplier or the local conservation district for their recom- mendations. Increase the application rate until the ground is 95% covered (i.e. not visible under the mulch layer). Note: Thickness may be increased for disturbed areas in or near sensitive areas or other areas highly susceptible to erosion. Where the option of “Compost” is selected, it should be a coarse compost that meets the size grad- ations listed in Table II-3.5: Size Gradations of Compost as Mulch Material when tested in accord- ance with Test Method 02.02-B found in Test Methods for the Examination of Composting and Compost (Thompson, 2001). Sieve Size Percent Passing 3"100% 1"90% - 100% 3/4"70% - 100% 1/4"40% - 100% Table II-3.5: Size Gradations of Compost as Mulch Material Mulch used within the ordinary high-water mark of surface waters should be selected to minimize potential flotation of organic matter. Composted organic materials have higher specific gravities (densities) than straw, wood, or chipped material. Consult the Hydraulic Permit Authority (HPA) for mulch mixes if applicable. Maintenance Standards The thickness of the mulch cover must be maintained. Any areas that experience erosion shall be remulched and/or protected with a net or blanket. If the erosion problem is drainage related, then the problem shall be fixed and the eroded area remulched. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 290 Mulch Mater- ial Guideline Description Straw Quality Standards Air-dried; free from undesirable seed and coarse material. Application Rates 2"-3" thick; 5 bales per 1,000 sf or 2-3 tons per acre Remarks Cost-effective protection when applied with adequate thickness. Hand- application generally requires greater thickness than blown straw. The thickness of straw may be reduced by half when used in conjunction with seeding. In windy areas straw must be held in place by crimping, using a tackifier, or covering with netting. Blown straw always has to be held in place with a tackifier as even light winds will blow it away. Straw, however, has several deficiencies that should be considered when selecting mulch materials. It often introduces and/or encourages the propagation of weed species and it has no significant long-term benefits It should also not be used within the ordinary high-water elevation of surface waters (due to flot- ation). Hydromulch Quality Standards No growth inhibiting factors. Application Rates Approx. 35-45 lbs per 1,000 sf or 1,500 - 2,000 lbs per acre Remarks Shall be applied with hydromulcher. Shall not be used without seed and tackifier unless the application rate is at least doubled. Fibers longer than about 3/4 - 1 inch clog hydromulch equipment. Fibers should be kept to less than 3/4 inch. Compost Quality Standards No visible water or dust during handling. Must be produced per WAC 173- 350, Solid Waste Handling Standards, but may have up to 35% biosolids. Application Rates 2" thick min.; approx. 100 tons per acre (approx. 750 lbs per cubic yard) Remarks More effective control can be obtained by increasing thickness to 3". Excel- lent mulch for protecting final grades until landscaping because it can be dir- ectly seeded or tilled into soil as an amendment. Compost used for mulch has a coarser size gradation than compost used for BMP C125: Topsoiling / Composting or BMP T5.13: Post-Construction Soil Quality and Depth. It is more stable and practical to use in wet areas and during rainy weather conditions. Do not use near wetlands or near phosphorous impaired water bodies. Chipped Site Veget- ation Quality Standards Gradations from fines to 6 inches in length for texture, variation, and inter- locking properties. Include a mix of various sizes so that the average size is between 2- and 4- inches. Application Rates 2" thick min.; Table II-3.6: Mulch Standards and Guidelines 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 291 Mulch Mater- ial Guideline Description Remarks This is a cost-effective way to dispose of debris from clearing and grub- bing, and it eliminates the problems associated with burning. Generally, it should not be used on slopes above approx. 10% because of its tendency to be transported by runoff. It is not recommended within 200 feet of sur- face waters. If permanent seeding or planting is expected shortly after mulch, the decomposition of the chipped vegetation may tie up nutrients important to grass establishment. Note: thick application of this material over existing grass, herbaceous spe- cies, and some groundcovers could smother and kill vegetation. Wood- Based Mulch Quality Standards No visible water or dust during handling. Must be purchased from a supplier with a Solid Waste Handling Permit or one exempt from solid waste reg- ulations. Application Rates 2" thick min.; approx. 100 tons per acre (approx. 750 lbs. per cubic yard) Remarks This material is often called "wood straw" or "hog fuel". The use of mulch ultimately improves the organic matter in the soil. Special caution is advised regarding the source and composition of wood-based mulches. Its preparation typically does not provide any weed seed control, so evidence of residual vegetation in its composition or known inclusion of weed plants or seeds should be monitored and prevented (or minimized). Wood Strand Mulch Quality Standards A blend of loose, long, thin wood pieces derived from native conifer or deciduous trees with high length-to-width ratio. Application Rates 2" thick min. Remarks Cost-effective protection when applied with adequate thickness. A min- imum of 95-percent of the wood strand shall have lengths between 2 and 10-inches, with a width and thickness between 1/16 and 1/2-inches. The mulch shall not contain resin, tannin, or other compounds in quantities that would be detrimental to plant life. Sawdust or wood shavings shall not be used as mulch. [Specification 9-14.4(4) from the Standard Specifications for Road, Bridge, and Municipal Construction (WSDOT, 2016) Table II-3.6: Mulch Standards and Guidelines (continued) BMP C122: Nets and Blankets Purpose Erosion control nets and blankets are intended to prevent erosion and hold seed and mulch in place on steep slopes and in channels so that vegetation can become well established. In addition, some nets and blankets can be used to permanently reinforce turf to protect drainage ways during high flows. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 292 Nets (commonly called matting) are strands of material woven into an open, but high-tensile strength net (for example, coconut fiber matting). Blankets are strands of material that are not tightly woven, but instead form a layer of interlocking fibers, typically held together by a biodegradable or pho- todegradable netting (for example, excelsior or straw blankets). They generally have lower tensile strength than nets, but cover the ground more completely. Coir (coconut fiber) fabric comes as both nets and blankets. Conditions of Use Erosion control netting and blankets shall be made of natural plant fibers unaltered by synthetic materials. Erosion control nets and blankets should be used: l To aid permanent vegetated stabilization of slopes 2H:1V or greater and with more than 10 feet of vertical relief. l For drainage ditches and swales (highly recommended). The application of appropriate net- ting or blanket to drainage ditches and swales can protect bare soil from channelized runoff while vegetation is established. Nets and blankets also can capture a great deal of sediment due to their open, porous structure. Nets and blankets can be used to permanently stabilize channels and may provide a cost-effective, environmentally preferable alternative to riprap. Disadvantages of nets and blankets include: l Surface preparation is required. l On slopes steeper than 2.5H:1V, net and blanket installers may need to be roped and har- nessed for safety. l They cost at least $4,000-6,000 per acre installed. Advantages of nets and blankets include: l Installation without mobilizing special equipment. l Installation by anyone with minimal training l Installation in stages or phases as the project progresses. l Installers can hand place seed and fertilizer as they progress down the slope. l Installation in any weather. l There are numerous types of nets and blankets that can be designed with various parameters in mind. Those parameters include: fiber blend, mesh strength, longevity, biodegradability, cost, and availability. An alternative to nets and blankets in some limited conditions is BMP C202: Riprap Channel Lining. Ensure that BMP C202: Riprap Channel Lining is appropriate before using it as a substitute for nets and blankets. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 293 Design and Installation Specifications l See Figure II-3.3: Channel Installation (Clackamas County et al., 2008) and Figure II-3.4: Slope Installation for typical orientation and installation of nets and blankets used in channels and as slope protection. Note: these are typical only; all nets and blankets must be installed per manufacturer’s installation instructions. l Installation is critical to the effectiveness of these products. If good ground contact is not achieved, runoff can concentrate under the product, resulting in significant erosion. l Installation of nets and blankets on slopes: 1. Complete final grade and track walk up and down the slope. 2. Install hydromulch with seed and fertilizer. 3. Dig a small trench, approximately 12 inches wide by 6 inches deep along the top of the slope. 4. Install the leading edge of the net/blanket into the small trench and staple approximately every 18 inches. NOTE: Staples are metal, “U”-shaped, and a minimum of 6 inches long. Longer staples are used in sandy soils. Biodegradable stakes are also available. 5. Roll the net/blanket slowly down the slope as the installer walks backward. NOTE: The net/blanket rests against the installer’s legs. Staples are installed as the net/blanket is unrolled. It is critical that the proper staple pattern is used for the net/blanket being installed. The net/blanket is not to be allowed to roll down the slope on its own as this stretches the net/blanket, making it impossible to maintain soil contact. In addition, no one is allowed to walk on the net/blanket after it is in place. 6. If the net/blanket is not long enough to cover the entire slope length, the trailing edge of the upper net/blanket should overlap the leading edge of the lower net/blanket and be stapled. On steeper slopes, this overlap should be installed in a small trench, stapled, and covered with soil. l With the variety of products available, it is impossible to cover all the details of appropriate use and installation. Therefore, it is critical that the designer consult the manufacturer's inform- ation and that a site visit takes place in order to ensure that the product specified is appro- priate. Information is also available in WSDOT's Standard Specifications for Road, Bridge, and Municipal Construction Division 8-01 and Division 9-14 (WSDOT, 2016). l Use jute matting in conjunction with mulch (BMP C121: Mulching). Excelsior, woven straw blankets and coir (coconut fiber) blankets may be installed without mulch. There are many other types of erosion control nets and blankets on the market that may be appropriate in cer- tain circumstances. l In general, most nets (e.g., jute matting) require mulch in order to prevent erosion because they have a fairly open structure. Blankets typically do not require mulch because they usually provide complete protection of the surface. l Extremely steep, unstable, wet, or rocky slopes are often appropriate candidates for use of synthetic blankets, as are riverbanks, beaches and other high-energy environments. If 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 294 synthetic blankets are used, the soil should be hydromulched first. l 100-percent biodegradable blankets are available for use in sensitive areas. These organic blankets are usually held together with a paper or fiber mesh and stitching which may last up to a year. l Most netting used with blankets is photodegradable, meaning it breaks down under sunlight (not UV stabilized). However, this process can take months or years even under bright sun. Once vegetation is established, sunlight does not reach the mesh. It is not uncommon to find non-degraded netting still in place several years after installation. This can be a problem if maintenance requires the use of mowers or ditch cleaning equipment. In addition, birds and small animals can become trapped in the netting. Maintenance Standards l Maintain good contact with the ground. Erosion must not occur beneath the net or blanket. l Repair and staple any areas of the net or blanket that are damaged or not in close contact with the ground. l Fix and protect eroded areas if erosion occurs due to poorly controlled drainage. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 295 Figure II-3.3: Channel Installation 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 296 Figure II-3.4: Slope Installation 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 297 BMP C123: Plastic Covering Purpose Plastic covering provides immediate, short-term erosion protection to slopes and disturbed areas. Conditions of Use Plastic covering may be used on disturbed areas that require cover measures for less than 30 days, except as stated below. l Plastic is particularly useful for protecting cut and fill slopes and stockpiles. However, the rel- atively rapid breakdown of most polyethylene sheeting makes it unsuitable for applications greater than six months. l Due to rapid runoff caused by plastic covering, do not use this method upslope of areas that might be adversely impacted by concentrated runoff. Such areas include steep and/or unstable slopes. l Plastic sheeting may result in increased runoff volumes and velocities, requiring additional on- site measures to counteract the increases. Creating a trough with wattles or other material can convey clean water away from these areas. l To prevent undercutting, trench and backfill rolled plastic covering products. l Although the plastic material is inexpensive to purchase, the cost of installation, maintenance, removal, and disposal add to the total costs of this BMP. l Whenever plastic is used to protect slopes, install water collection measures at the base of the slope. These measures include plastic-covered berms, channels, and pipes used to convey clean rainwater away from bare soil and disturbed areas. Do not mix clean runoff from a plastic covered slope with dirty runoff from a project. l Other uses for plastic include: o Temporary ditch liner. o Pond liner in temporary sediment pond. o Liner for bermed temporary fuel storage area if plastic is not reactive to the type of fuel being stored. o Emergency slope protection during heavy rains. o Temporary drainpipe (“elephant trunk”) used to direct water. Design and Installation Specifications l Plastic slope cover must be installed as follows: 1. Run plastic up and down the slope, not across the slope. 2. Plastic may be installed perpendicular to a slope if the slope length is less than 10 feet. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 298 3.Provide a minimum of 8-inch overlap at the seams. 4.On long or wide slopes, or slopes subject to wind, tape all seams. 5.Place plastic into a small (12-inch wide by 6-inch deep) slot trench at the top of the slope and backfill with soil to keep water from flowing underneath. 6.Place sand filled burlap or geotextile bags every 3 to 6 feet along seams and tie them together with twine to hold them in place. 7.Inspect plastic for rips, tears, and open seams regularly and repair immediately. This prevents high velocity runoff from contacting bare soil, which causes extreme erosion. 8.Sandbags may be lowered into place tied to ropes. However, all sandbags must be staked in place. l Plastic sheeting shall have a minimum thickness of 0.06 millimeters. l If erosion at the toe of a slope is likely, a gravel berm, riprap, or other suitable protection shall be installed at the toe of the slope in order to reduce the velocity of runoff. Maintenance Standards l Torn sheets must be replaced and open seams repaired. l Completely remove and replace the plastic if it begins to deteriorate due to ultraviolet radi- ation. l Completely remove plastic when no longer needed. l Dispose of old tires used to weight down plastic sheeting appropriately. Approved as Functionally Equivalent Ecology has approved products as able to meet the requirements of this BMP. The products did not pass through the Technology Assessment Protocol – Ecology (TAPE) process. Local jurisdictions may choose not to accept these products, or may require additional testing prior to consideration for local use. Products that Ecology has approved as functionally equivalent are available for review on Ecology’s website at: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-per- mittee-guidance-resources/Emerging-stormwater-treatment-technologies 2019 Stormwater Management Manual for Western Washington Volume II - Chapter 3 - Page 299 BMP C140: Dust Control Purpose Dust control prevents wind transport of dust from disturbed soil surfaces onto roadways, drainage ways, and surface waters. Conditions of Use Use dust control in areas (including roadways) subject to surface and air movement of dust where on-site or off-site impacts to roadways, drainage ways, or surface waters are likely. Design and Installation Specifications l Vegetate or mulch areas that will not receive vehicle traffic. In areas where planting, mulching, or paving is impractical, apply gravel or landscaping rock. l Limit dust generation by clearing only those areas where immediate activity will take place, leaving the remaining area(s) in the original condition. Maintain the original ground cover as long as practical. l Construct natural or artificial windbreaks or windscreens. These may be designed as enclos- ures for small dust sources. l Sprinkle the site with water until the surface is wet. Repeat as needed. To prevent carryout of mud onto the street, refer to BMP C105: Stabilized Construction Access and BMP C106: Wheel Wash. l Irrigation water can be used for dust control. Irrigation systems should be installed as a first step on sites where dust control is a concern. l Spray exposed soil areas with a dust palliative, following the manufacturer’s instructions and cautions regarding handling and application. Used oil is prohibited from use as a dust sup- pressant. Local governments may approve other dust palliatives such as calcium chloride or PAM. l PAM (BMP C126: Polyacrylamide (PAM) for Soil Erosion Protection) added to water at a rate of 0.5 pounds per 1,000 gallons of water per acre and applied from a water truck is more effect- ive than water alone. This is due to increased infiltration of water into the soil and reduced evaporation. In addition, small soil particles are bonded together and are not as easily trans- ported by wind. Adding PAM may reduce the quantity of water needed for dust control. Note that the application rate specified here applies to this BMP, and is not the same application rate that is specified in BMP C126: Polyacrylamide (PAM) for Soil Erosion Protection, but the downstream protections still apply. Refer to BMP C126: Polyacrylamide (PAM) for Soil Erosion Protection for conditions of use. PAM shall not be directly applied to water or allowed to enter a water body. l Contact your local Air Pollution Control Authority for guidance and training on other dust con- trol measures. Compliance with the local Air Pollution Control Authority constitutes 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 313 compliance with this BMP. l Use vacuum street sweepers. l Remove mud and other dirt promptly so it does not dry and then turn into dust. l Techniques that can be used for unpaved roads and lots include: o Lower speed limits. High vehicle speed increases the amount of dust stirred up from unpaved roads and lots. o Upgrade the road surface strength by improving particle size, shape, and mineral types that make up the surface and base materials. o Add surface gravel to reduce the source of dust emission. Limit the amount of fine particles (those smaller than .075 mm) to 10 to 20 percent. o Use geotextile fabrics to increase the strength of new roads or roads undergoing recon- struction. o Encourage the use of alternate, paved routes, if available. o Apply chemical dust suppressants using the admix method, blending the product with the top few inches of surface material. Suppressants may also be applied as surface treatments. o Limit dust-causing work on windy days. o Pave unpaved permanent roads and other trafficked areas. Maintenance Standards Respray area as necessary to keep dust to a minimum. BMP C150: Materials on Hand Purpose Keep quantities of erosion prevention and sediment control materials on the project site at all times to be used for regular maintenance and emergency situations such as unexpected heavy rains. Hav- ing these materials on-site reduces the time needed to replace existing or implement new BMPs when inspections indicate that existing BMPs are not meeting the Construction SWPPP require- ments. In addition, contractors can save money by buying some materials in bulk and storing them at their office or yard. Conditions of Use l Construction projects of any size or type can benefit from having materials on hand. A small commercial development project could have a roll of plastic and some gravel available for immediate protection of bare soil and temporary berm construction. A large earthwork project, such as highway construction, might have several tons of straw, several rolls of plastic, flexible 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 314 pipe, sandbags, geotextile fabric and steel “T” posts. l Materials should be stockpiled and readily available before any site clearing, grubbing, or earthwork begins. A large contractor or project proponent could keep a stockpile of materials that are available for use on several projects. l If storage space at the project site is at a premium, the contractor could maintain the materials at their office or yard. The office or yard must be less than an hour from the project site. Design and Installation Specifications Depending on project type, size, complexity, and length, materials and quantities will vary. A good minimum list of items that will cover numerous situations includes: l Clear Plastic, 6 mil l Drainpipe, 6 or 8 inch diameter l Sandbags, filled l Straw Bales for mulching l Quarry Spalls l Washed Gravel l Geotextile Fabric l Catch Basin Inserts l Steel "T" Posts l Silt fence material l Straw Wattles Maintenance Standards l All materials with the exception of the quarry spalls, steel “T” posts, and gravel should be kept covered and out of both sun and rain. l Re-stock materials as needed. BMP C151: Concrete Handling Purpose Concrete work can generate process water and slurry that contain fine particles and high pH, both of which can violate water quality standards in the receiving water. Concrete spillage or concrete dis- charge to waters of the State is prohibited. Use this BMP to minimize and eliminate concrete, con- crete process water, and concrete slurry from entering waters of the State. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 315 Conditions of Use Any time concrete is used, utilize these management practices. Concrete construction project com- ponents include, but are not limited to: l Curbs l Sidewalks l Roads l Bridges l Foundations l Floors l Runways Disposal options for concrete, in order of preference are: 1. Off-site disposal 2. Concrete wash-out areas (see BMP C154: Concrete Washout Area) 3. De minimus washout to formed areas awaiting concrete Design and Installation Specifications l Wash concrete truck drums at an approved off-site location or in designated concrete washout areas only. Do not wash out concrete trucks onto the ground (including formed areas awaiting concrete), or into storm drains, open ditches, streets, or streams. Refer to BMP C154: Concrete Washout Area for information on concrete washout areas. o Return unused concrete remaining in the truck and pump to the originating batch plant for recycling. Do not dump excess concrete on site, except in designated concrete washout areas as allowed in BMP C154: Concrete Washout Area. l Wash small concrete handling equipment (e.g. hand tools, screeds, shovels, rakes, floats, trowels, and wheelbarrows) into designated concrete washout areas or into formed areas awaiting concrete pour. l At no time shall concrete be washed off into the footprint of an area where an infiltration fea- ture will be installed. l Wash equipment difficult to move, such as concrete paving machines, in areas that do not dir- ectly drain to natural or constructed stormwater conveyance or potential infiltration areas. l Do not allow washwater from areas, such as concrete aggregate driveways, to drain directly (without detention or treatment) to natural or constructed stormwater conveyances. l Contain washwater and leftover product in a lined container when no designated concrete washout areas (or formed areas, allowed as described above) are available. Dispose of con- tained concrete and concrete washwater (process water) properly. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 316 l Always use forms or solid barriers for concrete pours, such as pilings, within 15-feet of surface waters. l Refer to BMP C252: Treating and Disposing of High pH Water for pH adjustment require- ments. l Refer to the Construction Stormwater General Permit (CSWGP) for pH monitoring require- ments if the project involves one of the following activities: o Significant concrete work (as defined in the CSWGP). o The use of soils amended with (but not limited to) Portland cement-treated base, cement kiln dust or fly ash. o Discharging stormwater to segments of water bodies on the 303(d) list (Category 5) for high pH. Maintenance Standards Check containers for holes in the liner daily during concrete pours and repair the same day. BMP C152: Sawcutting and Surfacing Pollution Prevention Purpose Sawcutting and surfacing operations generate slurry and process water that contains fine particles and high pH (concrete cutting), both of which can violate the water quality standards in the receiving water. Concrete spillage or concrete discharge to waters of the State is prohibited. Use this BMP to minimize and eliminate process water and slurry created through sawcutting or surfacing from enter- ing waters of the State. Conditions of Use Utilize these management practices anytime sawcutting or surfacing operations take place. Saw- cutting and surfacing operations include, but are not limited to: l Sawing l Coring l Grinding l Roughening l Hydro-demolition l Bridge and road surfacing 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 317 Design and Installation Specifications l Vacuum slurry and cuttings during cutting and surfacing operations. l Slurry and cuttings shall not remain on permanent concrete or asphalt pavement overnight. l Slurry and cuttings shall not drain to any natural or constructed drainage conveyance includ- ing stormwater systems. This may require temporarily blocking catch basins. l Dispose of collected slurry and cuttings in a manner that does not violate ground water or sur- face water quality standards. l Do not allow process water generated during hydro-demolition, surface roughening or similar operations to drain to any natural or constructed drainage conveyance including stormwater systems. Dispose of process water in a manner that does not violate ground water or surface water quality standards. l Handle and dispose of cleaning waste material and demolition debris in a manner that does not cause contamination of water. Dispose of sweeping material from a pick-up sweeper at an appropriate disposal site. Maintenance Standards Continually monitor operations to determine whether slurry, cuttings, or process water could enter waters of the state. If inspections show that a violation of water quality standards could occur, stop operations and immediately implement preventive measures such as berms, barriers, secondary containment, and/or vacuum trucks. BMP C153: Material Delivery, Storage, and Containment Purpose Prevent, reduce, or eliminate the discharge of pollutants to the stormwater system or watercourses from material delivery and storage. Minimize the storage of hazardous materials on-site, store mater- ials in a designated area, and install secondary containment. Conditions of Use Use at construction sites with delivery and storage of the following materials: l Petroleum products such as fuel, oil and grease l Soil stabilizers and binders (e.g., Polyacrylamide) l Fertilizers, pesticides and herbicides l Detergents l Asphalt and concrete compounds 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 318 l Hazardous chemicals such as acids, lime, adhesives, paints, solvents, and curing compounds l Any other material that may be detrimental if released to the environment Design and Installation Specifications l The temporary storage area should be located away from vehicular traffic, near the con- struction entrance(s), and away from waterways or storm drains. l Safety Data Sheets (SDS) should be supplied for all materials stored. Chemicals should be kept in their original labeled containers. l Hazardous material storage on-site should be minimized. l Hazardous materials should be handled as infrequently as possible. l During the wet weather season (Oct 1 – April 30), consider storing materials in a covered area. l Materials should be stored in secondary containments, such as an earthen dike, horse trough, or even a children’s wading pool for non-reactive materials such as detergents, oil, grease, and paints. Small amounts of material may be secondarily contained in “bus boy” trays or con- crete mixing trays. l Do not store chemicals, drums, or bagged materials directly on the ground. Place these items on a pallet and, when possible, within secondary containment. l If drums must be kept uncovered, store them at a slight angle to reduce ponding of rainwater on the lids to reduce corrosion. Domed plastic covers are inexpensive and snap to the top of drums, preventing water from collecting. l Liquids, petroleum products, and substances listed in 40 CFR Parts 110, 117, or 302 shall be stored in approved containers and drums and shall not be overfilled. Containers and drums shall be stored in temporary secondary containment facilities. l Temporary secondary containment facilities shall provide for a spill containment volume able to contain 10% of the total enclosed container volume of all containers, or 110% of the capa- city of the largest container within its boundary, whichever is greater. l Secondary containment facilities shall be impervious to the materials stored therein for a min- imum contact time of 72 hours. l Sufficient separation should be provided between stored containers to allow for spill cleanup and emergency response access. l During the wet weather season (Oct 1 – April 30), each secondary containment facility shall be covered during non-working days, prior to and during rain events. l Keep material storage areas clean, organized and equipped with an ample supply of appro- priate spill clean-up material (spill kit). l The spill kit should include, at a minimum: 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 319 o 1-Water Resistant Nylon Bag o 3-Oil Absorbent Socks 3”x 4’ o 2-Oil Absorbent Socks 3”x 10’ o 12-Oil Absorbent Pads 17”x19” o 1-Pair Splash Resistant Goggles o 3-Pair Nitrile Gloves o 10-Disposable Bags with Ties o Instructions Maintenance Standards l Secondary containment facilities shall be maintained free of accumulated rainwater and spills. In the event of spills or leaks, accumulated rainwater and spills shall be collected and placed into drums. These liquids shall be handled as hazardous waste unless testing determines them to be non-hazardous. l Re-stock spill kit materials as needed. BMP C154: Concrete Washout Area Purpose Prevent or reduce the discharge of pollutants from concrete waste to stormwater by conducting washout off-site, or performing on-site washout in a designated area. Conditions of Use Concrete washout areas are implemented on construction projects where: l Concrete is used as a construction material l It is not possible to dispose of all concrete wastewater and washout off-site (ready mix plant, etc.). l Concrete truck drums are washed on-site. Note that auxiliary concrete truck components (e.g. chutes and hoses) and small concrete handling equipment (e.g. hand tools, screeds, shovels, rakes, floats, trowels, and wheel- barrows) may be washed into formed areas awaiting concrete pour. At no time shall concrete be washed off into the footprint of an area where an infiltration feature will be installed. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 320 Design and Installation Specifications Implementation l Perform washout of concrete truck drums at an approved off-site location or in designated con- crete washout areas only. l Do not wash out concrete onto non-formed areas, or into storm drains, open ditches, streets, or streams. l Wash equipment difficult to move, such as concrete paving machines, in areas that do not dir- ectly drain to natural or constructed stormwater conveyance or potential infiltration areas. l Do not allow excess concrete to be dumped on-site, except in designated concrete washout areas as allowed above. l Concrete washout areas may be prefabricated concrete washout containers, or self-installed structures (above-grade or below-grade). l Prefabricated containers are most resistant to damage and protect against spills and leaks. Companies may offer delivery service and provide regular maintenance and disposal of solid and liquid waste. l If self-installed concrete washout areas are used, below-grade structures are preferred over above-grade structures because they are less prone to spills and leaks. l Self-installed above-grade structures should only be used if excavation is not practical. l Concrete washout areas shall be constructed and maintained in sufficient quantity and size to contain all liquid and concrete waste generated by washout operations. Education l Discuss the concrete management techniques described in this BMP with the ready-mix con- crete supplier before any deliveries are made. l Educate employees and subcontractors on the concrete waste management techniques described in this BMP. l Arrange for the contractor’s superintendent or Certified Erosion and Sediment Control Lead (CESCL) to oversee and enforce concrete waste management procedures. l A sign should be installed adjacent to each concrete washout area to inform concrete equip- ment operators to utilize the proper facilities. Contracts Incorporate requirements for concrete waste management into concrete supplier and subcontractor agreements. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 321 Location and Placement l Locate concrete washout areas at least 50 feet from sensitive areas such as storm drains, open ditches, water bodies, or wetlands. l Allow convenient access to the concrete washout area for concrete trucks, preferably near the area where the concrete is being poured. l If trucks need to leave a paved area to access the concrete washout area, prevent track-out with a pad of rock or quarry spalls (see BMP C105: Stabilized Construction Access). These areas should be far enough away from other construction traffic to reduce the likelihood of acci- dental damage and spills. l The number of concrete washout areas you install should depend on the expected demand for storage capacity. l On large sites with extensive concrete work, concrete washout areas should be placed in mul- tiple locations for ease of use by concrete truck drivers. Concrete Truck Washout Procedures l Washout of concrete truck drums shall be performed in designated concrete washout areas only. l Concrete washout from concrete pumper bins can be washed into concrete pumper trucks and discharged into designated concrete washout areas or properly disposed of off-site. Concrete Washout Area Installation l Concrete washout areas should be constructed as shown in the figures below, with a recom- mended minimum length and minimum width of 10 ft, but with sufficient quantity and volume to contain all liquid and concrete waste generated by washout operations. l Plastic lining material should be a minimum of 10 mil polyethylene sheeting and should be free of holes, tears, or other defects that compromise the impermeability of the material. l Lath and flagging should be commercial type. l Liner seams shall be installed in accordance with manufacturers’ recommendations. l Soil base shall be prepared free of rocks or other debris that may cause tears or holes in the plastic lining material. Maintenance Standards Inspection and Maintenance l Inspect and verify that concrete washout areas are in place prior to the commencement of con- crete work. l Once concrete wastes are washed into the designated washout area and allowed to harden, 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 322 the concrete should be broken up, removed, and disposed of per applicable solid waste reg- ulations. Dispose of hardened concrete on a regular basis. l During periods of concrete work, inspect the concrete washout areas daily to verify continued performance. o Check overall condition and performance. o Check remaining capacity (% full). o If using self-installed concrete washout areas, verify plastic liners are intact and side- walls are not damaged. o If using prefabricated containers, check for leaks. l Maintain the concrete washout areas to provide adequate holding capacity with a minimum freeboard of 12 inches. l Concrete washout areas must be cleaned, or new concrete washout areas must be con- structed and ready for use once the concrete washout area is 75% full. l If the concrete washout area is nearing capacity, vacuum and dispose of the waste material in an approved manner. l Do not discharge liquid or slurry to waterways, storm drains or directly onto ground. l Do not discharge to the sanitary sewer without local approval. l Place a secure, non-collapsing, non-water collecting cover over the concrete washout area prior to predicted wet weather to prevent accumulation and overflow of pre- cipitation. l Remove and dispose of hardened concrete and return the structure to a functional con- dition. Concrete may be reused on-site or hauled away for disposal or recycling. l When you remove materials from a self-installed concrete washout area, build a new struc- ture; or, if the previous structure is still intact, inspect for signs of weakening or damage, and make any necessary repairs. Re-line the structure with new plastic after each cleaning. Removal of Concrete Washout Areas l When concrete washout areas are no longer required for the work, the hardened concrete, slurries and liquids shall be removed and properly disposed of. l Materials used to construct concrete washout areas shall be removed from the site of the work and disposed of or recycled. l Holes, depressions or other ground disturbance caused by the removal of the concrete washout areas shall be backfilled, repaired, and stabilized to prevent erosion. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 323 Figure II-3.7: Concrete Washout Area with Wood Planks 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 324 Figure II-3.8: Concrete Washout Area with Straw Bales 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 325 Figure II-3.9: Prefabricated Concrete Washout Container w/Ramp 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 326 BMP C160: Certified Erosion and Sediment Control Lead Purpose The project proponent designates at least one person as the responsible representative in charge of erosion and sediment control (ESC), and water quality protection. The designated person shall be responsible for ensuring compliance with all local, state, and federal erosion and sediment control and water quality requirements. Construction sites one acre or larger that discharge to waters of the State must designate a Certified Erosion and Sediment Control Lead (CESCL) as the responsible representative. Conditions of Use A CESCL shall be made available on projects one acre or larger that discharge stormwater to sur- face waters of the state. Sites less than one acre may have a person without CESCL certification conduct inspections. The CESCL shall: l Have a current certificate proving attendance in an erosion and sediment control training course that meets the minimum ESC training and certification requirements established by Ecology. Ecology has provided the minimum requirements for CESCL course training, as well as a list of ESC training and certification providers at: https://ecology.wa.gov/Regulations-Permits/Permits-certifications/Certified-erosion-sed- iment-control OR l Be a Certified Professional in Erosion and Sediment Control (CPESC). For additional inform- ation go to: http://www.envirocertintl.org/cpesc/ Specifications l CESCL certification shall remain valid for three years. l The CESCL shall have authority to act on behalf of the contractor or project proponent and shall be available, or on-call, 24 hours per day throughout the period of construction. l The Construction SWPPP shall include the name, telephone number, fax number, and address of the designated CESCL. See II-2 Construction Stormwater Pollution Prevention Plans (Construction SWPPPs). l A CESCL may provide inspection and compliance services for multiple construction projects in the same geographic region, but must be on site whenever earthwork activities are 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 327 occurring that could generate release of turbid water. l Duties and responsibilities of the CESCL shall include, but are not limited to the following: o Maintaining a permit file on site at all times which includes the Construction SWPPP and any associated permits and plans. o Directing BMP installation, inspection, maintenance, modification, and removal. o Updating all project drawings and the Construction SWPPP with changes made. o Completing any sampling requirements including reporting results using electronic Dis- charge Monitoring Reports (WebDMR). o Facilitate, participate in, and take corrective actions resulting from inspections per- formed by outside agencies or the owner. o Keeping daily logs, and inspection reports. Inspection reports should include: n Inspection date/time. n Weather information; general conditions during inspection and approximate amount of precipitation since the last inspection. n Visual monitoring results, including a description of discharged stormwater. The presence of suspended sediment, turbid water, discoloration, and oil sheen shall be noted, as applicable. n Any water quality monitoring performed during inspection. n General comments and notes, including a brief description of any BMP repairs, maintenance or installations made as a result of the inspection. n A summary or list of all BMPs implemented, including observations of all erosion/sediment control structures or practices. The following shall be noted: 1. Locations of BMPs inspected. 2. Locations of BMPs that need maintenance. 3. Locations of BMPs that failed to operate as designed or intended. 4. Locations of where additional or different BMPs are required. BMP C162: Scheduling Purpose Sequencing a construction project reduces the amount and duration of soil exposed to erosion by wind, rain, runoff, and vehicle tracking. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 328 Conditions of Use The construction sequence schedule is an orderly listing of all major land-disturbing activities together with the necessary erosion and sedimentation control measures planned for the project. This type of schedule guides the contractor on work to be done before other work is started so that serious erosion and sedimentation problems can be avoided. Following a specified work schedule that coordinates the timing of land-disturbing activities and the installation of control measures is perhaps the most cost-effective way of controlling erosion during construction. The removal of ground cover leaves a site vulnerable to erosion. Construction sequen- cing that limits land clearing, provides timely installation of erosion and sedimentation controls, and restores protective cover quickly can significantly reduce the erosion potential of a site. Design Considerations l Minimize construction during rainy periods. l Schedule projects to disturb only small portions of the site at any one time. Complete grading as soon as possible. Immediately stabilize the disturbed portion before grading the next por- tion. Practice staged seeding in order to revegetate cut and fill slopes as the work progresses. II-3.3 Construction Runoff BMPs BMP C200: Interceptor Dike and Swale Purpose Provide a dike of compacted soil or a swale at the top or base of a disturbed slope or along the peri- meter of a disturbed construction area to convey stormwater. Use the dike and/or swale to intercept the runoff from unprotected areas and direct it to areas where erosion can be controlled. This can prevent storm runoff from entering the work area or sediment-laden runoff from leaving the con- struction site. Conditions of Use Use an interceptor dike or swale where runoff from an exposed site or disturbed slope must be con- veyed to an erosion control BMP which can safely convey the stormwater. l Locate upslope of a construction site to prevent runoff from entering the disturbed area. l When placed horizontally across a disturbed slope, it reduces the amount and velocity of run- off flowing down the slope. l Locate downslope to collect runoff from a disturbed area and direct it to a sediment BMP (e.g. BMP C240: Sediment Trap or BMP C241: Sediment Pond (Temporary)). 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 329 Design and Installation Specifications l Dike and/or swale and channel must be stabilized with temporary or permanent vegetation or other channel protection during construction. l Steep grades require channel protection and check dams. l Review construction for areas where overtopping may occur. l Can be used at the top of new fill before vegetation is established. l May be used as a permanent diversion channel to carry the runoff. l Contributing area for an individual dike or swale should be one acre or less. l Design the dike and/or swale to contain flows calculated by one of the following methods: o Single Event Hydrograph Method: The peak volumetric flow rate calculated using a 10- minute time step from a Type 1A, 10-year, 24-hour frequency storm for the worst-case land cover condition. OR o Continuous Simulation Method: The 10-year peak flow rate, as determined by an approved continuous runoff model with a 15-minute time step for the worst-case land cover condition. Worst-case land cover conditions (i.e., producing the most runoff) should be used for analysis (in most cases, this would be the land cover conditions just prior to final landscaping). Interceptor Dikes Interceptor dikes shall meet the following criteria: l Top Width: 2 feet minimum. l Height: 1.5 feet minimum on berm. l Side Slope: 2H:1V or flatter. l Grade: Depends on topography, however, dike system minimum is 0.5%, and maximum is 1%. l Compaction: Minimum of 90 percent ASTM D698 standard proctor. l Stabilization: Depends on velocity and reach. Inspect regularly to ensure stability. l Ground Slopes <5%: Seed and mulch applied within 5 days of dike construction (see BMP C121: Mulching). l Ground Slopes 5 - 40%: Dependent on runoff velocities and dike materials. Stabilization should be done immediately using either sod or riprap, or other measures to avoid erosion. l The upslope side of the dike shall provide positive drainage to the dike outlet. No erosion shall 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 330 occur at the outlet. Provide energy dissipation measures as necessary. Sediment-laden runoff must be released through a sediment trapping facility. l Minimize construction traffic over temporary dikes. Use temporary cross culverts for channel crossing. l See Table II-3.8: Horizontal Spacing of Interceptor Dikes Along Ground Slope for recom- mended horizontal spacing between dikes. Average Slope Slope Percent Flowpath Length 20H:1V or less 3-5%300 feet (10 to 20)H:1V 5-10%200 feet (4 to 10)H:1V 10-25%100 feet (2 to 4)H:1V 25-50%50 feet Table II-3.8: Horizontal Spacing of Interceptor Dikes Along Ground Slope Interceptor Swales Interceptor swales shall meet the following criteria: l Bottom Width: 2 feet minimum; the cross-section bottom shall be level. l Depth: 1-foot minimum. l Side Slope: 2H:1V or flatter. l Grade: Maximum 5 percent, with positive drainage to a suitable outlet (such as BMP C241: Sediment Pond (Temporary)). l Stabilization: Seed as per BMP C120: Temporary and Permanent Seeding, or BMP C202: Riprap Channel Lining, 12 inches thick riprap pressed into the bank and extending at least 8 inches vertical from the bottom. Maintenance Standards l Inspect diversion dikes and interceptor swales once a week and after every rainfall. Imme- diately remove sediment from the flow area. l Damage caused by construction traffic or other activity must be repaired before the end of each working day. l Check outlets and make timely repairs as needed to avoid gully formation. When the area below the temporary diversion dike is permanently stabilized, remove the dike and fill and sta- bilize the channel to blend with the natural surface. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 331 BMP C202: Riprap Channel Lining Purpose To protect channels by providing a channel liner using riprap. Conditions of Use Use this BMP when natural soils or vegetated stabilized soils in a channel are not adequate to pre- vent channel erosion. Use this BMP when a permanent ditch or pipe system is to be installed and a temporary measure is needed. An alternative to riprap channel lining is BMP C122: Nets and Blankets. The Federal Highway Administration recommends not using geotextile liners whenever the slope exceeds 10 percent or the shear stress exceeds 8 lbs/ft2. Design and Installation Specifications l Since riprap is typically used where erosion potential is high, construction must be sequenced so that the riprap is put in place with the minimum possible delay. l Disturb areas awaiting riprap only when final preparation and placement of the riprap can fol- low immediately behind the initial disturbance. Where riprap is used for outlet protection, the riprap should be placed before or in conjunction with the construction of the pipe or channel so that it is in place when the pipe or channel begins to operate. l The designer, after determining the riprap size that will be stable under the flow conditions, shall consider that size to be a minimum size and then, based on riprap gradations actually available in the area, select the size or sizes that equal or exceed the minimum size. The pos- sibility of drainage structure damage by others shall be considered in selecting a riprap size, especially if there is nearby water or a gully in which to toss the stones. l Stone for riprap shall consist of field stone or quarry stone of approximately rectangular shape. The stone shall be hard and angular and of such quality that it will not disintegrate on exposure to water or weathering and it shall be suitable in all respects for the purpose inten- ded. See Section 9-13 of WSDOT's Standard Specifications for Road, Bridge, and Municipal Construction (WSDOT, 2016). l A lining of engineering filter fabric (geotextile) shall be placed between the riprap and the underlying soil surface to prevent soil movement into or through the riprap. The geotextile should be keyed in at the top of the bank. l Filter fabric shall not be used on slopes greater than 1.5H:1V as slippage may occur. It should be used in conjunction with a layer of coarse aggregate (granular filter blanket) when the riprap to be placed is 12 inches and larger. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 337 Maintenance Standards Replace riprap as needed. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 338 BMP C207: Check Dams Purpose Construction of check dams across a swale or ditch reduces the velocity of concentrated flow and dis- sipates energy at the check dam. Conditions of Use Use check dams where temporary or permanent channels are not yet vegetated, channel lining is infeasible, and/or velocity checks are required. l Check dams may not be placed in streams unless approved by the State Department of Fish and Wildlife. l Check dams may not be placed in wetlands without approval from a permitting agency. l Do not place check dams below the expected backwater from any salmonid bearing water between October 1 and May 31 to ensure that there is no loss of high flow refuge habitat for overwintering juvenile salmonids and emergent salmonid fry. Design and Installation Specifications l Construct rock check dams from appropriately sized rock. The rock used must be large enough to stay in place given the expected design flow through the channel. The rock must be placed by hand or by mechanical means (do not dump the rock to form the dam) to achieve complete coverage of the ditch or swale and to ensure that the center of the dam is lower than the edges. l Check dams may also be constructed of either rock or pea-gravel filled bags. Numerous new products are also available for this purpose. They tend to be re-usable, quick and easy to install, effective, and cost efficient. l Place check dams perpendicular to the flow of water. l The check dam should form a triangle when viewed from the side. This prevents undercutting as water flows over the face of the check dam rather than falling directly onto the ditch bottom. l Before installing check dams, impound and bypass upstream water flow away from the work area. Options for bypassing include pumps, siphons, or temporary channels. l Check dams combined with sumps work more effectively at slowing flow and retaining sed- iment than a check dam alone. A deep sump should be provided immediately upstream of the check dam. l In some cases, if carefully located and designed, check dams can remain as permanent install- ations with very minor regrading. They may be left as either spillways, in which case accu- mulated sediment would be graded and seeded, or as check dams to prevent further sediment from leaving the site. l The maximum spacing between check dams shall be such that the downstream toe of the 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 351 upstream dam is at the same elevation as the top of the downstream dam. l Keep the maximum height at 2 feet at the center of the check dam. l Keep the center of the check dam at least 12 inches lower than the outer edges at natural ground elevation. l Keep the side slopes of the check dam at 2H:1V or flatter. l Key the stone into the ditch banks and extend it beyond the abutments a minimum of 18 inches to avoid washouts from overflow around the dam. l Use filter fabric foundation under a rock or sand bag check dam. If a blanket ditch liner is used, filter fabric is not necessary. A piece of organic or synthetic blanket cut to fit will also work for this purpose. l In the case of grass-lined ditches and swales, all check dams and accumulated sediment shall be removed when the grass has matured sufficiently to protect the ditch or swale - unless the slope of the swale is greater than 4 percent. The area beneath the check dams shall be seeded and mulched immediately after dam removal. l Ensure that channel appurtenances, such as culvert entrances below check dams, are not subject to damage or blockage from displaced stones. l See Figure II-3.16: Rock Check Dam. Maintenance Standards Check dams shall be monitored for performance and sediment accumulation during and after each rainfall that produces runoff. Sediment shall be removed when it reaches one half the sump depth. l Anticipate submergence and deposition above the check dam and erosion from high flows around the edges of the dam. l If significant erosion occurs between dams, install a protective riprap liner in that portion of the channel. See BMP C202: Riprap Channel Lining. Approved as Functionally Equivalent Ecology has approved products as able to meet the requirements of this BMP. The products did not pass through the Technology Assessment Protocol – Ecology (TAPE) process. Local jurisdictions may choose not to accept these products, or may require additional testing prior to consideration for local use. Products that Ecology has approved as functionally equivalent are available for review on Ecology’s website at: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-per- mittee-guidance-resources/Emerging-stormwater-treatment-technologies 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 352 Figure II-3.16: Rock Check Dam 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 353 BMP C209: Outlet Protection Purpose Outlet protection prevents scour at conveyance outlets and minimizes the potential for downstream erosion by reducing the velocity of concentrated stormwater flows. Conditions of Use Use outlet protection at the outlets of all ponds, pipes, ditches, or other conveyances that discharge to a natural or manmade drainage feature such as a stream, wetland, lake, or ditch. Design and Installation Specifications l The receiving channel at the outlet of a pipe shall be protected from erosion by lining a min- imum of 6 feet downstream and extending up the channel sides a minimum of 1–foot above the maximum tailwater elevation, or 1-foot above the crown, whichever is higher. For pipes lar- ger than 18 inches in diameter, the outlet protection lining of the channel shall be four times the diameter of the outlet pipe. l Standard wingwalls, tapered outlets, and paved channels should also be considered when appropriate for permanent culvert outlet protection (WSDOT, 2015). l BMP C122: Nets and Blankets or BMP C202: Riprap Channel Lining provide suitable options for lining materials. l With low flows, BMP C201: Grass-Lined Channels can be an effective alternative for lining material. l The following guidelines shall be used for outlet protection with riprap: o If the discharge velocity at the outlet is less than 5 fps, use 2-inch to 8-inch riprap. Min- imum thickness is 1-foot. o For 5 to 10 fps discharge velocity at the outlet, use 24-inch to 48-inch riprap. Minimum 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 355 thickness is 2 feet. o For outlets at the base of steep slope pipes (pipe slope greater than 10 percent), use an engineered energy dissipator. o Filter fabric or erosion control blankets should always be used under riprap to prevent scour and channel erosion. See BMP C122: Nets and Blankets. l Bank stabilization, bioengineering, and habitat features may be required for disturbed areas. This work may require a Hydraulic Project Approval (HPA) from the Washington State Depart- ment of Fish and Wildlife. See I-2.11 Hydraulic Project Approvals. Maintenance Standards l Inspect and repair as needed. l Add rock as needed to maintain the intended function. l Clean energy dissipator if sediment builds up. BMP C220: Inlet Protection Purpose Inlet protection prevents coarse sediment from entering drainage systems prior to permanent sta- bilization of the disturbed area. Conditions of Use Use inlet protection at inlets that are operational before permanent stabilization of the disturbed areas that contribute runoff to the inlet. Provide protection for all storm drain inlets downslope and within 500 feet of a disturbed or construction area, unless those inlets are preceded by a sediment trapping BMP. Also consider inlet protection for lawn and yard drains on new home construction. These small and numerous drains coupled with lack of gutters can add significant amounts of sediment into the roof drain system. If possible, delay installing lawn and yard drains until just before landscaping, or cap these drains to prevent sediment from entering the system until completion of landscaping. Provide 18-inches of sod around each finished lawn and yard drain. Table II-3.10: Storm Drain Inlet Protection lists several options for inlet protection. All of the methods for inlet protection tend to plug and require a high frequency of maintenance. Limit contributing drain- age areas for an individual inlet to one acre or less. If possible, provide emergency overflows with additional end-of-pipe treatment where stormwater ponding would cause a hazard. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 356 Type of Inlet Pro- tection Emergency Overflow Applicable for Paved/ Earthen Sur- faces Conditions of Use Drop Inlet Protection Excavated drop inlet protection Yes, temporary flooding may occur Earthen Applicable for heavy flows. Easy to maintain. Large area requirement: 30'x30'/acre Block and gravel drop inlet pro- tection Yes Paved or Earthen Applicable for heavy concentrated flows. Will not pond. Gravel and wire drop inlet pro- tection No Paved or Earthen Applicable for heavy concentrated flows. Will pond. Can withstand traffic. Catch basin filters Yes Paved or Earthen Frequent maintenance required. Curb Inlet Protection Curb inlet pro- tection with wooden weir Small capacity overflow Paved Used for sturdy, more compact install- ation. Block and gravel curb inlet pro- tection Yes Paved Sturdy, but limited filtration. Culvert Inlet Protection Culvert inlet sed- iment trap N/A N/A 18 month expected life. Table II-3.10: Storm Drain Inlet Protection Design and Installation Specifications Excavated Drop Inlet Protection Excavated drop inlet protection consists of an excavated impoundment around the storm drain inlet. Sediment settles out of the stormwater prior to entering the storm drain. Design and installation spe- cifications for excavated drop inlet protection include: l Provide a depth of 1-2 ft as measured from the crest of the inlet structure. l Slope sides of excavation should be no steeper than 2H:1V. l Minimum volume of excavation is 35 cubic yards. l Shape the excavation to fit the site, with the longest dimension oriented toward the longest inflow area. l Install provisions for draining to prevent standing water. l Clear the area of all debris. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 357 l Grade the approach to the inlet uniformly. l Drill weep holes into the side of the inlet. l Protect weep holes with screen wire and washed aggregate. l Seal weep holes when removing structure and stabilizing area. l Build a temporary dike, if necessary, to the down slope side of the structure to prevent bypass flow. Block and Gravel Filter A block and gravel filter is a barrier formed around the inlet with standard concrete blocks and gravel. See Figure II-3.17: Block and Gravel Filter. Design and installation specifications for block gravel fil- ters include: l Provide a height of 1 to 2 feet above the inlet. l Recess the first row of blocks 2-inches into the ground for stability. l Support subsequent courses by placing a pressure treated wood 2x4 through the block open- ing. l Do not use mortar. l Lay some blocks in the bottom row on their side to allow for dewatering the pool. l Place hardware cloth or comparable wire mesh with ½-inch openings over all block openings. l Place gravel to just below the top of blocks on slopes of 2H:1V or flatter. l An alternative design is a gravel berm surrounding the inlet, as follows: o Provide a slope of 3H:1V on the upstream side of the berm. o Provide a slope of 2H:1V on the downstream side of the berm. o Provide a 1-foot wide level stone area between the gravel berm and the inlet. o Use stones 3 inches in diameter or larger on the upstream slope of the berm. o Use gravel ½- to ¾-inch at a minimum thickness of 1-foot on the downstream slope of the berm. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 358 Figure II-3.17: Block and Gravel Filter 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 359 Gravel and Wire Mesh Filter Gravel and wire mesh filters are gravel barriers placed over the top of the inlet. This method does not provide an overflow. Design and installation specifications for gravel and wire mesh filters include: l Use a hardware cloth or comparable wire mesh with ½-inch openings. o Place wire mesh over the drop inlet so that the wire extends a minimum of 1-foot bey- ond each side of the inlet structure. o Overlap the strips if more than one strip of mesh is necessary. l Place coarse aggregate over the wire mesh. o Provide at least a 12-inch depth of aggregate over the entire inlet opening and extend at least 18-inches on all sides. Catch Basin Filters Catch basin filters are designed by manufacturers for construction sites. The limited sediment stor- age capacity increases the amount of inspection and maintenance required, which may be daily for heavy sediment loads. To reduce maintenance requirements, combine a catch basin filter with another type of inlet protection. This type of inlet protection provides flow bypass without overflow and therefore may be a better method for inlets located along active rights-of-way. Design and install- ation specifications for catch basin filters include: l Provides 5 cubic feet of storage. l Requires dewatering provisions. l Provides a high-flow bypass that will not clog under normal use at a construction site. l Insert the catch basin filter in the catch basin just below the grating. Curb Inlet Protection with Wooden Weir Curb inlet protection with wooden weir is an option that consists of a barrier formed around a curb inlet with a wooden frame and gravel. Design and installation specifications for curb inlet protection with wooden weirs include: l Use wire mesh with ½-inch openings. l Use extra strength filter cloth. l Construct a frame. l Attach the wire and filter fabric to the frame. l Pile coarse washed aggregate against the wire and fabric. l Place weight on the frame anchors. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 360 Block and Gravel Curb Inlet Protection Block and gravel curb inlet protection is a barrier formed around a curb inlet with concrete blocks and gravel. See Figure II-3.18: Block and Gravel Curb Inlet Protection. Design and installation spe- cifications for block and gravel curb inlet protection include: l Use wire mesh with ½-inch openings. l Place two concrete blocks on their sides abutting the curb at either side of the inlet opening. These are spacer blocks. l Place a 2x4 stud through the outer holes of each spacer block to align the front blocks. l Place blocks on their sides across the front of the inlet and abutting the spacer blocks. l Place wire mesh over the outside vertical face. l Pile coarse aggregate against the wire to the top of the barrier. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 361 Figure II-3.18: Block and Gravel Curb Inlet Protection 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 362 Curb and Gutter Sediment Barrier Curb and gutter sediment barrier is a sandbag or rock berm (riprap and aggregate) 3 feet high and 3 feet wide in a horseshoe shape. See Figure II-3.19: Curb and Gutter Barrier. Design and installation specifications for curb and gutter sediment barrier include: l Construct a horseshoe shaped berm, faced with coarse aggregate if using riprap, 3 feet high and 3 feet wide, at least 2 feet from the inlet. l Construct a horseshoe shaped sedimentation trap on the upstream side of the berm. Size the trap to sediment trap standards for protecting a culvert inlet. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 363 Figure II-3.19: Curb and Gutter Barrier 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 364 Maintenance Standards l Inspect all forms of inlet protection frequently, especially after storm events. Clean and replace clogged catch basin filters. For rock and gravel filters, pull away the rocks from the inlet and clean or replace. An alternative approach would be to use the clogged rock as fill and put fresh rock around the inlet. l Do not wash sediment into storm drains while cleaning. Spread all excavated material evenly over the surrounding land area or stockpile and stabilize as appropriate. Approved as Functionally Equivalent Ecology has approved products as able to meet the requirements of this BMP. The products did not pass through the Technology Assessment Protocol – Ecology (TAPE) process. Local jurisdictions may choose not to accept these products, or may require additional testing prior to consideration for local use. Products that Ecology has approved as functionally equivalent are available for review on Ecology’s website at: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-per- mittee-guidance-resources/Emerging-stormwater-treatment-technologies 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 365 BMP C233: Silt Fence Purpose Silt fence reduces the transport of coarse sediment from a construction site by providing a temporary physical barrier to sediment and reducing the runoff velocities of overland flow. Conditions of Use Silt fence may be used downslope of all disturbed areas. l Silt fence shall prevent sediment carried by runoff from going beneath, through, or over the top of the silt fence, but shall allow the water to pass through the fence. l Silt fence is not intended to treat concentrated flows, nor is it intended to treat substantial amounts of overland flow. Convey any concentrated flows through the drainage system to a sediment trapping BMP. l Do not construct silt fences in streams or use in V-shaped ditches. Silt fences do not provide an adequate method of silt control for anything deeper than sheet or overland flow. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 370 Figure II-3.22: Silt Fence 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 371 Design and Installation Specifications l Use in combination with other construction stormwater BMPs. l Maximum slope steepness (perpendicular to the silt fence line) 1H:1V. l Maximum sheet or overland flow path length to the silt fence of 100 feet. l Do not allow flows greater than 0.5 cfs. l Use geotextile fabric that meets the following standards. All geotextile properties listed below are minimum average roll values (i.e., the test result for any sampled roll in a lot shall meet or exceed the values shown in Table II-3.11: Geotextile Fabric Standards for Silt Fence): Geotextile Property Minimum Average Roll Value Polymeric Mesh AOS (ASTM D4751) 0.60 mm maximum for slit film woven (#30 sieve). 0.30 mm maximum for all other geotextile types (#50 sieve). 0.15 mm minimum for all fabric types (#100 sieve). Water Permittivity (ASTM D4491) 0.02 sec-1 minimum Grab Tensile Strength (ASTM D4632) 180 lbs. Minimum for extra strength fabric. 100 lbs minimum for standard strength fabric. Grab Tensile Strength (ASTM D4632) 30% maximum Ultraviolet Resistance (ASTM D4355) 70% minimum Table II-3.11: Geotextile Fabric Standards for Silt Fence l Support standard strength geotextiles with wire mesh, chicken wire, 2-inch x 2-inch wire, safety fence, or jute mesh to increase the strength of the geotextile. Silt fence materials are available that have synthetic mesh backing attached. l Silt fence material shall contain ultraviolet ray inhibitors and stabilizers to provide a minimum of six months of expected usable construction life at a temperature range of 0°F to 120°F. l One-hundred percent biodegradable silt fence is available that is strong, long lasting, and can be left in place after the project is completed, if permitted by the local jurisdiction. l Refer to Figure II-3.22: Silt Fence for standard silt fence details. Include the following Stand- ard Notes for silt fence on construction plans and specifications: 1. The Contractor shall install and maintain temporary silt fences at the locations shown in the Plans. 2. Construct silt fences in areas of clearing, grading, or drainage prior to starting those activities. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 372 3. The silt fence shall have a 2-feet min. and a 2½-feet max. height above the original ground surface. 4. The geotextile fabric shall be sewn together at the point of manufacture to form fabric lengths as required. Locate all sewn seams at support posts. Alternatively, two sections of silt fence can be overlapped, provided that the overlap is long enough and that the adjacent silt fence sections are close enough together to prevent silt laden water from escaping through the fence at the overlap. 5. Attach the geotextile fabric on the up-slope side of the posts and secure with staples, wire, or in accordance with the manufacturer's recommendations. Attach the geotextile fabric to the posts in a manner that reduces the potential for tearing. 6. Support the geotextile fabric with wire or plastic mesh, dependent on the properties of the geotextile selected for use. If wire or plastic mesh is used, fasten the mesh securely to the up-slope side of the posts with the geotextile fabric up-slope of the mesh. 7. Mesh support, if used, shall consist of steel wire with a maximum mesh spacing of 2- inches, or a prefabricated polymeric mesh. The strength of the wire or polymeric mesh shall be equivalent to or greater than 180 lbs. grab tensile strength. The polymeric mesh must be as resistant to the same level of ultraviolet radiation as the geotextile fabric it supports. 8. Bury the bottom of the geotextile fabric 4-inches min. below the ground surface. Backfill and tamp soil in place over the buried portion of the geotextile fabric, so that no flow can pass beneath the silt fence and scouring cannot occur. When wire or polymeric back-up support mesh is used, the wire or polymeric mesh shall extend into the ground 3-inches min. 9. Drive or place the silt fence posts into the ground 18-inches min. A 12–inch min. depth is allowed if topsoil or other soft subgrade soil is not present and 18-inches cannot be reached. Increase fence post min. depths by 6 inches if the fence is located on slopes of 3H:1V or steeper and the slope is perpendicular to the fence. If required post depths cannot be obtained, the posts shall be adequately secured by bracing or guying to pre- vent overturning of the fence due to sediment loading. 10. Use wood, steel or equivalent posts. The spacing of the support posts shall be a max- imum of 6-feet. Posts shall consist of either: l Wood with minimum dimensions of 2 inches by 2 inches by 3 feet. Wood shall be free of defects such as knots, splits, or gouges. l No. 6 steel rebar or larger. l ASTM A 120 steel pipe with a minimum diameter of 1-inch. l U, T, L, or C shape steel posts with a minimum weight of 1.35 lbs./ft. l Other steel posts having equivalent strength and bending resistance to the post sizes listed above. 11. Locate silt fences on contour as much as possible, except at the ends of the fence, 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 373 where the fence shall be turned uphill such that the silt fence captures the runoff water and prevents water from flowing around the end of the fence. 12. If the fence must cross contours, with the exception of the ends of the fence, place check dams perpendicular to the back of the fence to minimize concentrated flow and erosion. The slope of the fence line where contours must be crossed shall not be steeper than 3H:1V. l Check dams shall be approximately 1-foot deep at the back of the fence. Check dams shall be continued perpendicular to the fence at the same elevation until the top of the check dam intercepts the ground surface behind the fence. l Check dams shall consist of crushed surfacing base course, gravel backfill for walls, or shoulder ballast. Check dams shall be located every 10 feet along the fence where the fence must cross contours. l Refer to Figure II-3.23: Silt Fence Installation by Slicing Method for slicing method details. The following are specifications for silt fence installation using the slicing method: 1. The base of both end posts must be at least 2- to 4-inches above the top of the geo- textile fabric on the middle posts for ditch checks to drain properly. Use a hand level or string level, if necessary, to mark base points before installation. 2. Install posts 3- to 4-feet apart in critical retention areas and 6- to 7-feet apart in standard applications. 3. Install posts 24-inches deep on the downstream side of the silt fence, and as close as possible to the geotextile fabric, enabling posts to support the geotextile fabric from upstream water pressure. 4. Install posts with the nipples facing away from the geotextile fabric. 5. Attach the geotextile fabric to each post with three ties, all spaced within the top 8- inches of the fabric. Attach each tie diagonally 45 degrees through the fabric, with each puncture at least 1-inch vertically apart. Each tie should be positioned to hang on a post nipple when tightening to prevent sagging. 6. Wrap approximately 6-inches of the geotextile fabric around the end posts and secure with 3 ties. 7. No more than 24-inches of a 36-inch geotextile fabric is allowed above ground level. 8. Compact the soil immediately next to the geotextile fabric with the front wheel of the tractor, skid steer, or roller exerting at least 60 pounds per square inch. Compact the upstream side first and then each side twice for a total of four trips. Check and correct the silt fence installation for any deviation before compaction. Use a flat-bladed shovel to tuck the fabric deeper into the ground if necessary. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 374 Figure II-3.23: Silt Fence Installation by Slicing Method 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 375 Maintenance Standards l Repair any damage immediately. l Intercept and convey all evident concentrated flows uphill of the silt fence to a sediment trap- ping BMP. l Check the uphill side of the silt fence for signs of the fence clogging and acting as a barrier to flow and then causing channelization of flows parallel to the fence. If this occurs, replace the fence and remove the trapped sediment. l Remove sediment deposits when the deposit reaches approximately one-third the height of the silt fence, or install a second silt fence. l Replace geotextile fabric that has deteriorated due to ultraviolet breakdown. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 376 BMP C241: Sediment Pond (Temporary) Purpose Sediment ponds are temporary ponds used during construction to remove sediment from runoff ori- ginating from disturbed areas of the project site. Sediment ponds are typically designed to remove sediment no smaller than medium silt (0.02 mm). Consequently, they usually reduce turbidity only slightly. Conditions of Use l Use a sediment pond where the contributing drainage area to the pond is 3 acres or more. Ponds must be used in conjunction with other Construction Stormwater BMPs to reduce the amount of sediment flowing into the pond. l Do not install sediment ponds on sites where failure of the BMP would result in loss of life, damage to homes or buildings, or interruption of use or service of public roads or utilities. Also, sediment ponds are attractive to children and can be dangerous. Compliance with local ordin- ances regarding health and safety must be addressed. If fencing of the pond is required, show the type of fence and its location on the drawings in the Construction SWPPP. l Sediment ponds that can impound 10 acre-ft (435,600 cu-ft, or 3.26 million gallons) or more, or have an embankment of more than 6 feet, are subject to the Washington Dam Safety Regu- lations (Chapter 173-175 WAC). See BMP D.1: Detention Ponds for more information regard- ing dam safety considerations for detention ponds. l Projects that are constructing permanent Flow Control BMPs or Runoff Treatment BMPs that use ponding for treatment may use the rough-graded or final-graded permanent BMP foot- print for the temporary sediment pond. When permanent BMP footprints are used as tem- porary sediment ponds, the surface area requirement of the temporary sediment pond must be met. If the surface area requirement of the sediment pond is larger than the surface area of the permanent BMP, then the sediment pond shall be enlarged beyond the permanent BMP footprint to comply with the surface area requirement. The permanent control structure must be temporarily replaced with a control structure that only allows water to leave the temporary sediment pond from the surface or by pumping. Alternatively, the permanent control structure may used if it is temporarily modified by plug- ging any outlet holes below the riser. The permanent control structure must be installed as part of the permanent BMP after the site is fully stabilized. Design and Installation Specifications General l See Figure II-3.28: Sediment Pond Plan View, Figure II-3.29: Sediment Pond Cross Section, and Figure II-3.30: Sediment Pond Riser Detail for details. l Use of permanent infiltration BMP footprints for temporary sediment ponds during 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 388 construction tends to clog the soils and reduce their capacity to infiltrate. If permanent infilt- ration BMP footprints are used, the sides and bottom of the temporary sediment pond must only be rough excavated to a minimum of 2 feet above final grade of the permanent infiltration BMP. Final grading of the permanent infiltration BMP shall occur only when all contributing drainage areas are fully stabilized. Any proposed permanent pretreatment BMP prior to the infiltration BMP should be fully constructed and used with the temporary sediment pond to help prevent clogging of the soils. See Element 13: Protect Low Impact Development BMPs for more information about protecting permanent infiltration BMPs. l The pond shall be divided into two roughly equal volume cells by a permeable divider that will reduce turbulence while allowing movement of water between the cells. The divider shall be at least one-half the height of the riser, and at least one foot below the top of the riser. Wire- backed, 2- to 3-foot high, high strength geotextile fabric supported by treated 4"x4"s can be used as a divider. Alternatively, staked straw bales wrapped with geotextile fabric may be used. If the pond is more than 6 feet deep, a different divider design must be proposed. A riprap embankment is one acceptable method of separation for deeper ponds. Other designs that satisfy the intent of this provision are allowed as long as the divider is permeable, struc- turally sound, and designed to prevent erosion under and around the divider. l The most common structural failure of sediment ponds is caused by piping. Piping refers to two phenomena: (1) water seeping through fine-grained soil, eroding the soil grain by grain and forming pipes or tunnels; and, (2) water under pressure flowing upward through a gran- ular soil with a head of sufficient magnitude to cause soil grains to lose contact and capability for support. The most critical construction practices to prevent piping are: o Tight connections between the riser and outlet pipe, and other pipe connections. o Adequate anchoring of the riser. o Proper soil compaction of the embankment and riser footing. o Proper construction of anti-seep devices. Sediment Pond Geometry To determine the sediment pond geometry, first calculate the design surface area (SA) of the pond, measured at the top of the riser pipe. Use the following equation: SA = 2 x Q2/0.00096 or 2080 square feet per cfs of inflow See BMP C240: Sediment Trap for more information on the above equation. The basic geometry of the pond can now be determined using the following design criteria: l Required surface area SA (from the equation above) at the top of the riser. l Minimum 3.5-foot depth from the top of the riser to the bottom of the pond. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 389 l Maximum 3H:1V interior side slopes and maximum 2H:1V exterior slopes. The interior slopes can be increased to a maximum of 2H:1V if fencing is provided at or above the maximum water surface. l One foot of freeboard between the top of the riser and the crest of the emergency spillway. l Flat bottom. l Minimum 1-foot deep spillway. l Length-to-width ratio between 3:1 and 6:1. Sediment Pond Discharge The outlet for the pond consists of a combination of principal and emergency spillways. These out- lets must pass the peak runoff expected from the contributing drainage area for a 100-year storm. If, due to site conditions and basin geometry, a separate emergency spillway is not feasible, the prin- cipal spillway must pass the entire peak runoff expected from the 100-year storm. However, an attempt to provide a separate emergency spillway should always be made. Base the runoff cal- culations on the site conditions during construction. The flow through the dewatering orifice cannot be utilized when calculating the 100-year storm elevation because of its potential to become clogged; therefore, available spillway storage must begin at the principal spillway riser crest. The principal spillway designed by the procedures described below will result in some reduction in the peak rate of runoff. However, the design will not control the discharge flow rates to the extent required to comply with I-3.4.7 MR7: Flow Control. The size of the contributing basin, the expected life of the construction project, the anticipated downstream effects, and the anticipated weather con- ditions during construction should be considered to determine the need for additional discharge con- trol. Principal Spillway: Determine the required diameter for the principal spillway (riser pipe). The dia- meter shall be the minimum necessary to pass the peak volumetric flow rate using a 15-minute time step from a Type 1A, 10-year, 24-hour frequency storm for the developed condition. Use Figure II- 3.31: Riser Inflow Curves to determine the riser diameter. To aid in determining sediment depth, one-foot intervals shall be prominently marked on the riser. Emergency Overflow Spillway: Size the emergency overflow spillway for the peak volumetric flow rate using a 10-minute time step from a Type 1A, 100-year, 24-hour frequency storm for the developed condition. See BMP D.1: Detention Ponds for additional guidance for Emergency Over- flow Spillway design Dewatering Orifice: Size of the dewatering orifice(s) (minimum 1-inch diameter) using a modified version of the discharge equation for a vertical orifice and a basic equation for the area of a circular orifice. Determine the required area of the orifice with the following equation: where Ao = orifice area (square feet) 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 390 AS = pond surface area (square feet) h = head of water above orifice (height of riser in feet) T = dewatering time (24 hours) g = acceleration of gravity (32.2 feet/second2) Convert the orifice area (in square feet) to the orifice diameter D (in inches): The vertical, perforated tubing connected to the dewatering orifice must be at least 2 inches larger in diameter than the orifice to improve flow characteristics. The size and number of perforations in the tubing should be large enough so that the tubing does not restrict flow. The orifice should control the flow rate. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 391 Figure II-3.28: Sediment Pond Plan View 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 392 Figure II-3.29: Sediment Pond Cross Section 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 393 Figure II-3.30: Sediment Pond Riser Detail 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 394 Figure II-3.31: Riser Inflow Curves 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 395 Maintenance Standards l Remove sediment from the pond when it reaches 1 foot in depth. l Repair any damage to the pond embankments or slopes. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 396 Construction Stormwater Pollution Prevention Plan The Summit at Thompson Creek 2220471.11 Exhibit 5 Geotechnical Engineering Report South Sound Geotechnical Consulting, Inc. June 16, 2022 Geotechnical Seasonal Groundwater Monitoring (Winter 2022-2023) South Sound Geotechnical Consulting, Inc. May 12, 2023 South Sound Geotechnical Consulting P.O. Box 39500, Lakewood, WA 98496 (253) 973-0515 June 16, 2022 Mr. Matt Weber c/o AHBL 2215 North 30th Street, Suite 200 Tacoma, Washington 98403-3350 Subject: Geotechnical Engineering Report The Summit at Thompson Creek 14444 Berry Valley Road Yelm, Washington SSGC Project No. 22029 Mr. Weber, South Sound Geotechnical Consulting (SSGC) has completed a geotechnical assessment for the planned development at the above addressed property in Yelm, Washington. Our services have been completed in general conformance with our proposal P21150 (dated March 31, 2022) and authorized per signature of our agreement for services. Our scope of services included completion of twenty test pits and one infiltration test on the site, laboratory testing, engineering analyses, and preparation of this report. PROJECT INFORMATION The site is composed of several individual parcels totaling about 30 acres, west of the current terminus of Berry Valley Road. Development plans include dividing the properties into 93 individual single-family lots. SITE CONDITIONS The eastern portion of the site is lower in elevation and generally level. This area is grass field. The western portion is characterized by a southwest-northeast trending knoll. An existing residence and several outbuildings are in the eastern side of the western portion. The remainder of the western portion is undeveloped and partially forested. Overall elevation change across the site is on the order of 70 (+/-) feet from the highest point on the knoll in the western portion to the lower eastern portion of the site. Elevation change in the western portion is on the order of 50 feet (+/-). SUBSURFACE CONDITIONS Subsurface conditions were characterized by completing twenty test pits and one infiltration test on April 14, 2022. Explorations were advanced to final depths between 2.5 and 11 feet below existing ground surface. Approximate locations of the explorations are shown on Figure 1, Exploration Plan. A summary description of observed subgrade conditions is provided below. Logs of the test holes are provided in Appendix A. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 2 Soil Conditions Topsoil and isolated fill was at the surface of the test pits and extended to depths between 6 inches and 2.5 feet. Native soils below the topsoil/fill varied across the site. In general, the lower eastern portion contains loose sandy gravel with cobbles, occasional boulders and minor fines. These soils are considered to be outwash deposits and extended to the termination depth of the test holes in this area. Native soils in the western portion consisted of upper layers of soft to stiff clayey silt (interpreted as glacial lake deposits) or outwash over glacial till/drift consisting of silty sand with gravel and variable cobbles. Upper soils extended to depths from less than 1 foot to about 5 feet. Till/drift was in a medium dense grading to very dense condition and extended to the termination depth of the test pits, where encountered. Groundwater Conditions Groundwater was observed in test pit TP-17 (adjacent infiltration test PIT-1 in the lower eastern portion) near Thompson Creek at an elevation of about 7 feet at the time of excavation. Seepage was observed in several of the test pits in the western portion above the glacial till/drift. Groundwater levels and seepage will vary throughout the year due to seasonal precipitation and on- and off-site drainage patterns. Geologic Setting Soil types mapped on the site per the USDA Soil Conservation Service map of Thurston County consist of both glacial outwash and glacial drift. Outwash soils are generally mapped in the lower eastern portion of the site, with drift in the higher western portion. Native soils appear to generally conform to the mapped soil types. GEOTECHNICAL DESIGN CONSIDERATIONS The planned development is considered feasible based on observed soil conditions in the test pits. Properly prepared native soils are considered suitable for support of conventional spread footing foundations, slab- on-grade floors, and conventional pavements. Native (outwash) soils are suitable for infiltration, where encountered. Soils with higher fines content and drift/till are predominant in the higher elevated western portion. These soils will limit infiltration facilities to shallow dispersion or detention systems. Infiltration is not feasible in the silty (lake deposits) or dense drift/till soils. Depth to seasonally high groundwater levels should be considered in the design of infiltration facilities in the lower eastern portion. We anticipate grading (cuts and fills) will be used to modify existing topography for final design grades of the development, particularly in the western portion. Final grades should be considered relative to the feasibility of infiltration systems. Recommendations presented in the following sections should be considered general and may require modifications when earthwork and grading occur. They are based upon the subsurface conditions observed Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 3 in the test pits and the assumption that finish site grades will be similar to existing grades. It should be noted that subsurface conditions across the site may vary from those depicted on the exploration logs and can change with time. Therefore, proper site preparation will depend upon the weather and soil conditions encountered at the time of construction. We recommend SSGC review final plans and assess subgrade conditions at the time of construction. General Site Preparation Site grading and earthwork should include procedures to control surface water runoff. Grading the site without adequate drainage control measures may negatively impact site soils, resulting in increased export of impacted soil and import of fill materials, thereby potentially increasing the cost of the earthwork and subgrade preparation phases of the project. Site grading should include removal of all fill and any organic materials in future building and pavement areas. Subgrades should consist of firm native soils following stripping. Final stripping depths can only be determined at the time of earthwork. General Subgrade Preparation Subgrades in building and pavement areas should consist of firm native soil or compacted structural fill. We recommend exposed subgrades in building and conventional pavement areas are proofrolled using a large roller, loaded dump truck, or other mechanical equipment to assess subgrade conditions following stripping. Proofrolling efforts should result in the upper 1 foot of subgrade soils achievin g a firm and unyielding condition and a compaction level of at least 92 percent of the maximum dry density (MDD) per the ASTM D1557 test method. Wet, loose, or soft subgrades that cannot achieve this compaction level should be removed (over-excavated) and replaced with structural fill. The depth of over-excavation should be based on soil conditions at the time of construction. A representative of SSGC should be present to assess subgrade conditions during proofrolling. Grading and Drainage Positive drainage should be provided during construction and maintained throughout the life of the development. Surface water should not be allowed into cut/fill areas, utility trenches, building footprints, or pavement areas. Structural Fill Materials The suitability of soil for use as structural fill will depend on the gradation and moisture content of the soil when it is placed. Soils with higher fines content (soil fraction passing the U.S. No. 200 sieve) will become sensitive with higher moisture content. It is often difficult to achieve adequate compaction if soil moisture is outside of optimum ranges for soils that contain more than about 5 percent fines. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 4 Site Soils: Topsoil is not considered suitable for structural fill and should be wasted from the site or used in landscape areas. The clayey silt and glacial till/drift will be difficult to use due to higher fines content of these soils. Native outwash soil is generally considered suitable for use as structural. Soils with higher fines content could be used provided they can be moisture conditioned to within optimal ranges. Silt content will vary in the native soils and can make them moisture sensitive, requiring conditioning (drying or wetting) to obtain optimum moisture content. Optimum moisture is considered within about +/- 2 percent of the moisture content required to achieve the maximum density per the ASTM D-1557 test method. Import Fill Materials: We recommend imported structural fill placed during dry weather consist of material which meets the specifications for Gravel Borrow as described in Section 9-03.14(1) of the Washington State Department of Transportation (WSDOT) Specifications for Road, Bridge, and Municipal Construction manual (Publication M41-10). Gravel Borrow should be protected from disturbance if exposed to wet conditions after placement. During wet weather, or for backfill on wet subgrades, import soil suitable for compaction in wetter conditions should be provided. Imported fill for use in wet conditions should conform to specifications for Select Borrow as described in Section 9-03.14(2), or Crushed Surfacing per Section 9-03.9(3) of the WSDOT M41-10 manual, with the modification that a maximum of 5 percent by weight shall pass the U.S. No. 200 sieve for these soil types. Structural fill placement and compaction is weather-dependent. Delays due to inclement weather are common, even when using select granular fill. We recommend site grading and earthwork be scheduled for the drier months of the year. Structural fill should not consist of frozen material. Structural Fill Placement We recommend structural fill is placed in lifts not exceeding about 10 inches in loose measure. It may be necessary to adjust lift thickness based on site and fill conditions, and type of compaction equipment used during placement and compaction. Finer grained soil used as structural fill and/or lighter weight compaction equipment may require significantly thinner lifts to attain required compaction levels. Coarser granular soil with lower fines contents could potentially be placed in thicker lifts (1 foot maximum) if they can be adequately compacted. Structural fill should be compacted to attain the recommended levels presented in Table 1, Compaction Criteria. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 5 Table 1. Compaction Criteria Fill Application Compaction Criteria* Footing areas (below structures and retaining walls) 95 % Upper 2 feet in pavement areas, slabs and sidewalks, and utility trenches 95 % Below 2 feet in pavement areas, slabs and sidewalks, and utility trenches 92 % Utility trenches or general fill in non-paved or -building areas 90 % *Per the ASTM D 1557 test method. Trench backfill within about 2 feet of utility lines should not be over-compacted to reduce the risk of damage to the line. In some instances the top of the utility line may be within 2 feet of the surface. Backfill in these circumstances should be compacted to a firm and unyielding condition. We recommend fill procedures include maintaining grades that promote drainage and do not allow ponding of water within the fill area. The contractor should protect compacted fill subgrades from disturbance during wet weather. In the event of rain during structural fill placement, the exposed fill surface should be allowed to dry prior to placement of additional fill. Alternatively, the we t soil can be removed. We recommend consideration be given to protecting haul routes and other high traffic areas with free-draining granular fill material (i.e. sand and gravel containing less than 5 percent fines) or quarry spalls to reduce the potential for disturbance to the subgrade during inclement weather. Structural fill placed on sloping ground should be constructed using a benched (stairstep) methodology. Benches should be cut level or with a slight downward incline into the slope in firm native soil. Benches should be at least 3 feet wide and wide enough to accommodate compaction equipment, and be a maximum of two feet high. Earthwork Procedures Conventional earthmoving equipment should be suitable for earthwork at this site. Earthwork may be difficult during periods of wet weather or if elevated soil moisture is present. Excavated site soils may not be suitable as structural fill depending on the soil moisture content and weather condit ions at the time of earthwork. If soils are stockpiled and wet weather is anticipated, the stockpile should be protected with securely anchored plastic sheeting. If stockpiled soils become unusable, it may become necessary to import clean, granular soils to complete wet weather site work. Wet or disturbed subgrade soils should be over-excavated to expose firm, non-yielding, non-organic soils and backfilled with compacted structural fill. We recommend the earthwork portion of this project be completed during extended periods of dry weather. If earthwork is completed during the wet season (typically late October through May) it may be necessary to take extra measures to protect subgrade soils. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 6 If earthwork takes place during freezing conditions, we recommend the exposed subgrade be allowed to thaw and be re-compacted prior to placing subsequent lifts of structural fill. Alternatively, the frozen soil can be removed to unfrozen soil and replaced with structural fill. The contractor is responsible for designing and constructing stable, temporary excavations (such as utility trenches) to maintain stability of excavation sides and bottoms. Excavations should be sloped or shored in the interest of safety following local and federal regulations, including current OSHA excavation and trench safety standards. Temporary excavation cuts should be sloped at inclinations of 1.5H:1V (Horizontal:Vertical) or flatter, unless the contractor can demonstrate the safety of steeper inclinations. Deeper excavations may require shoring. Permanent cut and fill slopes should be graded at inclinations of 2H:1V, or flatter. A geotechnical engineer and accredited testing material laboratory should be retained during the construction phase of the project to observe earthwork operations and to perform necessary tests and observations during subgrade preparation, placement and compaction of structural fill, and backfilling of excavations. Foundations Foundations can be placed on native soils or on structural fill above prepared native subgrades as described in this report. The following recommendations are for conventional spread footing foundations: Bearing Capacity (net allowable): 2,000 pounds per square foot (psf) for footings supported firm native soils or structural fill over native subgrades prepared as described in this report. Footing Width (Minimum): 18 inches (Strip) 24 inches (Column) Embedment Depth (Minimum): 18 inches (Exterior) 12 inches (Interior) Settlement: Total: < 1 inch Differential: < 1/2 inch (over 30 feet) Allowable Lateral Passive Resistance: 300 psf/ft* (below 18 inches) Allowable Coefficient of Friction: 0.35* *These values include a factor of safety of approximately 1.5. The net allowable bearing pressures presented above may be increased by one-third to resist transient, dynamic loads such as wind or seismic forces. Lateral resistance to footings should be ignored in the upper 12-inches from exterior finish grade. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 7 Foundation Construction Considerations All foundation subgrades should be free of water and loose soil prior to placing concrete, and should be prepared as recommended in this report. Concrete should be placed soon after excavating and compaction to reduce disturbance to bearing soils. Should soils at foundation level become excessively dry, disturbed, saturated, or frozen, the affected soil should be removed prior to placing concrete. We recommend SSGC observe all foundation subgrades prior to placement of concrete. Foundation Drainage Ground surface adjacent foundations should be sloped away from buildings. We recommend footing drains are installed around perimeter footings. Footing drains should include a minimum 4-inch diameter perforated rigid plastic or metal drain line installed at the base of the footing. The perforated drain lines should be connected to a tight line pipe that discharges to an approved storm drain receptor. The drain line should be surrounded by a zone of clean, free-draining granular material having less than 5 percent passing the No. 200 sieve or meeting the requirements of section 9-03.12(2) “Gravel Backfill for Walls” in the WSDOT M41-10 manual. The free-draining aggregate zone should be at least 12 inches wide and wrapped in filter fabric. The granular fill should extend to within 6 inches of final grade where it should be capped with compacted fill containing sufficient fines to reduce infiltration of surface water into the footing drains. Alternately, the ground surface can be paved with asphalt or concrete. Cleanouts are recommended for maintenance of the drain system. On-Grade Floor Slabs On-grade floor slabs should be placed on native soils or structural fill prepared as described in this report. We recommend a modulus subgrade reaction of 150 pounds per square inch per inch (psi/in) for native soil or compacted granular structural fill over native soil. We recommend a capillary break is provided between the prepared subgrade and bottom of slab. Capillary break material should be a minimum of 4 inches thick and consist of compacted clean, free-draining, well graded course sand and gravel. The capillary break material should contain less than 5 percent fines, based on that soil fraction passing the U.S. No. 4 sieve. Alternatively, a clean angular gravel such as No. 7 aggregate per Section 9-03.1(4) C of the WSDOT (M41-10) manual could be used for this purpose. We recommend positive separations and/or isolation joints are provided between slabs and foundations, and columns or utility lines to allow independent movement where needed. Backfill in interior trenches beneath slabs should be compacted in accordance with recommendations presented in this report. A vapor retarder should be considered beneath concrete slabs that will be covered with moisture sensitive or impervious coverings (such as tile, wood, etc.), or when the slab will support equipment or stored materials sensitive to moisture. We recommend the slab designer refer to ACI 302 and/or ACI 360 for procedures and limitations regarding the use and placement of vapor retarders. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 8 Seismic Considerations Seismic parameters and values in Table 2 are recommended based on the 2015 International Building Code (IBC). Table 2. Seismic Parameters PARAMETER VALUE 2018 International Building Code (IBC) Site Classification1 D Ss Spectral Acceleration for a Short Period 1.39 S1 Spectral Acceleration for a 1-Second Period 0.521g 1 Note: In general accordance with the 2018 International Building Code for risk categories I,II,III. IBC Site Class is based on the estimated characteristics of the upper 100 feet of the subsurface profile. Ss, and S1 values based on the OSHPD Seismic Design Maps website. Liquefaction Soil liquefaction is a condition where loose, typically granular soils located below the groundwater surface lose strength during ground shaking, and is often associated with earthquakes. The risk of liquefaction at this site is low for the design level earthquake based on the Thurston County Liquefaction Hazard Map, dated April 2011. A site-specific liquefaction analyses was not part of our scope and would be required to fully assess liquefaction potential and impacts. Infiltration Characteristics Infiltration facilities will be used to assist in control of stormwater, where feasible. An assessment of infiltration potential of the outwash in the lower eastern portion of the site was completed by performing one Pilot Infiltration Test per procedures in the Thurston County Drainage Design and Erosion Control Manual. Results of the test is presented in Table 3. Table 3. Infiltration Rates Sample ID and Depth (ft) Soil Type Measured Infiltration Rate (in/hr) Corrected Infiltration Rate (in/hr) Correction Factors* (Fg/Ft/Fp) PIT-1, 3 ft Outwash 62 28 (1.0/0.5/0.9) *Correction Factors from the Thurston County Drainage Design and Erosion Control Manual. The measured infiltration rate is considered appropriate for the soil tested and are similar to infiltration tests completed at other sites with similar soil throughout Thurston County. We recommend a design infiltration rate of 28 inches per hour (in/hr) for native outwash soils. However, groundwater was encountered at about Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 9 7 feet below the surface in this area and should be considered in the design of infiltration facilities. Infiltration in the clayey silt and till soils is not considered feasible. Additional infiltration assessment may be necessary in other portions of the site to assess infiltration potential once final grades have been established. Cation Exchange Capacity (CEC) and organic content tests were completed on samples of the outwash soil. Test results are summarized in the table below. Table 4. CEC and Organic Content Results Test Site, Sample Number, Depth CEC Results (milliequivalents) CEC Required* (milliequivalents) Organic Content Results (%) Organic Content Required* (%) TP-1, S-1, 3 ft 13.3 ≥ 5 5.30 ≥1.0 PIT-1, S-1, 3 ft 13.5 5.45 * Values from the Thurston County Drainage Design and Erosion Control Manual. Organic content and CEC results of the tested samples satisfy County requirements. Conventional Pavement Sections Subgrades for conventional pavements should be prepared as described in the “Subgrade Preparation” and “Structural Fill” sections of this report. Subgrades below pavement sections should be graded or crowned to promote drainage and not allow for ponding of water beneath the section. If drainage is not provided and ponding occurs, subgrade soils could become saturated, lose strength, and result in premature distress or failure of the section. In addition, the pavement surfacing should also be graded to promote drainage and reduce the potential for ponding of water on the pavement surface. We recommend a separation fabric (such as Mirafi N180, or other) is placed on roads underlain by clayey silt subgrades prior to placement of pavement section materials. The purpose of the fabric is to maintain segregation of the coarser fill and the lower finer grained native soil. Coarser fill will have the tendency to migrate into the looser native soil over time which can compromise the structural integrity of the pavement section fill and result in premature distress in the pavement without the separation fabric. Separation fabric is not considered necessary in roadways underlain by till or outwash. Minimum recommended pavement sections for conventional asphalt or concrete pavements are presented in Table 5. Pavement sections in public right-of-ways should be designed per City of Yelm (or Thurston County) standards. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 10 Table 5. Preliminary Pavement Sections Traffic Area Minimum Recommended Pavement Section Thickness (inches) Asphalt Concrete Surface1 Portland Cement Concrete2 Aggregate Base Course3,4 Subbase Aggregate5 Driveways 2 4 4 12 Access Roads 3 6 6 12 1 1/2 –inch nominal aggregate hot-mix asphalt (HMA) per WSDOT 9-03.8(1) 2 A 28 day minimum compressive strength of 4,000 psi and an allowable flexural strength of at least 250 psi 3 Crushed Surfacing Base Course per WSDOT 9-03.9(3) 4Although not required for structural support under concrete pavements, a minimum four-inch thick base course layer is recommended to help reduce potentials for slab curl, shrinkage cracking, and subgrade “pumping” through joints 5 Gravel Borrow per WSDOT 9-03.14(1) or Permeable Ballast WSDOT 9-03.9(1) or native soil compacted to 95% of ASTM D-1557 Conventional Pavement Maintenance The performance and lifespan of pavements can be significantly impacted by future maintenance. The above pavement sections represent minimum recommended thicknesses and, as such, periodic maintenance should be completed. Proper maintenance will slow the rate of pavement deterioration, and will improve pavement performance and life. Preventive maintenance consists of both localized maintenance (crack and joint sealing and patching) and global maintenance (surface sealing). Added maintenance measures and reduced pavement life should be anticipated over the lifetime of pavements if any existing fill or topsoil is left in-place beneath pavement sections. REPORT CONDITIONS This report has been prepared for the exclusive use of Mr. Matt Weber for specific application to the project discussed, and has been prepared in accordance with generally accepted geotechnical engineering practices in the area. No warranties, either express or implied, are intended or made. The analysis and recommendations presented in this report are based on observed soil conditions and test results at the indicated locations, and from other geologic information discussed. This report does not reflect variations that may occur across the site, or due to the modifying effects of construction, or weather. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplemental recommendations can be provided. This report was prepared for the planned type of development of the site as discussed herein. It is not valid for third party entities or alternate types of development on the site without the express written consent of Geotechnical Engineering Report The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 SSGC SSGC. If development plans change we should be notified to review those changes and modify our recommendations as necessary. The scope of services for this project does not include any environmental or biological assessment of the site including identification or prevention of pollutants, hazardous materials, or conditions. Other studies should be completed if the owner is concerned about the potential for contamination or pollution. We appreciate the opportunity to work with you on this project. Please contact us if additional information is required or we can be of further assistance. Respectfully, South Sound GeoteMchnical Consulting RO Timothy H. Roberts, P.E. Member/Geotechnical Engineer Attachments: Figure 1 — Exploration Plan Appendix A — Field Exploration Procedures and Test Pit Logs Appendix B — Laboratory Testing and Results Unified Soil Classification System TP-1 South Sound Geotechnical Consulting P.O. Box 39500 Lakewood, WA 98496 (253) 973-0515 Figure 1 – Exploration Plan The Summit at Thompson Creek Yelm, WA SSGC Project #22029 Base map from sheet drawing titled “Preliminary Site Plan - The Summit at Thompson Creek ”, by Informed AHBL, dated 3-16-22. N Legend Approximate Test Pit Location No Scale PIT - 1 PIT - 1 Approximate Infiltration Test Location TP-10 TP-4 TP-1 TP-5 TP-6 TP-3 TP-2 TP-9 TP-8 TP-11 TP-12 TP-13 TP-14 TP-15 TP-16 TP-17 TP-18 TP-19 TP-20 TP-7 Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 Appendix A Field Exploration Procedures and Test Pit Logs Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 A-1 Field Exploration Procedures Our field exploration for this project included twenty test pits and one Pilot Infiltration Test completed on April 14, 2022. The approximate locations of the explorations are shown on Figure 1, Exploration Plan. Test pit locations were determined by pacing from site features. Ground surface elevations referenced on the logs were inferred from topographic data from Google Earth satellite imagery and the Thurston County GIS. Test pit locations and elevations should be considered accurate only to the degree implied by the means and methods used. A private excavation company dug the test pits. Soil samples were collected and stored in moisture tight containers for further assessment and laboratory testing. Explorations were backfilled with excavated soils and tamped when completed. Please note that backfill in the explorations may settle with time. Backfill material located in roads or building areas should be re-excavated and recompacted, or replaced with structural fill. The following logs indicate the observed lithology of soils and other materials observed in the explorations at the time of excavation. Where a soil contact was observed to be gradational, our log indicates the average contact depth. Our logs also indicate the approximate depth to groundwater (where observed at the time of excavation), along with sample numbers and approximate sample depths. Soil descriptions on the logs are based on the Unified Soil Classification System. Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 1 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-1 Depth (feet) Material Description 0 – 1 1 – 4.5 4.5 – 5 Topsoil Fine to coarse sandy GRAVEL with some cobbles: Loose, moist, brown. (GP)(Glacial Outwash)(Sample S-1 @ 3 feet) Silty SAND with some gravel and a few cobbles: Medium dense, moist, gray. (SM) (Glacial Till). Test pit completed at approximately 5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 380 feet Test Pit TP-2 Depth (feet) Material Description 0 – 1 1 – 4.5 4.5 – 5.5 Topsoil Clayey SILT/Silty CLAY with some fine gravel: Soft to stiff, moist, mottled reddish brown to gray. (ML/CL) (Glacial Lake Deposits). Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 5.5 feet on 4/14/22. Slight seepage at 4 feet at time of excavation. Approximate surface elevation: 360 feet. Test Pit TP-3 Depth (feet) Material Description 0 – 1.0 1.0 – 4.5 4.5 – 5.5 Topsoil Sandy GRAVEL with trace to some silt and occasional cobble: Loose, moist, brown grading gray. (GP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, gray. (SM) (Glacial Till). Test pit completed at approximately 5.5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 360 feet Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 2 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-4 Depth (feet) Material Description 0 – 1 1 – 3 3 – 4 Topsoil Sandy GRAVEL with trace to some silt and occasional cobble: Loose, moist, brown grading gray. (GP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, gray. (SM) (Glacial Till). Test pit completed at approximately 4 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 365 feet Test Pit TP-5 Depth (feet) Material Description 0 – 1 1 – 4 4 – 5.5 Topsoil Sandy GRAVEL with trace to some silt and occasional cobble: Loose, moist, reddish brown. (GP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, gray. (SM) (Glacial Till). Test pit completed at approximately 5.5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 380 feet Test Pit TP-6 Depth (feet) Material Description 0 – 0.5 0.5 – 1.5 1.5 – 2.5 Topsoil Sandy GRAVEL with trace to some silt and occasional cobble: Loose, moist, reddish brown. (GP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 2.5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 395 feet Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 3 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-7 Depth (feet) Material Description 0 – 2.5 2.5 – 4 4 – 4.5 Topsoil/fill Organic silt, sand, gravel: Loose, moist dark brown. Silty SAND with gravel and some cobbles: Medium dense to dense, moist, mottled brown to gray. (SM) (Weathered Glacial Till). Silty SAND with gravel and cobbles: Dense to very dense, moist, gray, massive. (SM) (Glacial Till) Test pit completed at approximately 4.5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 380 feet Test Pit TP-8 Depth (feet) Material Description 0 – 0.5 0.5 – 4 Topsoil Silty SAND with gravel and a few cobbles: Medium dense to dense, moist, brown grading gray. (SM) (Glacial Till). Test pit completed at approximately 4 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 360 feet. Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 4 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-9 Depth (feet) Material Description 0 – 0.5 0.5 – 4 4 – 5 Topsoil Sandy GRAVEL with trace to some silt and occasional cobble: Loose, dry to moist, light brown grading gray. (GP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 360 feet Test Pit TP-10 Depth (feet) Material Description 0 – 0.5 0.5 – 5 5 – 5.5 Topsoil Sandy GRAVEL with trace to some silt and occasional cobble: Loose, dry to moist, light brown grading gray. (GP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 5.5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 360 feet Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 5 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-11 Depth (feet) Material Description 0 – 0.5 0.5 – 3 3 – 5 5 – 6 Topsoil Fine to coarse sandy GRAVEL and a few cobbles grades medium to coarse gravelly fine to medium sand at 2.0-3.0 feet: Loose, moist, brown. (SP/GP). (Outwash) Fine to medium SAND with a trace of gravel: Loose, moist, gray. (SP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 6 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 358 feet Test Pit TP-12 Depth (feet) Material Description 0 – 0.5 0.5 – 3.5 3.5 – 4 Topsoil Clayey SILT/Silty CLAY with some fine gravel: Soft to stiff, moist, reddish brown grading to gray at 4.0 feet (ML/CL) (Glacial Lake Deposits). Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 4 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 350 feet Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 6 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-13 Depth (feet) Material Description 0 – 0.5 0.5 – 2 2 – 5.5 Topsoil SILT with variable clay and fine gravel: Soft to stiff, moist, reddish brown grading to gray at 4.0 feet (ML) (Glacial Lake Deposits). Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 5.5 feet on 4/14/22. Moderate seepage at 2 feet observed at time of excavation. Approximate surface elevation: 350 feet Test Pit TP-14 Depth (feet) Material Description 0 – 0.5 0.5 – 5 5 – 6 Topsoil SILT with variable clay and fine gravel: Soft to stiff, moist, reddish brown grading to gray at 4.0 feet (ML) (Glacial Lake Deposits). Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 6 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 355 feet. Test Pit TP-15 Depth (feet) Material Description 0 – 0.5 0.5 – 4 4 – 5 Topsoil Sandy GRAVEL with some cobbles and a few boulders: Loose, dry to moist, light brown grading to gray at 3.0 feet. (GP) (Outwash) Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 5 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 355 feet Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 7 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-16 Depth (feet) Material Description 0 – 0.5 0.5 – 6 6 – 7 Topsoil SILT with variable clay and fine gravel: Soft to stiff, moist, reddish brown grading to gray at 4.0 feet (ML) (Glacial Lake Deposits). Silty SAND with gravel and a few cobbles: Dense, moist, brownish gray. (SM) (Glacial Till). Test pit completed at approximately 7 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 380 feet. Test Pit TP-17 Depth (feet) Material Description 0 – 0.5 0.5 – 11 Topsoil Cobbly fine to coarse GRAVEL with sand: Loose to medium dense, moist to wet, black grading to light brown below 3.0 feet. (GP) (Outwash) Test pit completed at approximately 11 feet on 4/14/22. Groundwater and sloughing observed at 7 feet at time of excavation. Approximate surface elevation: 335 feet. Test Pit TP-18 Depth (feet) Material Description 0 – 0.5 0.5 – 10 Topsoil Medium to coarse sandy GRAVEL with cobbles and boulders: Loose to medium dense, moist to wet at 9 feet, black grading to light brown below 3.0 feet. (GP) (Outwash). Test pit completed at approximately 10 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 335 feet. Project: Summit at Thompson Creek SSGC Job # 22029 EXPLORATION LOGS PAGE 8 OF 8 Location: 14444 Berry Valley Rd, Yelm, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 to TP-20, PIT-1 Logged by: THR Test Pit TP-19 Depth (feet) Material Description 0 – 0.5 0.5 – 8 Topsoil Medium to coarse sandy GRAVEL with cobbles and boulders: Loose, moist, black grading to light brown below 3.0 feet. (GP) (Outwash). Test pit completed at approximately 8 feet on 4/14/22. Groundwater not observed at time of excavation. Approximate surface elevation: 340 feet. Test Pit TP-20 Depth (feet) Material Description 0 – 0.5 0.5 – 5.0 Topsoil Medium to coarse sandy GRAVEL with cobbles and boulders: Loose, moist, black grading to light brown below 3.0 feet. (GP) (Outwash). Test pit completed at approximately 5 feet on 4/14/22 due to caving. Groundwater not observed at time of the excavation. Approximate surface elevation: 340 feet. Infiltration Test PIT-1 Depth (feet) Material Description 0 – 0.5 0.5 – 5 Topsoil Cobbly fine to coarse GRAVEL with boulders with some sand: Loose, moist, black grading to light brown below 3.0 feet. (GP) (Glacial Outwash) (Sample S-1 @ 3 feet). Test hole completed at approximately 5 feet on 4/14/22. Groundwater not observed at time of excavation. Infiltration test completed at 3 feet. Approximate surface elevation: 335 feet. Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 Appendix B Laboratory Testing and Results Geotechnical Engineering Report SSGC The Summit at Thompson Creek Yelm, Washington SSGC Project No. 22029 June 16, 2022 B-1 Laboratory Testing Select soil samples were tested for organic content and cation exchange capacity (CEC) by Northwest Agricultural Consultants of Kennewick, Washington. Results of the laboratory testing are included in this appendix. 2545 W Falls Avenue Kennewick, WA 99336 509.783.7450 www.nwag.com lab@nwag.com Sample ID Organic Matter Cation Exchange Capacity Pit-1, S-1 5.45% 13.5 meq/100g TP-1, S-1 5.30% 13.3 meq/100g Method ASTM D2974 EPA 9081 South Sound Geotechnical Consulting PO Box 39500 Lakewood, WA 98496 Report: 59040-1-1 Date: April 25, 2022 Project No: 22029 Project Name: Berry Valley UNIFIED SOIL CLASSIFICATION SYSTEM Criteria for Assigning Group Symbols and Group Names Using Laboratory TestsA Soil Classification Group Symbol Group NameB Coarse Grained Soils More than 50% retained on No. 200 sieve Gravels More than 50% of coarse fraction retained on No. 4 sieve Clean Gravels Less than 5% finesC Cu  4 and 1  Cc  3E GW Well-graded gravelF Cu  4 and/or 1  Cc  3E GP Poorly graded gravelF Gravels with Fines More than 12% finesC Fines classify as ML or MH GM Silty gravelF,G, H Fines classify as CL or CH GC Clayey gravelF,G,H Sands 50% or more of coarse fraction passes No. 4 sieve Clean Sands Less than 5% finesD Cu  6 and 1  Cc  3E SW Well-graded sandI Cu  6 and/or 1  Cc  3E SP Poorly graded sandI Sands with Fines More than 12% finesD Fines classify as ML or MH SM Silty sandG,H,I Fines Classify as CL or CH SC Clayey sandG,H,I Fine-Grained Soils 50% or more passes the No. 200 sieve Silts and Clays Liquid limit less than 50 inorganic PI  7 and plots on or above “A” lineJ CL Lean clayK,L,M PI  4 or plots below “A” lineJ ML SiltK,L,M organic Liquid limit - oven dried  0.75 OL Organic clayK,L,M,N Liquid limit - not dried Organic siltK,L,M,O Silts and Clays Liquid limit 50 or more inorganic PI plots on or above “A” line CH Fat clayK,L,M PI plots below “A” line MH Elastic SiltK,L,M organic Liquid limit - oven dried  0.75 OH Organic clayK,L,M,P Liquid limit - not dried Organic siltK,L,M,Q Highly organic soils Primarily organic matter, dark in color, and organic odor PT Peat A Based on the material passing the 3-in. (75-mm) sieve B If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name. C Gravels with 5 to 12% fines require dual symbols: GW -GM well-graded gravel with silt, GW -GC well-graded gravel with clay, GP-GM poorly graded gravel with silt, GP-GC poorly graded gravel with clay. D Sands with 5 to 12% fines require dual symbols: SW -SM well-graded sand with silt, SW -SC well-graded sand with clay, SP-SM poorly graded sand with silt, SP-SC poorly graded sand with clay E Cu = D60/D10 Cc = 6010 2 30 DxD )(D F If soil contains  15% sand, add “with sand” to group name. G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM. HIf fines are organic, add “with organic fines” to group name. I If soil contains  15% gravel, add “with gravel” to group name. J If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay. K If soil contains 15 to 29% plus No. 200, add “with sand” or “with gravel,” whichever is predominant. L If soil contains  30% plus No. 200 predominantly sand, add “sandy” to group name. M If soil contains  30% plus No. 200, predominantly gravel, add “gravelly” to group name. N PI  4 and plots on or above “A” line. O PI  4 or plots below “A” line. P PI plots on or above “A” line. Q PI plots below “A” line. South Sound Geotechnical Consulting P.O. Box 39500, Lakewood, WA 98496 (253) 973-0515 May 12, 2023 Mr. Matt Weber c/o AHBL 2215 North 30th Street, Suite 200 Tacoma, Washington 98403-3350 Subject: Seasonal Groundwater Monitoring (Winter 2022-2023) The Summit at Thompson Creek 14444 Berry Valley Road Yelm, Washington SSGC Project No. 22029 Mr. Weber, South Sound Geotechnical Consulting (SSGC) has completed monitoring of groundwater levels through the 2022-2023 winter season at the planned Summit at Thompson Creek development in Yelm, Washington. A piezometer was installed in one of the test pits (TP-17) in the proposed stormwater facility to monitor seasonal groundwater fluctuations through the 2022 – 2023 winter season. Approximate locations of all test holes are shown on Figure 1, Exploration Plan. Groundwater levels measured from the original date of setting the piezometers to April 2023 are presented in the table below. Test Hole Date Groundwater Level (Below Surface) TP-17 4/14/22 7’ 12/10/22 7’ 6” 1/15/23 7’ 4” 2/18/23 6’ 9” 3/18/23 6’ 10” 4/8/23 6’8” We recommend the April 2023 level observed in test pit TP-17 is used in design of stormwater control facilities for this site. Adjustments to groundwater levels for infiltration facilities should account for final grades of the site (cuts and fills that will impact existing surface grades). REPORT CONDITIONS This report has been prepared for the exclusive use of Matt Weber for specific application to the project discussed, and has been prepared in accordance with generally accepted geotechnical engineering practices in the area. No warranties, either express or implied, are intended or made of future groundwater conditions. Seasonal Groundwater Monitoring (Winter 2022-2023) The Summit at Thompson Ridge Yelm, Washington SSGC Project No. 22029 May 12, 2023 SSGC We appreciate the opportunity to work with you on this project. Please contact us if additional information is required or we can be of further assistance. Respectfully, South Sound Geotechnical Consulting Timothy H. Roberts, P.E. Member/Geotechnical Engineer Attachment: Figure 1 — Exploration Plan TP-1 South Sound Geotechnical Consulting P.O. Box 39500 Lakewood, WA 98496 (253) 973-0515 Figure 1 – Exploration Plan The Summit at Thompson Creek Yelm, WA SSGC Project #22029 Base map from sheet drawing titled “Preliminary Site Plan - The Summit at Thompson Creek ”, by Informed AHBL, dated 3-16-22. N Legend Approximate Test Pit Location No Scale PIT - 1 PIT - 1 Approximate Infiltration Test Location TP-10 TP-4 TP-1 TP-5 TP-6 TP-3 TP-2 TP-9 TP-8 TP-11 TP-12 TP-13 TP-14 TP-15 TP-16 TP-17 TP-18 TP-19 TP-20 TP-7