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9904 - SWPPP Report - 11-19-2024a Larson & Associates surveyors, engineers and planners 9027 Pacific Avenue, Suite 4 Tacoma, WA 98444 STORMWATER POLLUTION PREVENTION PLAN PROPONENT: C&E DEVELOPMENTS,, LLC P.O. BOX 2983 YELM, WA. 98597 PH: (360) 400-0432 �.'l f PREPARED BY: Larson & Associates surveyors, engineers and planners 9027 Pacific Avenue, Suite 4 Tacoma, WA 98444 (253) 474-3404 November 19, 2024 PROJECT ENGINEER'S CERTIFICATION.......................................................................................................... 1 STORMWATER POLLUTION PREVENTION PLAN SECTION 1 - CONSTRUCTION STORMWATER POLLUTION PREVENTION ELEMENTS ......................2-10 SECTION 2 - PROJECT DESCRIPTION ............................................................................................................... 11 SECTION 3 - EXISTING SITE CONDITIONS ...................................................................................................... 11 SECTION 4 - ADJACENT AREAS ........................................................................................................................ 11 SECTION 5 - CRITICAL AREAS ..................................................................................................................... 11-12 SECTION 6 - SOILS ............................................................................................................................................... 12 SECTION 7 - EROSION PROBLEM AREAS ........................................................................................................ 12 SECTION 8 - CONSTRUCTION PHASING ..................................................................................................... 12-13 SECTION 9 - CONSTRUCTION SCHEDULE ...................................................................................................... 13 SECTION 10 - FINANCIAL/OWNERSHIP RESPONSIBILITIES ....................................................................... 13 SECTION 11 - ENGINEERING CALCULATIONS ......................................................................................... 13-16 SECTION 12 - EROSION CONTROL SPECIALIST ............................................................................................. 17 APPENDIX "A" ............................................................... STORMWATER POLLUTION PREVENTION BMPs 1 SECTION 1 – CONSTRUCTION STORMWATER POLLUTION PREVENTION ELEMENTS Stormwater pollution prevention will be maintained during the construction of this site by incorporating standard erosion control methods such as a temporary construction entrance and mirafi siltation fences. The following devices will be used to trap sediment from the cleared areas and prevent it from leaving the site. A construction entrance will be installed at the entrance to the site to keep sediment from being tracked out of the site and onto the City roads. Mirafi silt fences will be installed along the perimeter to prevent sediment runoff from exiting the project limits. The following general Washington State Dept. of Ecology Construction Stormwater Pollution Prevention Elements shall be upheld at all times during the construction process. Please reference the Best Management Practices (BMPs) in Appendix A of this report. Element #1: Preserve Vegetation/Mark Clearing Limits • Prior to beginning land disturbing activities, including clearing and grading, clearly mark all clearing limits, sensitive areas and their buffers, and trees that are to be preserved within the construction area. These shall be marked, both in the field and on the plans, to prevent damage and offsite impacts. • The duff layer, native topsoil, and natural vegetation shall be retained in an undisturbed state to the maximum degree practicable. If it is not practical to retain the native topsoil or duff layer in place, then stockpile it onsite or at an approved location, cover it to prevent erosion, and replace it immediately when site disturbance is complete. See SOIL PRESERVATION AND AMENDMENT NOTES on the site development plans. • Plastic, metal, or stake wire fence may be used to mark the clearing limits. • Suggested BMPs: - BMP C103: High Visibility Plastic or Metal Fence - BMP C233: Silt Fence Element #2: Establish Construction Access • Construction vehicle access and exit shall be limited to one route, if possible. • Access points shall be stabilized with a pad of quarry spalls, crushed rock or other equivalent BMP, to minimize the tracking of sediment onto public roads. • Wheel wash or tire baths should be located on site, if the stabilized construction entrance is not effective in preventing sediment from being tracked onto public roads. 2 • If sediment is tracked off site, public roads shall be cleaned thoroughly at the end of each day, or more frequently during wet weather. Sediment shall be removed from roads by shoveling or pickup sweeping and shall be transported to a controlled sediment disposal area. • Street washing is allowed only after sediment is removed in accordance with the above bullet. Street wash wastewater shall be controlled by pumping back on site or otherwise be prevented from discharging into systems tributary to waters of the state. • Suggested BMPs: - BMP C105: Stabilized Construction Entrance Element #3: Control Flow Rates • Protect properties and waterways downstream of the proposed development from erosion and the associated discharge of turbid waters due to increases in the velocity and peak volumetric flow rate of stormwater runoff from the project site. • Due to the native soil’s significant infiltration rate, it is the intent of the project storm drainage design to infiltrate 100% of the stormwater runoff with no surface discharge offsite. The site topography is also relatively flat, so very little, if any, stormwater runoff is expected to discharge offsite during construction. However, if necessary, flow control BMPs can be placed along the perimeter of the project, adjacent to the silt fence. • Suggested BMPs: - BMP C207: Check Dams - BMP C241: Sediment Pond Element #4: Install Sediment Controls • Install and maintain effective erosion controls and sediment controls to minimize the discharge of pollutants. • Construct sediment control BMPs as one of the first steps in grading. These BMPs shall be functional before other land disturbing activities take place. • Suggested BMPs: - BMP C233: Silt Fence - BMP C241: Sediment Pond Element #5: Stabilize Soils • Exposed and unworked soils shall be stabilized by application of effective BMPs that prevent erosion. Applicable BMPs include, but are not limited to: temporary and permanent seeding, sodding, mulching, plastic covering, the early application of gravel base on areas to be paved, and dust control. 3 • Soils must not remain exposed and unworked for more than the time periods set forth below to prevent erosion: - During the dry season (May 1 – Sept. 30): 7 days - During the wet season (October 1 – April 30): 2 days • Stabilize soils at the end of the shift before a holiday or weekend if needed based on the weather forecast. • Stabilize soil stockpiles from erosion, protected with sediment trapping measures, and where possible, locate away from storm drain inlets, waterways, and drainage channels. • Suggested BMPs: - BMP C120: Temporary and Permanent Seeding - BMP C121: Mulching - BMP C123: Plastic Covering - BMP C124: Sodding - BMP C125: Topsoiling/Composting - BMP C140: Dust Control Element #6: Protect Slopes • Construct cut-and-fill slopes in a manner to minimize erosion. • Divert offsite stormwater (run-on) or groundwater away from slopes and disturbed areas with interceptor dikes, pipes, and/or swales. Offsite stormwater must be managed separately from stormwater generated on the site. • At the top of slopes, collect drainage in pipe slope drains or protected channels to prevent erosion. • Provide drainage to remove groundwater intersecting the slope surface of exposed soil areas. • Place excavated material on the uphill side of trenches, consistent with safety and space considerations. • Place check dams at regular intervals within constructed channels that are cut down a slope. • Stabilize soils on slopes, as specified in Element #5 above. • Suggested BMPs: - BMP C120: Temporary and Permanent Seeding - BMP C121: Mulching - BMP C200: Interceptor Dike and Swale 4 - BMP C207: Check Dams Element #7: Protect Drain Inlets • Protect all storm drain inlets made operable during construction, including offsite inlets adjacent to the project site, so that stormwater runoff does not enter the conveyance system without first being filtered or treated to remove sediment. • Clean or remove and replace inlet protection devices when sediment has filled one- third of the available storage (unless a different standard is specified by the product manufacturer). • Inlets shall be inspected weekly at a minimum and daily during storm events. • Keep all approach roads clean. Sediment and street wash wastewater shall be controlled as specified above in Element #2. • Suggested BMPs: - BMP C220: Storm Drain Inlet Protection Element #8: Stabilize Channels and Outlets • Provide stabilization, including armoring material, adequate to prevent erosion of outlets, adjacent streambanks, slopes, and downstream reaches at the outlets of all conveyance systems. • The preferred method for stabilizing channels is to completely line the channel with a blanket product first, then add check dams as necessary to function as an anchor and to slow the flow of water. • Suggested BMPs: - BMP C207: Check Dams Element #9: Control Pollutants • Handle and dispose of all pollutants, including waste materials and demolition debris that occurs onsite in a manner that does not cause contamination of stormwater. Woody debris may be chopped and spread onsite. • Provide cover, containment, and protection from vandalism for all chemicals, liquid products, petroleum products, and other materials that have the potential to pose a threat to human health or the environment. Onsite fueling tanks must include secondary containment, i.e. – placing tanks or containers within an impervious structure capable of containing 110% of the volume contained in the largest tank within the containment structure. Double-walled tanks do not require additional secondary containment. 5 • Conduct maintenance, fueling, and repair of heavy equipment and vehicles using spill prevention and control measures. Clean contaminated surfaces immediately following any spill incident. • Conduct oil changes, hydraulic system drain down, solvent and de-greasing cleaning operations, fuel tank drain down and removal, and other activities which may result in discharge or spillage of pollutants to the ground or into stormwater runoff using spill prevention measures, such as drip pans. • Discharge wheel wash or tire bath wastewater shall be discharged to a separate onsite treatment system that prevents discharge to surface water, such as closed-loop recirculation or to the sanitary sewer. For discharges to the sanitary sewer, permits must be obtained from the County Industrial Pretreatment Program at (253) 798-3013. • Apply fertilizers and pesticides in a manner and at application rates that will not result in loss of chemical to stormwater runoff. Follow manufacturers’ label requirements for application rates and procedures. • Use BMPs to prevent contamination of stormwater runoff by pH-modifying sources. The sources for this contamination include, but are not limited to: bulk cement, cement kiln dust, fly ash, new concrete washing and curing waters, waste streams generated from concrete grinding and sawing, exposed aggregate processes, dewatering concrete vaults, concrete pumping and mixer washout waters. Adjust the pH of stormwater if necessary, to prevent violations of water quality standards. • Obtain written approval from Ecology before using chemical treatment, other than CO2 or dry ice to adjust pH. • Wheel wash or tire bath wastewater should not include wastewater from concrete washout areas. • Clean contaminated surfaces immediately following any discharge or spill incident. Emergency repairs may be performed onsite using temporary plastic placed beneath and, if raining, over the vehicle. • Suggested BMPs: - BMP C151: Concrete Handling - BMP C153: Material Delivery, Storage and Containment Element #10: Control Dewatering - This element is not applicable as there was no groundwater found in any of the geotechnical test pit explorations. 6 Element #11: Maintain BMPs • Maintain and repair all temporary and permanent Construction SWPPP BMPs as needed to ensure continued performance of their intended function in accordance with BMP specifications. • Remove all temporary Construction SWPPP BMPs within 30 days after achieving final site stabilization or after the temporary BMPs are no longer needed. • Provide protection to all BMPs installed for the permanent control of stormwater from sediment and compaction. All BMPs that are to remain in place following completion of construction shall be examined and placed in full operating conditions. If sediment enters the BMPs during construction, it shall be removed and the facility shall be returned to the conditions specified in the site development plans. • Suggested BMPs: - BMP C160: Certified Erosion and Sediment Control Lead Element #12: Manage the Project • Inspection and monitoring – Inspect, maintain, and repair all BMPs as needed to ensure continued performance of their intended function. Conduct site inspections and monitoring in accordance with all applicable county and Construction Stormwater General Permit requirements. • Maintaining an updated Construction SWPPP – Maintain, update, and implement the Construction SWPPP in accordance with the Construction Stormwater General Permit requirements and the requirements outlined in this Element (#12). • Project sites disturbing less than 1 acre may have a CESCL or a person without CESCL certification conduct inspections. By the initiation of construction, the Construction SWPPP must identify the CESCL or inspector, who shall be present onsite or on-call at all times. Additional Guidance for Site Inspections: • The CESCL (or other inspector for sites disturbing less than 1 acre) must have the skills to assess the: o Site conditions and construction activities that could impact the quality of stormwater. o Effectiveness of Construction SWPPP measures used to control the quality of stormwater discharges. 7 • The CESCL or inspector must examine stormwater visually for the presence of suspended sediment, turbidity, discoloration, and oil sheen. They must evaluate the effectiveness of BMPs and determine if it is necessary to install, maintain, or repair BMPs to improve the quality of stormwater discharges. • Based on the results of the inspection, construction site operators must correct the problems identified by: o Reviewing the Construction SWPPP for compliance with the 13 Construction SWPPP elements and making appropriate revisions within 7 days of the inspection. o Immediately begin the process of fully implementing and maintaining appropriate source control and/or treatment BMPs as soon as possible, addressing the problems no later than within 10 days of the inspection. If installation of necessary treatment BMPs is not feasible within 10 days, the construction site operator may request an extension within the initial 10-day response period o Documenting BMP implementation and maintenance in the site log book (applies only to sites that have coverage under the Construction Stormwater General Permit). • The site inspector must inspect all areas disturbed by construction activities, all BMPs, and all stormwater discharge locations at least once every calendar week and within 24 hours of any discharge from the site. (For purposes of this condition, individual discharge events that last more than 1 day do not require daily inspections. For example, if a stormwater pond discharges continuously over the course of a week, only one inspection is required that week.) Note that for projects that require a CESCL per BMP C160, additional requirements may apply. The inspector may reduce the inspection frequency for temporary stabilized, inactive sites to once every calendar month during the dry season only (May 1 through September 30). Additional Guidance: • Seasonal Work Limitations: From October 1 through April 30, clearing, grading, and other soil disturbing activities is permitted only if shown to the satisfaction of the county that the site operator will prevent silt-laden runoff from leaving the site through a combination of the following: o Compliance with Construction SWPPP Element #5 to Stabilize Soil and BMP Usage. o Site conditions including existing vegetative coverage, slope, soil type, and proximity to receiving waters. 8 o Limit activities and the extent of disturbed areas. o Proposed Construction SWPPP measures. • Based on the information provided and/or local weather conditions, the county may expand or restrict the seasonal limitation onsite disturbance. The county shall take enforcement action –such as a notice of violation, administrative order, penalty, or stop-work order under the following circumstances: o If, during the course of any construction activity or soil disturbance during the seasonal limitation period, sediment leaves the construction site causing a violation of the surface water quality standard. o If clearing and grading limits or Construction SWPPP measures shown in the approved plan are not maintained. The following activities are exempt from the seasonal clearing and grading limitations: o Routine maintenance and necessary repair of Construction SWPPP BMPs. o Routine maintenance of public facilities or existing utility structures that do not expose the soil or result in the removal of the vegetative cover to soil. o Activities where there is 100 percent infiltration of surface water runoff within the site in approved and installed Construction SWPPP facilities. • Coordination with Utilities and Other Contractors: The primary project applicant shall evaluate, with input from utilities and other contractors, the stormwater management requirements for the entire project, including the utilities, when preparing the Construction SWPPP. • Inspection and Monitoring: All BMPs must be inspected, maintained, and repaired as needed to ensure continued performance of their intended function. Site inspections must be conducted by a person knowledgeable in the principles and practices of erosion and sediment control. The person must have the skills to 1) assess the site conditions and construction activities that could impact the quality of stormwater, and 2) assess the effectiveness of Construction SWPPP measures used to control the quality of stormwater discharges. Appropriate BMPs or design changes shall be implemented as soon as possible whenever inspection and/or monitoring reveals that the BMPs identified in the Construction SWPPP are inadequate, due to the actual discharge of/or potential to discharge a significant amount of any pollutant. 9 Inspection reports and daily logs must be available onsite with the Construction SWPPP and shall be submitted to the county upon request at any time during the course of the project. • Maintaining an Updated Construction SWPPP: Retain the Construction SWPPP onsite or within reasonable access to the site. Modify the Construction SWPPP whenever there is a change in the design, construction, operation, or maintenance at the construction site that has, or could have, a significant effect on the discharge of pollutants to waters of the State. The Construction SWPPP must be modified if, during inspections or investigations conducted by the owner/operator, or the applicable county or state regulatory authority, it is determined that the Construction SWPPP is ineffective in eliminating or significantly minimizing pollutants in stormwater discharges from the site. Modify the Construction SWPPP as necessary to include additional or modified BMPs designed to correct problems identified. Complete revisions to the Construction SWPPP within seven (7) days following the inspection. Element #13: Protect Low Impact Development BMPs • Keep all heavy equipment off existing soils under LID facilities that have been excavated to final grade to retain the infiltration rate of the soils. • See Section 3.3 for more details on protecting LID BMPs. Suggested BMPs: - BMP C103: High Visibility Fence - BMP C233: Silt Fence 10 SECTION 2 – PROJECT DESCRIPTION The “Rhoton Road Binding Site Plan” project is located in the SE1/4 of the NW1/4 of Section 19, Township 17 North, Range 2 East of the Willamette Meridian in the City of Yelm in Thurston County, Washington. The proposal consists of creating (7) building pads for uses such as warehousing, equipment storage, light manufacturing and potential hardware store. An internal paved access, parking stalls, sidewalk and associated utilities including onsite private storm drainage, public water main and sewer S.T.E.P. force main extensions. The property is approximately 3.28 acres in size located on parcel No's 22719240201 and 22719240203. All building roof runoff will be collected and infiltrated directly into the ground via corresponding infiltration trenches, since it is considered “clean” per Department of Ecology standards and no formal water quality treatment is required for these roof surfaces. Stormwater runoff from the access road, parking areas, sidewalk and curb will be collected via catch basins set along inverted “crown” pavement flowline and routed to their corresponding infiltration trench which will then infiltrate stormwater to the native soils below. Prior to conveyance to the infiltration trench, stormwater will be routed to and through an Oldcastle (or approved equal) perkfilter system providing basic treatment of stormwater. All other disturbed pervious areas will be amended to meet the BMPT5.13 per the DOE manual. Stormwater BMPS have been designed per the 2019 DOE Stormwater Management Manual. SECTION 3 – EXISTING SITE CONDITIONS The site is located northwest of the intersection of Rhoton Rd NW and Northern Pacific Rd NW. The project area is vacant with prairie grass and a few trees onsite. The site has an approximate high point elevation of 339 in the southwest. The site slopes from west to east with an approximate low point elevation of 332 in the east. Stormwater runoff presently sheet flows across the site from the west to east and is infiltrated through the “good” free draining soils located across the site. SECTION 4 – ADJACENT AREAS Surrounding properties consist of single family residence to the west, industrial/commercial to the south and north, a church to the south and Roton Rd SE to the east. SECTION 5 – CRITICAL AREAS To the best of our knowledge, there are no wetland located onsite. Per the Thurston County GIS, there is a wetland area located offsite to the southwest and we have reviewed Figure I-3.5 from the 2019 SWMMWW and plan to employ standard erosion control BMPs such as siltation fencing along the down gradient side of the site and installation of CB inlet protection within all proposed catchbasins onsite to ensure this very minimal development proposal does not have any negative impacts on any offsite areas. We would also like note to that the proposed impervious surface associated with this project proposal has been designed to fully infiltrate “design” storm events within proposed underground infiltration trench facilities as modeled with an approved continuous 11 runoff model (MGS FLOOD). Further, any disturbed pervious surfaces onsite associated with this project will be properly soil amended following D.O.E. soil amendment BMP T5.13, as required. SECTION 6 - SOILS Per the N.R.C.S. soils map, on-site soils consist of Spanaway gravelly sandy loam, 0 to 3 percent slopes and Spanaway stony sandy loam, 3 to 15 percent slopes. These soils will be very conducive to infiltration of stormwater for proposed improvements onsite. A geotechnical report was completed by Quality Geo NW, PLLC dated 8/24/2023. They recommend that a design infiltration rate of 20 in/hr be utilized to size in-ground infiltration facilities. SECTION 7 – EROSION PROBLEM AREAS To the best of our knowledge, there are no erosion problem areas on or in the direct vicinity of the project site. SECTION 8 – CONSTRUCTION PHASING The proposed construction sequence will be as follows: 1. Contact the City of Yelm inspector to schedule the pre-construction meeting. 2. Clearly flag all limits of clearing and grading per the approved site development plans. 3. Install a temporary construction entrance as shown and per the notes and details. 4. Install temporary filter fabric (silt) fences as shown and per the notes and details. 5. Construct the temporary sediment pond as shown and per the notes and details. 6. Install the temporary interceptor swales and rock check dams as shown and per the notes and details. 7. Clear and grade site per the approved plans, stockpiling duff and topsoil per the soil preservation and amendment notes. 8. Hydroseed and/or mulch slopes and other exposed areas immediately after grading is completed as outlined in “erosion control notes”. 9. Install underground utilities (i.e. – storm drainage, water, sewer, etc.) 10. Construct the infiltration trenches per the approved site development plans. Protect all onsite storm drainage facilities until all concrete and asphalt work is complete and all exposed areas 12 are seeded and stabilized for erosion and sedimentation control or final landscaping is complete. 11. Construct asphalt paving per the approved site development plans. 12. Install inlet protection on all catch basins with grates and implement other BMPs to prevent sediment and debris from entering the stormwater facilities. 13. Clean out and test all storm drain facilities. 14. Inspect and maintain all erosion control facilities at regular intervals & complete required report. Clean as required until risk of sedimentation has passed. 15. Until all construction work which produces surface runoff are completed and all exposed ground surfaces are stabilized by vegetation or landscaping, permanent stormwater facilities may not be operated and no surface runoff may be permitted to enter the permanent storm system. Maintain interceptor swales, check dams, temporary culverts and sediment traps until all stormwater facilities have been thoroughly cleaned of sediment and debris, and inspected. SECTION 9 – CONSTRUCTION SCHEDULE Construction of this project will likely begin in the spring of 2020, and will follow the above construction sequence. During the wet season from October 1 through March 31, no soils shall remain exposed and unworked for more than 2 days at a time. SECTION 10 – FINANCIAL/OWNERSHIP RESPONSIBLITIES The property owner responsible for the initiation of any necessary bonds and/or other financial securities is: C&E DEVELOPMENTS, LLC P.O. BOX 2983 YELM, WA. 98597 PH: (360) 400-0432 SECTION 11 – ENGINEERING CALCULATIONS Design of the temporary sediment pond follows BMP C241 found in Section 3.2, Volume II, of the PCSWSDM where the surface area of the sediment pond is a function of the peak inflow from the developed 2-year, 24-hour storm event. • The 2-year, 24-hour peak inflow was calculated for the 3.281 acres of disturbed project area in the developed condition. Using MGS Flood, Version 4.46, a 2- 13 year, 24-hour peak inflow (Q2) of 0.811 cubic feet per second (cfs) was calculated. • Using the following formula: SA = 2,080 x Q2 where SA is the surface area of the temporary sediment pond measured at the top of the temporary riser pipe, the resulting surface area is: SA = 2,080 x 0.811 = 1,686.88 square feet • The dewatering orifice was sized using the equation: Ao = (As(2h)0.5) / (0.6)(3600Tg0.5) where: Ao = Orifice area (sq. ft.) As = Sediment pond surface area (sq. ft.) h = Head of water above the orifice = 1.0 feet T = dewatering time = 24 hours g = acceleration of gravity = 32.2 feet/sec2 Ao = (1686.88(2(1))0.5) / (0.6)(3600(24)(32.2)0.5) = 0.008 sf The orifice diameter, Do, is equal to (4Ao / π)0.5 = 0.101 ft = 1.21 inches Following is the MGS Flood continuous modeling for the onsite stormwater runoff in the developed condition which shows the peak inflow from the 2-year, 24-hour storm event. Sediment in the pond shall be removed when the depth of sediment reaches 1 foot. ————————————————————————————————— MGS FLOOD PROJECT REPORT Program Version: MGSFlood 4.62 Program License Number: 200810005 Project Simulation Performed on: 11/12/2024 4:13 PM Report Generation Date: 11/12/2024 4:13 PM ————————————————————————————————— Input File Name: Temp Sediment Pond Sizing.fld Project Name: C&E Commercial Analysis Title: Temp Sediment Pond Comments: ———————————————— PRECIPITATION INPUT ———————————————— Computational Time Step (Minutes): 15 Extended Precipitation Time Series Selected Full Period of Record Available used for Routing Climatic Region Number: 16 Precipitation Station : 96004405 Puget East 44 in_5min 10/01/1939-10/01/2097 14 Evaporation Station : 961044 Puget East 44 in MAP Evaporation Scale Factor : 0.750 HSPF Parameter Region Number: 1 HSPF Parameter Region Name : Ecology Default ********** Default HSPF Parameters Used (Not Modified by User) *************** ********************** WATERSHED DEFINITION *********************** Predevelopment/Post Development Tributary Area Summary Predeveloped Post Developed Total Subbasin Area (acres) 3.281 3.281 Area of Links that Include Precip/Evap (acres) 0.000 0.000 Total (acres) 3.281 3.281 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Subbasin 1 ---------- -------Area (Acres) -------- A/B, Forest, Flat 3.281 ---------------------------------------------- Subbasin Total 3.281 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 4 ---------- Subbasin : Contr. to Trench #4 ---------- -------Area (Acres) -------- ROOF TOPS/FLAT 0.153 ---------------------------------------------- Subbasin Total 0.153 ---------- Subbasin : Contr. to Trench #1 ---------- -------Area (Acres) -------- A/B, Pasture, Mod 0.671 ROADS/FLAT 0.411 ROOF TOPS/FLAT 0.256 SIDEWALKS/FLAT 0.108 ---------------------------------------------- Subbasin Total 1.446 ---------- Subbasin : Contr. to Trench #2 ---------- -------Area (Acres) -------- A/B, Pasture, Mod 0.636 ROADS/FLAT 0.523 ROOF TOPS/FLAT 0.138 SIDEWALKS/FLAT 0.071 15 ---------------------------------------------- Subbasin Total 1.368 ---------- Subbasin : Contr. to Trench #3 ---------- -------Area (Acres) -------- ROOF TOPS/FLAT 0.314 ---------------------------------------------- Subbasin Total 0.314 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ************************* LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ------------------------------------------ Link Name: New Copy Lnk2 Link Type: Copy Downstream Link: None **********************FLOOD FREQUENCY AND DURATION STATISTICS******************* ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 Number of Links: 0 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 4 Number of Links: 1 ***********Compliance Point Results ************* Scenario Predeveloped Compliance Subbasin: Subbasin 1 Scenario Postdeveloped Compliance Link: New Copy Lnk2 *** Point of Compliance Flow Frequency Data *** Recurrence Interval Computed Using Gringorten Plotting Position Predevelopment Runoff Postdevelopment Runoff Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs) ---------------------------------------------------------------------------------------------------------------------- 2-Year 2.544E-03 2-Year 0.811  Used to Size Temp. Pond 5-Year 2.624E-03 5-Year 1.046 10-Year 2.643E-03 10-Year 1.230 25-Year 9.934E-03 25-Year 1.541 50-Year 1.242E-02 50-Year 1.847 100-Year 1.800E-02 100-Year 2.299 200-Year 2.704E-02 200-Year 2.451 500-Year 3.909E-02 500-Year 2.645 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals 16 SECTION 12 – EROSION CONTROL SPECIALIST No Certified Erosion and Sediment Control Lead (CESCL) has been appointed at this time. Once one is established, he/she will be reported to the City of Yelm and WA State Department of Ecology. End of Report 17 APPENDIX “A” STORMWATER POLLUTION PREVENTION BMPs A1 burying and smothering vegetation. l Vegetative buffer zones for streams, lakes or other waterways shall be established by the local permitting authority or other state or federal permits or approvals. Maintenance Standards Inspect the area frequently to make sure flagging remains in place and the area remains undis- turbed. Replace all damaged flagging immediately. Remove all materials located in the buffer area that may impede the ability of the vegetation to act as a filter. 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 274A2 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 275A3 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 276A4 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 277A5 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 278A6 Figure II-3.1: Stabilized Construction Access 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 279A7 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 C106: Wheel Wash Purpose Wheel washes reduce the amount of sediment transported onto paved roads by washing dirt from the wheels of motor vehicles prior to the motor vehicles leaving the construction site. Conditions of Use l Use a wheel wash when BMP C105: Stabilized Construction Access is not preventing sed- iment from being tracked off site. l Wheel washing is generally an effective BMP when installed with careful attention to topo- graphy. For example, a wheel wash can be detrimental if installed at the top of a slope abut- ting a right-of-way where the water from the dripping truck can run unimpeded into the street. l Pressure washing combined with an adequately sized and surfaced pad with direct drainage to a large 10-foot x 10-foot sump can be very effective. l Wheel wash wastewater is not stormwater. It is commonly called process water, and must be discharged to a separate on-site treatment system that prevents discharge to waters of the State, or to the sanitary sewer with local sewer district approval. l Wheel washes may use closed-loop recirculation systems to conserve water use. l Wheel wash wastewater shall not include wastewater from concrete washout areas. l When practical, the wheel wash should be placed in sequence with BMP C105: Stabilized Construction Access. Locate the wheel wash such that vehicles exiting the wheel wash will enter directly onto BMP C105: Stabilized Construction Access. In order to achieve this, BMP C105: Stabilized Construction Access may need to be extended beyond the standard install- ation to meet the exit of the wheel wash. Design and Installation Specifications Suggested details are shown in Figure II-3.2: Wheel Wash. The Local Permitting Authority may allow other designs. A minimum of 6 inches of asphalt treated base (ATB) over crushed base mater- ial or 8 inches over a good subgrade is recommended to pave the wheel wash. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 280A8 Crushed rock, gravel base, etc., shall be added as required to maintain a stable driving surface and to stabilize any areas that have eroded. Following construction, these areas shall be restored to pre-construction condition or better to pre- vent future erosion. Perform street cleaning at the end of each day or more often if necessary. 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 284A9 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 285A10 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 286A11 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 287A12 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 288A13 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 289A14 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 290A15 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 291A16 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 292A17 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 298A18 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 BMP C124: Sodding Purpose The purpose of sodding is to establish turf for immediate erosion protection and to stabilize drainage paths where concentrated overland flow will occur. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 299A19 Conditions of Use Sodding may be used in the following areas: l Disturbed areas that require short-term or long-term cover. l Disturbed areas that require immediate vegetative cover. l All waterways that require vegetative lining. Waterways may also be seeded rather than sod- ded, and protected with a net or blanket. Design and Installation Specifications Sod shall be free of weeds, of uniform thickness (approximately 1-inch thick), and shall have a dense root mat for mechanical strength. The following steps are recommended for sod installation: 1. Shape and smooth the surface to final grade in accordance with the approved grading plan. Consider any areas (such as swales) that need to be overexcavated below design elevation to allow room for placing soil amendment and sod. 2. Amend 4 inches (minimum) of compost into the top 8 inches of the soil if the organic content of the soil is less than ten percent or the permeability is less than 0.6 inches per hour. See https://ecology.wa.gov/Waste-Toxics/Reducing-recycling-waste/Organic-mater- ials/Managing-organics-compost for further information. 3. Fertilize according to the sod supplier's recommendations. 4. Work lime and fertilizer 1 to 2 inches into the soil, and smooth the surface. 5. Lay strips of sod beginning at the lowest area to be sodded and perpendicular to the direction of water flow. Wedge strips securely into place. Square the ends of each strip to provide for a close, tight fit. Stagger joints at least 12 inches. Staple on slopes steeper than 3H:1V. Staple the upstream edge of each sod strip. 6. Roll the sodded area and irrigate. 7. When sodding is carried out in alternating strips or other patterns, seed the areas between the sod immediately after sodding. Maintenance Standards If the grass is unhealthy, the cause shall be determined and appropriate action taken to reestablish a healthy groundcover. If it is impossible to establish a healthy groundcover due to frequent saturation, instability, or some other cause, the sod shall be removed, the area seeded with an appropriate mix, and protected with a net or blanket. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 300A20 BMP C125: Topsoiling / Composting Purpose Topsoiling and composting provide a suitable growth medium for final site stabilization with veget- ation. While not a permanent cover practice in itself, topsoiling and composting are an integral com- ponent of providing permanent cover in those areas where there is an unsuitable soil surface for plant growth. Use this BMP in conjunction with other BMPs such as BMP C120: Temporary and Per- manent Seeding, BMP C121: Mulching, or BMP C124: Sodding. Implementation of this BMP may meet the post-construction requirements of BMP T5.13: Post-Construction Soil Quality and Depth. Native soils and disturbed soils that have been organically amended not only retain much more stormwater, but also serve as effective biofilters for urban pollutants and, by supporting more vig- orous plant growth, reduce the water, fertilizer and pesticides needed to support installed land- scapes. Topsoil does not include any subsoils but only the material from the top several inches including organic debris. Conditions of Use l Permanent landscaped areas shall contain healthy topsoil that reduces the need for fertilizers, improves overall topsoil quality, provides for better vegetative health and vitality, improves hydrologic characteristics, and reduces the need for irrigation. l Leave native soils and the duff layer undisturbed to the maximum extent practicable. Stripping of existing, properly functioning soil system and vegetation for the purpose of topsoiling during construction is not acceptable. Preserve existing soil systems in undisturbed and uncom- pacted conditions if functioning properly. l Areas that already have good topsoil, such as undisturbed areas, do not require soil amend- ments. l Restore, to the maximum extent practical, native soils disturbed during clearing and grading to a condition equal to or better than the original site condition’s moisture-holding capacity. Use on-site native topsoil, incorporate amendments into on-site soil, or import blended topsoil to meet this requirement. l Topsoiling is a required procedure when establishing vegetation on shallow soils, and soils of critically low pH (high acid) levels. l Beware of where the topsoil comes from, and what vegetation was on site before disturbance. Invasive plant seeds may be included and could cause problems for establishing native plants, landscaped areas, or grasses. l Topsoil from the site will contain mycorrhizal bacteria that are necessary for healthy root growth and nutrient transfer. These native mycorrhiza are acclimated to the site and will provide optimum conditions for establishing grasses. Use commercially available mycorrhiza products when using off-site topsoil. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 301A21 Design and Installation Specifications Meet the following requirements for disturbed areas that will be developed as lawn or landscaped areas at the completed project site: l Maximize the depth of the topsoil wherever possible to provide the maximum possible infilt- ration capacity and beneficial growth medium. Topsoil shall have: o A minimum depth of 8-inches. Scarify subsoils below the topsoil layer at least 4-inches with some incorporation of the upper material to avoid stratified layers, where feasible. Ripping or re-structuring the subgrade may also provide additional benefits regarding the overall infiltration and interflow dynamics of the soil system. o A minimum organic content of 10% dry weight in planting beds, and 5% organic matter content in turf areas. Incorporate organic amendments to a minimum 8-inch depth except where tree roots or other natural features limit the depth of incorporation. o A pH between 6.0 and 8.0 or matching the pH of the undisturbed soil. o If blended topsoil is imported, then fines should be limited to 25 percent passing through a 200 sieve. l Mulch planting beds with 2 inches of organic material l Accomplish the required organic content, depth, and pH by returning native topsoil to the site, importing topsoil of sufficient organic content, and/or incorporating organic amendments. When using the option of incorporating amendments to meet the organic content requirement, use compost that meets the compost specification for Bioretention (See BMP T7.30: Biore- tention), with the exception that the compost may have up to 35% biosolids or manure. l Sections 3 through 7 of Building Soil: Guidelines and Resources for Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater Management Manual for Western Washington (Stenn et al., 2016), provides useful guidance for implementing whichever option is chosen. It includes guidance for pre-approved default strategies and guidance for custom strategies. Check with your local jurisdiction concerning its acceptance of this guidance. l The final composition and construction of the soil system will result in a natural selection or favoring of certain plant species over time. For example, incorporation of topsoil may favor grasses, while layering with mildly acidic, high-carbon amendments may favor more woody vegetation. l Allow sufficient time in scheduling for topsoil spreading prior to seeding, sodding, or planting. l Take care when applying top soil to subsoils with contrasting textures. Sandy topsoil over clayey subsoil is a particularly poor combination, as water creeps along the junction between the soil layers and causes the topsoil to slough. If topsoil and subsoil are not properly bonded, water will not infiltrate the soil profile evenly and it will be difficult to establish vegetation. The best method to promote bonding is to actually work the topsoil into the layer below for a depth of at least 6 inches. l Field exploration of the site shall be made to determine if there is surface soil of sufficient quantity and quality to justify stripping. Topsoil shall be friable and loamy (loam, sandy loam, 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 302A22 silt loam, sandy clay loam, and clay loam). Avoid areas of natural ground water recharge. l Stripping shall be confined to the immediate construction area. A 4-inch to 6-inch stripping depth is common, but depth may vary depending on the particular soil. All surface runoff con- trol structures shall be in place prior to stripping. l Do not place topsoil while in a frozen or muddy condition, when the subgrade is excessively wet, or when conditions exist that may otherwise be detrimental to proper grading or pro- posed sodding or seeding. l In any areas requiring grading, remove and stockpile the duff layer and topsoil on site in a des- ignated, controlled area, not adjacent to public resources and critical areas. Reapply stock- piled topsoil to other portions of the site where feasible. l Locate the topsoil stockpile so that it meets specifications and does not interfere with work on the site. It may be possible to locate more than one pile in proximity to areas where topsoil will be used. l Stockpiling of topsoil shall occur in the following manner: o Side slopes of the stockpile shall not exceed 2H:1V. o Between October 1 and April 30: n An interceptor dike with gravel outlet and silt fence shall surround all topsoil. n Within 2 days complete erosion control seeding, or covering stockpiles with clear plastic, or other mulching materials. o Between May 1 and September 30: n An interceptor dike with gravel outlet and silt fence shall surround all topsoil if the stockpile will remain in place for a longer period of time than active construction grading. n Within 7 days complete erosion control seeding, or covering stockpiles with clear plastic, or other mulching materials. l When native topsoil is to be stockpiled and reused the following should apply to ensure that the mycorrhizal bacterial, earthworms, and other beneficial organisms will not be destroyed: o Re-install topsoil within 4 to 6 weeks. o Do not allow the saturation of topsoil with water. o Do not use plastic covering. Maintenance Standards l Inspect stockpiles regularly, especially after large storm events. Stabilize any areas that have eroded. l Establish soil quality and depth toward the end of construction and once established, protect from compaction, such as from large machinery use, and from erosion. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 303A23 l Plant and mulch soil after installation. l Leave plant debris or its equivalent on the soil surface to replenish organic matter. l Reduce and adjust, where possible, the use of irrigation, fertilizers, herbicides and pesticides, rather than continuing to implement formerly established practices. BMP C126: Polyacrylamide (PAM) for Soil Erosion Protection Purpose Polyacrylamide (PAM) is used on construction sites to prevent soil erosion. Applying PAM to bare soil in advance of a rain event significantly reduces erosion and controls sed- iment in two ways. First, PAM increases the soil’s available pore volume, thus increasing infiltration and reducing the quantity of stormwater runoff. Second, it increases flocculation of suspended particles and aids in their deposition, thus reducing stormwater runoff turbidity and improving water quality. Conditions of Use PAM shall not be directly applied to water or allowed to enter a water body. Stormwater runoff shall pass through a sediment pond prior to discharging to surface waters. PAM can be applied to bare soil under the following conditions: l During rough grading operations. l In Staging areas. l Balanced cut and fill earthwork. l Haul roads prior to placement of crushed rock surfacing. l Compacted soil roadbase. l Stockpiles. l After final grade and before paving or final seeding and planting. l Pit sites. l Sites having a winter shut down. In the case of winter shut down, or where soil will remain unworked for several months, PAM should be used together with mulch. Design and Installation Specifications l Do not use PAM on a slope that flows directly into a stream or wetland. l Do not add PAM to water discharging from the site. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 304A24 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 313A25 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 314A26 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 315A27 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 316A28 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 317A29 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 318A30 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 319A31 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 320A32 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 327A33 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 328A34 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 329A35 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 330A36 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 331A37 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 351A38 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 352A39 Figure II-3.16: Rock Check Dam 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 353A40 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 356A41 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 357A42 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 358A43 Figure II-3.17: Block and Gravel Filter 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 359A44 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 360A45 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 361A46 Figure II-3.18: Block and Gravel Curb Inlet Protection 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 362A47 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 363A48 Figure II-3.19: Curb and Gutter Barrier 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 364A49 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 BMP C231: Brush Barrier Purpose The purpose of brush barriers is to reduce 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 l Brush barriers may be used downslope of disturbed areas that are less than one-quarter acre. l Brush barriers are not intended to treat concentrated flows, nor are they intended to treat sub- stantial amounts of overland flow. Any concentrated flows must be directed to a sediment trap- ping BMP. The only circumstance in which overland flow can be treated solely by a brush barrier, rather than by a sediment trapping BMP, is when the area draining to the barrier is small. l Brush barriers should only be installed on contours. Design and Installation Specifications l Height: 2 feet (minimum) to 5 feet (maximum). l Width: 5 feet at base (minimum) to 15 feet (maximum). l Filter fabric (geotextile) may be anchored over the brush berm to enhance the filtration ability of the barrier. Ten-ounce burlap is an adequate alternative to filter fabric. 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 365A50 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 370A51 Figure II-3.22: Silt Fence 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 371A52 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 372A53 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 373A54 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 374A55 Figure II-3.23: Silt Fence Installation by Slicing Method 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 375A56 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. BMP C234: Vegetated Strip Purpose Vegetated strips reduce the transport of coarse sediment from a construction site by providing a physical barrier to sediment and reducing the runoff velocities of overland flow. Conditions of Use l Vegetated strips may be used downslope of all disturbed areas. l Vegetated strips are not intended to treat concentrated flows, nor are they intended to treat substantial amounts of overland flow. Any concentrated flows must be conveyed through the drainage system to BMP C241: Sediment Pond (Temporary) or other sediment trapping BMP. The only circumstance in which overland flow can be treated solely by a vegetated strip, rather than by a sediment trapping BMP, is when the following criteria are met (see Table II- 3.12: Contributing Drainage Area for Vegetated Strips): Average Contributing Area Slope Average Contributing Area Per- cent Slope Max Contributing area Flowpath Length 1.5H : 1V or flatter 67% or flatter 100 feet 2H : 1V or flatter 50% or flatter 115 feet 4H : 1V or flatter 25% or flatter 150 feet 6H : 1V or flatter 16.7% or flatter 200 feet 10H : 1V or flatter 10% or flatter 250 feet Table II-3.12: Contributing Drainage Area for Vegetated Strips 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 376A57 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 388A58 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 389A59 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 390A60 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 391A61 Figure II-3.28: Sediment Pond Plan View 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 392A62 Figure II-3.29: Sediment Pond Cross Section 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 393A63 Figure II-3.30: Sediment Pond Riser Detail 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 394A64 Figure II-3.31: Riser Inflow Curves 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 395A65 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. BMP C250: Construction Stormwater Chemical Treatment Purpose This BMP applies when using chemicals to treat turbidity in stormwater by either batch or flow- through chemical treatment. Turbidity is difficult to control once fine particles are suspended in stormwater runoff from a con- struction site. BMP C241: Sediment Pond (Temporary) is effective at removing larger particulate matter by gravity settling, but is ineffective at removing smaller particulates such as clay and fine silt. Traditional Construction Stormwater BMPs may not be adequate to ensure compliance with the water quality standards for turbidity in the receiving water. Chemical treatment can reliably provide exceptional reductions of turbidity and associated pol- lutants. Chemical treatment may be required to meet turbidity stormwater discharge requirements, especially when construction proceeds through the wet season. Conditions of Use Formal written approval from Ecology is required for the use of chemical treatment, regardless of site size. See https://fortress.wa.gov/ecy/publications/SummaryPages/ecy070258.html for a copy of the Request for Chemical Treatment form. The Local Permitting Authority may also require review and approval. When authorized, the chemical treatment systems must be included in the Con- struction Stormwater Pollution Prevention Plan (SWPPP). Chemically treated stormwater discharged from construction sites must be nontoxic to aquatic organ- isms. The Chemical Technology Assessment Protocol - Ecology (CTAPE) must be used to evaluate chemicals proposed for stormwater treatment. Only chemicals approved by Ecology under the CTAPE may be used for stormwater treatment. The approved chemicals, their allowable application techniques (batch treatment or flow-through treatment), allowable application rates, and conditions of use can be found at the Department of Ecology Emerging Technologies website: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee- guidance-resources/Emerging-stormwater-treatment-technologies Background on Chemical Treatment Systems Coagulation and flocculation have been used for over a century to treat water. The use of coagu- lation and flocculation to treat stormwater is a very recent application. Experience with the treatment of water and wastewater has resulted in a basic understanding of the process, in particular factors 2019 Stormwater Management Manual for Western Washington Volume II -Chapter 3 -Page 396A66