2020.0106.CI0005 SWPPP (Approved) 7.29.2020CONSTRUCTION STORMWATERPOLLUTION PREVENTIOI{ PLANFORWyndstoneYelm, \tashingtonJune 2020Prepared for:C&El)evelopments,LLCPO Box 2983Yelm, WA 98597Prepared by:Daniel P. Smith, PE, Project Managerf,,,rApproved By:Craig Deaver, PrincipalREPORT #06164.0"I hereby state that this Drainage and Erosion/Sediment Control Plan for Yyndsto{re ha¡ b9e.npr.pured by me or under my suþervision and meets the standard of care and expertise which isusuäl and 'customary in this community of professional engineers. I understand that City ofYelm does not and-will not assume liability for the suffìciency, suitability or performance ofdrainage facilities prepared by me."This analysis is based on data and records either supplied to, or obtained by, C.E.S. N'W, Inc.These doðuments are referenced within the text of the analysis. The analysis has been preparedutilizing procedures and practices within the standard accepted practices of the industry.1Pe
TABLE OF CONTENTS
PAGE
CONSTRUCTION STORMWATER POLLUTION PREVENTION .................................................................... 1
1. PROPOSED PROJECT DESCRIPTION ...................................................................................................................... 1
2. EXISTING SITE CONDITIONS ............................................................................................................................... 2
3. ADJACENT AREAS .............................................................................................................................................. 2
4. CRITICAL AREAS ................................................................................................................................................ 2
5. SOIL .................................................................................................................................................................... 2
6. POTENTIAL EROSION PROBLEM AREAS .............................................................................................................. 2
7. CONSTRUCTION PHASING ................................................................................................................................... 3
8. CONSTRUCTION SCHEDULE ................................................................................................................................ 4
9. THIRTEEN ELEMENTS ......................................................................................................................................... 4
10. FINANCIAL/OWNERSHIP RESPONSIBILITIES .................................................................................................... 6
11. ENGINEERING CALCULATIONS ....................................................................................................................... 6
Appendix A Maps
Vicinity Map .........................................................................................................................A-1
Soils Map and Descriptions ..................................................................................................A-2
FIRM Panel 53053C0604E ...................................................................................................A-3
Appendix B Construction Best Management Practices (BMPs) ..........................................................B-1
Appendix C Geotechnical Engineer’s Report ........................................................................................C-1
1
CONSTRUCTION STORMWATER POLLUTION PREVENTION
1. Proposed Project Description
This report accompanies the final engineering plans for the Wyndstone project as submitted to
the City of Yelm for site plan review. Pursuant to City of Yelm Municipal Code (YMC)
13.16.060 the methodology and design criteria for the project are established by the Department
of Ecology’s 2014 Stormwater Management Manual (Manual).
The Wyndstone project consists of 75 multifamily units across four building situated on parcel #
21724420300 totaling approximately 4.67 acres. The proposed project is made up of a
rectangular shaped parcel of land located at the intersection of Tahoma Blvd and Durant Street,
Yelm, Washington. The site is currently surrounded by Tahoma Blvd to the north, single family
homes and Durant Street to the west, and vacant parcels to the south and east. A vicinity map is
provided in Appendix “A” for reference.
Land Use Application – Site Plan Review
Address – 15025 Tahoma Blvd SE Yelm, WA 98597
Parcel Numbers – 21724420300
Zoning – R16-High Density
Legal Description – Parcel No. 21724420300; 15025 Tahoma Blvd. SE parcel a of City of
Yelm boundary line adjustment no. BLA 140153 YL as recorded July 18, 2014 under
Auditor's File No. 4400621. In Thurston County, Washington.
The Wyndstone project is proposed to be constructed in two separate phases with Buildings 1
and 2 and the site improvements constructed in Phase I, and Buildings 3 and 4 and utility
connects constructed in Phase II. A new public roadway extension from Tahoma Blvd SE is
proposed as part of Phase I and will extend the full length of the eastern boundary line. An
infiltration trench is proposed to fully infiltrate runoff from both phases with a FloGard Perk
Filter vault upstream that provides basic runoff treatment.
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2. Existing Site Conditions
The proposed project is comprised of one 4.67-acre parcel that is predominantly pasture and
brush with several scattered trees. Durant Street SE, an existing roadway, runs along the west
boundary of the site. The site is relatively flat sloping north towards Tahoma Blvd SE with
slopes between 2 – 10%.
According to FIRM Panel 53067C0353E the site is located within Zone X. This zone is
considered outside of a known flood plain. A copy of the FIRM Panel 53067C0353E can be
found within Appendix “B”.
3. Adjacent Areas
The site is located south of Tahoma Blvd SE, north of a vacant parcel, east of Tract B of Tahoma
Terra Phase 1, Division 1, and west of a single-family residence. Generally, the project site is
surrounded by single family residences. Construction is limited to parcel 21724420300 and its
frontage with Tahoma Blvd SE. Adjacent areas are protected from sediment with the erosion
control BMPs shown on the approved plans and described within this report.
4. Critical Areas
There are no known critical areas on site or in the immediate vicinity.
5. Soil
According to the Soil Survey of Thurston County, Washington, prepared by the United States
Department of Agriculture, the site’s soils is composed of Nisqually loamy fine sand (74) and
Spanaway gravelly sandy loam (110 & 111), which are a Type A soils having low erosion
potential, and high infiltration potential. A description of these soils and a copy of the soil map
for this project site is included within Appendix “A”. A geotechnical engineer’s report and
subsequent memo, dated July 12, 2019 and July 26, 2019 respectively, has been prepared by
Insight Geologic Inc. In the July 12th report they discovered that the site is underline by
recessional outwash sands and gravels to the bottom of the monitoring well (MW-1) at a depth of
31.5-feet. A copy of their report is provided in Appendix “C”.
3
6. Potential Erosion Problem Areas
There are no steep slopes on site. The project will not experience problems with erosion if the
BMPs described within this report and on the approved plans are implemented.
7. Construction Phasing
The proposed improvements include an erosion/sedimentation control plan designed to prevent
sediment-laden runoff from leaving the project site during construction. The design specifies a
combination of structural measures, cover measures and construction practices that are to be
implemented to maintain erosion control. Prior to the start of any clearing and grading of the site,
all erosion control measures should be constructed.
A general outline of the proposed construction sequence has been included. The contractor will
employ the best construction practices to properly clear and grade the site. The planned
construction sequence is as follows:
Construction Sequence
1. Arrange and attend pre-construction conference with the City of Yelm.
2. Stake clearing and grubbing limits.
3. Install filter fabric fence and construction entrance.
4. Flag infiltration area limit compaction and sediment laden runoff from this area.
5. Install/implement erosion control features.
6. Construct temporary sediment control pond per plan.
7. Clear and grub site as necessary to install site improvements. During wet season, do
not clear any more area than can be stabilized, per the grading and erosion control
plan, in a given workday. Do not compact infiltration areas.
8. Grade site per grading plan.
9. Install storm system per plans. Install inlet protection for new catch basins.
10. Install force main and step system tanks.
11. Install water main and fire suppression lines.
12. Construct public roadway and pave onsite drive areas in phase 1.
13. Install infiltration trench & Perkfilter vault. Do not place infiltration trench online
until the site is fully stabilized.
4
14. Stabilize unpaved areas of the site by hydro-seeding or other appropriate methods per
the erosion control notes.
15. Once the site has been fully stabilized place infiltration facilities online.
16. Remove all temporary erosion control facilities after site has been permanently
stabilized and approved by the city.
17. Clean storm drainage system piping and catch basins.
18. Call for final inspection(s).
8. Construction Schedule
Construction that exposes soils should be limited during the wet season (October 1 to April 30).
During this time erosion control BMPs should be checked regularly (once a week) and after each
major storm event. Generally, grading, utility installation, and paving should be completed prior
to the wet season.
9. Thirteen Elements
9.1 Mark Clearing Limits
The project proposes to clear areas onsite. Clearing limits are to be staked by a professional land
surveyor as shown on the approved plans. Clearing shall remain within these limits.
9.2 Establish Construction Access
A stabilized construction entrance (BMP C105) is proposed to protect Tahoma Blvd SE from
sediment. Adjacent paved surfaces must be cleaned daily, or if deemed necessary, more
frequently.
9.3 Control Flow Rates
The project will clear approximately 4.28 acres to construct the site improvements. The project
will mitigate runoff with cover measures (BMP C120 and C121), silt fences (BMP C233)
interceptor swales (BMP C200), check dams (BMP C207), and a temporary sediment pond
(BMP C241).
5
9.4 Install Sediment Controls
The project proposes silt fences (BMP C233) and interceptor swales (BMP C200) around the
perimeter of the site to trap sediment onsite.
9.5 Stabilize Soils
The project will stabilize exposed soils with the use of cover measures. These cover measures are
mulching, temporary seeding, and plastic sheeting (BMP C120, C121, C123).
9.6 Protect Slopes
Just like stabilizing the exposed soils the project’s exposed slopes will be controlled with the
same covering measures (BMP C120, C121, and BMP C123).
9.7 Protect Drain Inlets
Existing offsite drain inlets and proposed drain inlets will be protected from sediment with the
use of bag filters (BMP C220).
9.8 Stabilize Channels and Outlets
The project does not construct or modify channel and outlets therefore this element does not
apply to this project.
9.9 Control Pollutants
The project will require earth moving equipment. When vehicles are stored onsite care needs to
be taken to make sure that any fluid leaks are contained with drip pans and the fluids are
disposed of properly. All spills need to be cleaned up immediately as per the Department of
Ecology (ECY) and City of Yelm’s Standards.
9.10 Control Dewatering
This project proposal includes temporary dewatering with the use of a temporary swales (BMP
C200), check dams (BMP C207) and a temporary sediment pond (BMP C241).
9.11 Maintain BMPs
The proposed BMPs need to be maintained as per the approved plans notes and specifications. In
general, when sediment accumulation has reached 1/3 of the treatment device or one (1) foot of
depth it should be removed. Also, if there is a major storm event then the proposed BMPs should
6
be check and cleaned appropriately. If the sediment removed from these devices is approved by a
geotechnical engineer, they can be stabilized onsite. If not, they must be removed as per the ECY
and the City of Yelm’s requirements.
9.12 Manage the Project
A construction sequence is provided on the plans. This construction sequence needs to be
followed to ensure that sediment is not deposited downstream. The City and the Project Engineer
needs to inspect the erosion control BMPs after installation and during construction. The
contractor is to employ a Certified Erosion and Sediment Control Lead (CESL, BMP C160) as
described by the State to help manage and inspect the erosion control devices. Detailed
descriptions of each BMP listed above can be found in Appendix “B” of this report.
9.13 Protect Low Impact Development BMPs.
The project proposes an infiltration trench to control its runoff. Care should be taken to prevent
sediment laden runoff from entering the trench or the surrounding soils during construction.
Construction equipment and staging should be limited where the trench is proposed onsite.
10. Financial/Ownership Responsibilities
The owner and responsible party for the initiation of financial securities is C & E Developments,
LLC. Their contact information is as follows:
C&E Developments, LLC
Po Box 2983,
Yelm, WA 98597
11. Engineering Calculations
The project proposes a temporary sediment pond to mitigate runoff from the site during
construction. The project delineates the site into a single storm basin and the temporary sediment
pond is sized per BMP C241 of Vol. II of the SWWMM, 2014. To protect the downstream
Puyallup river, the sediment pond is sized to treat the 2-year storm event from the site as
calculated by the WWHM computer program. The following is the sediment pond sizing
calculations:
7
Sediment Pond Sizing Calculations
Required Surface Area
SA = 2080 *Q2
where;
Q2 = 1.16-cfs
SA = 2080 *1.16-cfs = 2,413-sq.ft. required; 2,700-sq.ft. provided
Dewatering Orifice
Ao = [As(2h)0.5]/[0.6x3600Tg0.5]
D = 13.54*(Ao) 0.5
where;
Ao = orifice area (square feet)
As = pond surface area = 2,700-sq.ft.
h = head of water above orifice from Figure 4.2.21 (height of riser in feet) = 0.30-feet
T = dewatering time (24 hours)
g = acceleration of gravity (32.2 feet/second2)
Ao = [2,700-sq.ft.*(2*0.30-ft)0.5]/[0.6*3600*24-hr*(32.2ft/s2)0.5] = 0.0071-sq.ft.
D = 13.54*(0.0071-sq.ft.) 0.5 = 1.142-inches; therefore, use 1.125-inches provided
8
APPENDIX A
MAPS
Vicinity Map A-1
Soils Map and Descriptions A-2
FIRM Panel 53067C0353E A-3
Soil Map—Thurston County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/13/2019
Page 1 of 351989405198960519898051990005199020519904051990605199080519910051991205199140519894051989605198980519900051990205199040519906051990805199100519912051991405199160528720528740528760528780528800528820528840528860528880
528720 528740 528760 528780 528800 528820 528840 528860 528880
46° 56' 43'' N 122° 37' 21'' W46° 56' 43'' N122° 37' 13'' W46° 56' 35'' N
122° 37' 21'' W46° 56' 35'' N
122° 37' 13'' WN
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84
0 50 100 200 300
Feet
0 15 30 60 90
Meters
Map Scale: 1:1,090 if printed on A portrait (8.5" x 11") sheet.
Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Map Unit Polygons
Soil Map Unit Lines
Soil Map Unit Points
Special Point Features
Blowout
Borrow Pit
Clay Spot
Closed Depression
Gravel Pit
Gravelly Spot
Landfill
Lava Flow
Marsh or swamp
Mine or Quarry
Miscellaneous Water
Perennial Water
Rock Outcrop
Saline Spot
Sandy Spot
Severely Eroded Spot
Sinkhole
Slide or Slip
Sodic Spot
Spoil Area
Stony Spot
Very Stony Spot
Wet Spot
Other
Special Line Features
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Thurston County Area, Washington
Survey Area Data: Version 13, Sep 16, 2019
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Mar 29, 2016—Oct
10, 2016
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Soil Map—Thurston County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/13/2019
Page 2 of 3
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
74 Nisqually loamy fine sand, 3 to
15 percent slopes
0.7 15.1%
110 Spanaway gravelly sandy
loam, 0 to 3 percent slopes
3.3 75.3%
111 Spanaway gravelly sandy
loam, 3 to 15 percent slopes
0.4 9.5%
Totals for Area of Interest 4.4 100.0%
Soil Map—Thurston County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/13/2019
Page 3 of 3
AREA OF MINIMAL FLOOD HAZARD Zone XT17N R1E S24
CITY OF YELM530310
USGS The National Map: Orthoimagery. Data refreshed April, 2019.
National Flood Hazard Layer FIRMette
0 500 1,000 1,500 2,000250Feet
Ü122°37'35.01"W 46°56'49.68"N 122°36'57.56"W 46°56'25.12"N
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOODHAZARD AR EAS
Without Base Flood Elevation (BFE)Zone A, V, A99With BFE or Depth Zone AE, AO, AH, VE, AR
Regulator y Floodway
0.2% Annual Chance Flood Hazard, Areasof 1% annual chance flood with averagedepth less than one foot or with drainageareas of less than one square mile Zone X
Future Conditions 1% AnnualChance Flood Hazard Zone XArea with Reduced Flood Risk due toLevee. See Notes.Zone X
Area with Flood Risk due to Levee Zone D
NO SCREE N Area of Minimal Flood Hazard Zone X
Area of Undetermined Flood Hazard Zone D
Channel, Culver t, or Storm SewerLevee, Dike, or Floodwall
Cross Sections with 1% Annual Chance17.5 Water Surface ElevationCoastal Transect
Coastal Transect BaselineProfile BaselineHydrographic Feature
Base Flood Elevation Line (BFE)
Effective LOMRs
Limit of StudyJurisdiction Boundar y
Digital Data Available
No Digital Data Available
Unmapped
This map complies with FEMA's standards for the use of digital flood maps if it is not void as described below. The basemap shown complies with FEMA's basemap accuracy standards
The flood hazard information is derived directly from theauthoritative NFHL web ser vices provided by FEMA. This mapwas exported on 11/13/2019 at 11:36:26 PM and does notreflect changes or amendments subsequent to this date andtime. The NFHL and effective information may change orbecome superseded by new data over time.
This map image is void if the one or more of the following mapelements do not appear: basemap imagery, flood zone labels,legend, scale bar, map creation date, community identifiers,FIRM panel number, and FIRM effective date. Map images forunmapped and unmodernized areas cannot be used forregulatory purposes.
Legend
OTHER AREAS OFFLOOD HAZARD
OTHER AREAS
GENERALSTRUCTURES
OTHERFEATURES
MAP PANELS
8
1:6,000
B 20.2
The pin displayed on the map is an approximate point selected by the user and does not represent an authoritative proper ty location.
9
APPENDIX B
Construction Best Management Practices (BMPs) B-1
BMP C105: Stabilized Construction Entrance
Purpose Construction entrances are stabilized to reduce the amount of sediment
transported onto paved roads by vehicles or equipment by constructing a
stabilized pad of quarry spalls at entrances to construction sites.
Conditions of Use Construction entrances shall be stabilized wherever traffic will be leaving
a construction site and traveling on paved roads or other paved areas
within 1,000 feet of the site.
On large commercial, highway, and road projects, the designer should
include enough extra materials in the contract to allow for additional
stabilized entrances 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 4.2 for details. Note: the 100’ minimum length of the
entrance shall be reduced to the maximum practicable size when the
size or configuration of the site does not allow the full length (100’).
• 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 following standards:
Grab Tensile Strength (ASTM D4751) 200 psi min.
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)
• Consider early installation of the first lift of asphalt in areas that will
paved; this can be used as a stabilized entrance. Also consider the
installation of excess concrete as a stabilized entrance. During large
concrete pours, excess concrete is often available for this purpose.
• Hog fuel (wood-based mulch) may be substituted for or combined with
quarry spalls in areas that will not be used for permanent roads. Hog
fuel is generally less effective at stabilizing construction entrances and
should be used only at sites where the amount of traffic is very limited.
Hog fuel is not recommended for entrance stabilization in urban areas.
The effectiveness of hog fuel is highly variable and it generally
requires more maintenance than quarry spalls. The inspector may at
any time require the use of quarry spalls if the hog fuel is not
preventing sediment from being tracked onto pavement or if the hog
fuel is being carried onto pavement. Hog fuel is prohibited in
permanent roadbeds because organics in the subgrade soils cause
degradation of the subgrade support over time.
• Fencing (see BMPs C103 and C104) shall be installed as necessary to
restrict traffic to the construction entrance.
4-8 Volume II – Construction Stormwater Pollution Prevention February 2005
• Whenever possible, the entrance shall be constructed on a firm,
compacted subgrade. This can substantially increase the effectiveness
of the pad and reduce the need for maintenance.
Maintenance
Standards
• Quarry spalls (or hog fuel) shall be added if the pad is no longer in
accordance with the specifications.
• If the entrance 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 street sweeping, an increase
in the dimensions of the entrance, or the installation of a wheel wash.
• Any sediment that is tracked onto pavement shall be removed by
shoveling or street sweeping. 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 sweeping is
ineffective and there is a threat to public safety. If it is necessary to
wash the streets, the construction of a small sump shall be considered.
The sediment would then be washed into the sump where it can be
controlled.
• Any quarry spalls that are loosened from the pad, which end up on the
roadway shall be removed immediately.
• If vehicles are entering or exiting the site at points other than the
construction entrance(s), fencing (see BMPs C103 and C104) shall be
installed to control traffic.
• Upon project completion and site stabilization, all construction
accesses intended as permanent access for maintenance shall be
permanently stabilized.
Figure 4.2 – Stabilized Construction Entrance 15’ min.February 2005 Volume II – Construction Stormwater Pollution Prevention 4-9
BMP C120: Temporary and Permanent Seeding
Purpose Seeding is intended to reduce erosion by stabilizing exposed soils. A
well-established vegetative cover is one of the most effective methods of
reducing erosion.
Conditions of Use • Seeding may be used throughout the project on disturbed areas that
have reached final grade or that will remain unworked for more than
30 days.
• Channels that will be vegetated should be installed before major
earthwork and hydroseeded with a Bonded Fiber Matrix. The
vegetation should be well established (i.e., 75 percent cover) before
water is allowed to flow in the ditch. With channels that will have
high flows, erosion control blankets should be installed over the
hydroseed. If vegetation cannot be established from seed before water
is allowed in the ditch, sod should be installed in the bottom of the
ditch over hydromulch and blankets.
• Retention/detention ponds should be seeded as required.
• Mulch is required at all times because it protects seeds from heat,
moisture loss, and transport due to runoff.
• All disturbed areas shall be reviewed in late August to early September
and all seeding should be completed by the end of September.
Otherwise, vegetation will not establish itself enough to provide more
than average protection.
• At final site stabilization, all disturbed areas not otherwise vegetated or
stabilized shall be seeded and mulched. Final stabilization 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.
Design and
Installation
Specifications
• Seeding should be done during those seasons most conducive to
growth and will vary with the climate conditions of the region.
Local experience should be used to determine the appropriate
seeding periods.
• The optimum seeding windows for western Washington are April 1
through June 30 and September 1 through October 1. Seeding that
occurs between July 1 and August 30 will require irrigation until 75
percent grass cover is established. Seeding that occurs between
October 1 and March 30 will require a mulch or plastic cover until
75 percent grass cover is established.
• To prevent seed from being washed away, confirm that all required
surface water control measures have been installed.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-13
• The seedbed should be firm and rough. All soil should be roughened
no matter what the slope. If compaction is required for engineering
purposes, slopes must be track walked before seeding. Backblading or
smoothing of slopes greater than 4:1 is not allowed if they are to be
seeded.
• New and more effective restoration-based landscape practices rely on
deeper incorporation than that provided by a simple single-pass
rototilling treatment. Wherever practical the subgrade should be
initially ripped to improve long-term permeability, infiltration, and
water inflow qualities. At a minimum, permanent areas shall use soil
amendments to achieve organic matter and permeability performance
defined in engineered soil/landscape systems. For systems that are
deeper than 8 inches the rototilling process should be done in multiple
lifts, or the prepared soil system shall be prepared properly and then
placed to achieve the specified depth.
• Organic matter is the most appropriate form of “fertilizer” because it
provides nutrients (including nitrogen, phosphorus, and potassium) in
the least water-soluble form. A natural system typically releases 2-10
percent of its nutrients annually. Chemical fertilizers have since been
formulated to simulate what organic matter does naturally.
• In general, 10-4-6 N-P-K (nitrogen-phosphorus-potassium) fertilizer
can be used at a rate of 90 pounds per acre. Slow-release fertilizers
should always be used because they are more efficient and have fewer
environmental impacts. It is recommended that areas being seeded for
final landscaping conduct soil tests to determine the exact type and
quantity of fertilizer needed. This will prevent the over-application of
fertilizer. Fertilizer should not be added to the hydromulch machine
and agitated more than 20 minutes before it is to be used. If agitated
too much, the slow-release coating is destroyed.
• There are numerous products available on the market 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 is a good source of
long-term, slow-release, available nitrogen.
• Hydroseed applications shall include a minimum of 1,500 pounds per
acre of mulch with 3 percent tackifier. Mulch may be made up of 100
percent: cottonseed meal; fibers made of wood, recycled cellulose,
hemp, and kenaf; compost; or blends of these. Tackifier shall be plant-
based, such as guar or alpha plantago, or chemical-based such as
polyacrylamide or polymers. Any mulch or tackifier product used
shall be installed per manufacturer’s instructions. Generally, mulches
come in 40-50 pound bags. Seed and fertilizer are added at time of
application.
4-14 Volume II – Construction Stormwater Pollution Prevention February 2005
• Mulch is always required for seeding. Mulch can be applied on top of
the seed or simultaneously by hydroseeding.
• On steep slopes, Bonded Fiber Matrix (BFM) or Mechanically Bonded
Fiber Matrix (MBFM) products should be used. BFM/MBFM
products are applied at a minimum rate of 3,000 pounds per acre of
mulch with approximately 10 percent tackifier. Application is made
so that a minimum of 95 percent soil coverage is achieved. Numerous
products are available commercially and should be installed per
manufacturer’s instructions. Most products require 24-36 hours to
cure before a rainfall and cannot be installed on wet or saturated soils.
Generally, these products come in 40-50 pound bags and include all
necessary ingredients except for seed and fertilizer.
BFMs and MBFMs have some advantages over blankets:
• No surface preparation required;
• Can be installed via helicopter in remote areas;
• On slopes steeper than 2.5:1, blanket installers may need to be roped
and harnessed for safety;
• They are at least $1,000 per acre cheaper installed.
In most cases, the shear strength of blankets is not a factor when used on
slopes, only when used in channels. BFMs and MBFMs are good
alternatives to blankets in most situations where vegetation establishment
is the goal.
• 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. One way to
overcome this is to increase seed quantities by up to 50 percent.
• Vegetation establishment can also be enhanced by dividing the
hydromulch operation into two phases:
1. Phase 1- Install all seed and fertilizer with 25-30 percent mulch
and tackifier onto soil in the first lift;
2. Phase 2- Install the rest of the mulch and tackifier over the first lift.
An alternative is to install the mulch, seed, fertilizer, and tackifier in one
lift. Then, spread or blow straw over the top of the hydromulch at a rate of
about 800-1000 pounds per acre. 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:
1. Irrigation
2. Reapplication of mulch
3. Repair of failed slope surfaces
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-15
This technique works with standard hydromulch (1,500 pounds per acre
minimum) and BFM/MBFMs (3,000 pounds per acre minimum).
• Areas to be permanently landscaped shall provide a healthy topsoil
that reduces the need for fertilizers, improves overall topsoil quality,
provides for better vegetal health and vitality, improves hydrologic
characteristics, and reduces the need for irrigation. This can be
accomplished in a number of ways:
Recent research has shown that the best method to improve till soils is
to amend these soils with compost. The optimum mixture is
approximately two parts soil to one part compost. This equates to 4
inches of compost mixed to a depth of 12 inches in till soils. Increasing
the concentration of compost beyond this level can have negative
effects on vegetal health, while decreasing the concentrations can
reduce the benefits of amended soils. Please note: The compost should
meet specifications for Grade A quality compost in Ecology
Publication 94-038.
Other soils, such as gravel or cobble outwash soils, may require
different approaches. Organics and fines easily migrate through the
loose structure of these soils. Therefore, the importation of at least 6
inches of quality topsoil, underlain by some type of filter fabric to
prevent the migration of fines, may be more appropriate for these soils.
Areas that already have good topsoil, such as undisturbed areas, do not
require soil amendments.
• Areas that will be seeded only and not landscaped may need compost
or meal-based mulch included in the hydroseed in order to establish
vegetation. Native topsoil should be re-installed on the disturbed soil
surface before application.
• Seed that is installed as a temporary measure may be installed by hand
if it will be covered by straw, mulch, or topsoil. Seed that is installed
as a permanent measure may be installed by hand on small areas
(usually less than 1 acre) that will be covered with mulch, topsoil, or
erosion blankets. The seed mixes listed below include recommended
mixes for both temporary and permanent seeding. These mixes, with
the exception of the wetland mix, shall be applied at a rate of 120
pounds per acre. This rate can be reduced if soil amendments or slow-
release fertilizers are used. Local suppliers or the local conservation
district should be consulted for their recommendations because 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.
4-16 Volume II – Construction Stormwater Pollution Prevention February 2005
Table 4.1 represents the standard mix for those areas where just a
temporary vegetative cover is required.
Table 4.1
Temporary Erosion Control Seed Mix
% Weight % Purity % Germination
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
Table 4.2 provides just one recommended possibility for landscaping seed.
Table 4.2
Landscaping Seed Mix
% Weight % Purity % Germination
Perennial rye blend
Lolium perenne
70 98 90
Chewings and red fescue blend
Festuca rubra var. commutata
or Festuca rubra
30 98 90
This turf seed mix in Table 4.3 is for dry situations where there is no need
for much water. The advantage is that this mix requires very little
maintenance.
Table 4.3
Low-Growing Turf Seed Mix
% Weight % Purity % Germination
Dwarf tall fescue (several varieties)
Festuca arundinacea 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
Table 4.4 presents a mix recommended for bioswales and other
intermittently wet areas.
Table 4.4
Bioswale Seed Mix*
% Weight % Purity % Germination
Tall or meadow fescue
Festuca arundinacea or Festuca elatior
75-80 98 90
Seaside/Creeping bentgrass
Agrostis palustris
10-15 92 85
Redtop bentgrass
Agrostis alba or Agrostis gigantea
5-10 90 80
* Modified Briargreen, Inc. Hydroseeding Guide Wetlands Seed Mix
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-17
The seed mix shown in Table 4.5 is a recommended low-growing,
relatively non-invasive seed mix appropriate for very wet areas that are
not regulated wetlands. Other mixes may be appropriate, depending on
the soil type and hydrology of the area. Recent research suggests that
bentgrass (agrostis sp.) should be emphasized in wet-area seed mixes.
Apply this mixture at a rate of 60 pounds per acre.
Table 4.5
Wet Area Seed Mix*
% Weight % Purity % Germination
Tall or meadow fescue
Festuca arundinacea or
Festuca elatior
60-70 98 90
Seaside/Creeping bentgrass
Agrostis palustris
10-15 98 85
Meadow foxtail
Alepocurus pratensis
10-15 90 80
Alsike clover
Trifolium hybridum
1-6 98 90
Redtop bentgrass
Agrostis alba
1-6 92 85
* Modified Briargreen, Inc. Hydroseeding Guide Wetlands Seed Mix
The meadow seed mix in Table 4.6 is recommended for areas that will be
maintained infrequently or not at all and where colonization by native
plants is desirable. Likely applications include rural road and utility right-
of-way. Seeding should take place in September or very early October in
order to obtain adequate establishment prior to the winter months. The
appropriateness of clover in the mix may need to be considered, as this can
be a fairly invasive species. If the soil is amended, the addition of clover
may not be necessary.
Table 4.6
Meadow Seed Mix
% Weight % Purity % Germination
Redtop or Oregon bentgrass
Agrostis alba or Agrostis oregonensis
20 92 85
Red fescue
Festuca rubra
70 98 90
White dutch clover
Trifolium repens
10 98 90
Maintenance
Standards
• Any seeded areas that fail to establish at least 80 percent cover (100
percent cover for areas that receive sheet or concentrated flows) shall
be reseeded. If reseeding is ineffective, an alternate method, such as
sodding, mulching, or nets/blankets, shall be used. If winter weather
prevents adequate grass growth, this time limit may be relaxed at the
discretion of the local authority when sensitive areas would otherwise
be protected.
4-18 Volume II – Construction Stormwater Pollution Prevention February 2005
• After adequate cover is achieved, any areas that experience erosion
shall be reseeded and protected by mulch. If the erosion problem is
drainage related, the problem shall be fixed and the eroded area
reseeded and protected by mulch.
• Seeded areas shall be supplied with adequate moisture, but not watered
to the extent that it causes runoff.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-19
BMP C121: Mulching
Purpose The purpose of mulching soils is to provide 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 is an enormous variety of mulches
that can be used. Only the most common types are discussed in this
section.
Conditions of Use As a temporary cover measure, mulch should be used:
• On disturbed areas that require cover measures for less than 30 days.
• As a cover for seed during the wet season and during the hot summer
months.
• 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.
Design and
Installation
Specifications
For mulch materials, application rates, and specifications, see Table 4.7.
Note: Thicknesses may be increased for disturbed areas in or near
sensitive areas or other areas highly susceptible to erosion.
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.
Maintenance
Standards
• The thickness of the 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.
4-20 Volume II – Construction Stormwater Pollution Prevention February 2005
Table 4.7
Mulch Standards and Guidelines
Mulch
Material Quality Standards
Application
Rates Remarks
Straw Air-dried; free from
undesirable seed and
coarse material.
2"-3" thick; 5
bales per 1000 sf
or 2-3 tons per
acre
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. Straw
should be used only if mulches with long-term benefits are
unavailable locally. It should also not be used within the
ordinary high-water elevation of surface waters (due to
flotation).
Hydromulch No growth
inhibiting factors.
Approx. 25-30
lbs per 1000 sf
or 1500 - 2000
lbs per acre
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 ¾-1 inch clog
hydromulch equipment. Fibers should be kept to less than ¾
inch.
Composted
Mulch and
Compost
No visible water or
dust during
handling. Must be
purchased from
supplier with Solid
Waste Handling
Permit (unless
exempt).
2" thick min.;
approx. 100 tons
per acre (approx.
800 lbs per yard)
More effective control can be obtained by increasing
thickness to 3". Excellent mulch for protecting final grades
until landscaping because it can be directly seeded or tilled
into soil as an amendment. Composted mulch has a coarser
size gradation than compost. It is more stable and practical
to use in wet areas and during rainy weather conditions.
Chipped Site
Vegetation
Average size shall
be several inches.
Gradations from
fines to 6 inches in
length for texture,
variation, and
interlocking
properties.
2" minimum
thickness
This is a cost-effective way to dispose of debris from
clearing and grubbing, 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 surface waters. If seeding is expected shortly after
mulch, the decomposition of the chipped vegetation may tie
up nutrients important to grass establishment.
Wood-based
Mulch
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 regulations.
2” thick; approx.
100 tons per acre
(approx. 800 lbs.
per cubic yard)
This material is often called “hog or hogged fuel.” It is
usable as a material for Stabilized Construction Entrances
(BMP C105) and as a mulch. 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).
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-21
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.
• Plastic is particularly useful for protecting cut and fill slopes and
stockpiles. Note: The relatively rapid breakdown of most polyethylene
sheeting makes it unsuitable for long-term (greater than six months)
applications.
• Clear plastic sheeting can be used over newly-seeded areas to create a
greenhouse effect and encourage grass growth if the hydroseed was
installed too late in the season to establish 75 percent grass cover, or if
the wet season started earlier than normal. Clear plastic should not be
used for this purpose during the summer months because the resulting
high temperatures can kill the grass.
• Due to rapid runoff caused by plastic sheeting, this method shall not be
used upslope of areas that might be adversely impacted by
concentrated runoff. Such areas include steep and/or unstable slopes.
• While plastic is inexpensive to purchase, the added cost of installation,
maintenance, removal, and disposal make this an expensive material,
up to $1.50-2.00 per square yard.
• Whenever plastic is used to protect slopes, water collection measures
must be installed at the base of the slope. These measures include
plastic-covered berms, channels, and pipes used to covey clean
rainwater away from bare soil and disturbed areas. At no time is clean
runoff from a plastic covered slope to be mixed with dirty runoff from
a project.
• Other uses for plastic include:
1. Temporary ditch liner;
2. Pond liner in temporary sediment pond;
3. Liner for bermed temporary fuel storage area if plastic is not
reactive to the type of fuel being stored;
4. Emergency slope protection during heavy rains; and,
5. Temporary drainpipe (“elephant trunk”) used to direct water.
4-26 Volume II – Construction Stormwater Pollution Prevention February 2005
Design and
Installation
Specifications
• Plastic slope cover must be installed as follows:
1. Run plastic up and down slope, not across slope;
2. Plastic may be installed perpendicular to a slope if the slope length
is less than 10 feet;
3. Minimum of 8-inch overlap at seams;
4. On long or wide slopes, or slopes subject to wind, all seams should
be taped;
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 pound a wooden stake through each 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.
• Plastic sheeting shall have a minimum thickness of 0.06 millimeters.
• 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
• Torn sheets must be replaced and open seams repaired.
• If the plastic begins to deteriorate due to ultraviolet radiation, it must
be completely removed and replaced.
• When the plastic is no longer needed, it shall be completely removed.
• Dispose of old tires appropriately.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-27
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 the Certified Erosion
and Sediment Control Lead (CESCL) who is responsible for ensuring
compliance with all local, state, and federal erosion and sediment control
and water quality requirements.
Conditions of Use A CESCL shall be made available on projects one acre or larger that
discharge stormwater to surface waters of the state
• The CESCL shall:
• 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
(see details below).
Ecology will maintain a list of ESC training and certification
providers at: www.ecy.wa.gov/programs/wq/stormwater.
OR
• Be a Certified Professional in Erosion and Sediment Control
(CPESC); for additional information go to: www.cpesc.net
Specifications • Certification shall remain valid for three years.
• The CESCL shall have authority to act on behalf of the contractor or
developer and shall be available, on call, 24 hours per day throughout
the period of construction.
• The Construction SWPPP shall include the name, telephone number,
fax number, and address of the designated CESCL.
• A CESCL may provide inspection and compliance services for
multiple construction projects in the same geographic region.
Duties and responsibilities of the CESCL shall include, but are not limited
to the following:
• Maintaining permit file on site at all times which includes the SWPPP
and any associated permits and plans.
• Directing BMP installation, inspection, maintenance, modification,
and removal.
• Updating all project drawings and the Construction SWPPP with
changes made.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-47
• Keeping daily logs, and inspection reports. Inspection reports should
include:
• Inspection date/time.
• Weather information; general conditions during inspection and
approximate amount of precipitation since the last inspection.
• 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, and
4) Locations of where additional or different BMPs are
required.
• 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.
• Any water quality monitoring performed during inspection.
• General comments and notes, including a brief description of any
BMP repairs, maintenance or installations made as a result of
the inspection.
• Facilitate, participate in, and take corrective actions resulting from
inspections performed by outside agencies or the owner.
4-48 Volume II – Construction Stormwater Pollution Prevention February 2005
Minimum Requirements for ESC Training and Certification Courses
General Requirements
1. The course shall teach the construction stormwater pollution prevention guidance
provided in the most recent version of:
a. The Washington State Dept. of Ecology Stormwater Management Manual for
Western Washington,
b. Other equivalent stormwater management manuals approved by Ecology.
2. Upon completion of course, each attendee shall receive documentation of certification,
including, at a minimum, a wallet-sized card that certifies completion of the course.
Certification shall remain valid for three years. Recertification may be obtained by
completing the 8-hour refresher course or by taking the initial 16-hour training course
again.
3. The initial certification course shall be a minimum of 16 hours (with a reasonable time
allowance for lunch, breaks, and travel to and from field) and include a field element and
test.
a. The field element must familiarize students with the proper installation,
maintenance and inspection of common erosion and sediment control BMPs
including, but not limited to, blankets, check dams, silt fence, straw mulch,
plastic, and seeding.
b. The test shall be open book and a passing score is not required for certification.
Upon completion of the test, the correct answers shall be provided and discussed.
4. The refresher course shall be a minimum of 8 hours and include a test.
a. The refresher course shall include:
i. Applicable updates to the Stormwater Management Manual that is used to
teach the course, including new or updated BMPs; and
ii. Applicable changes to the NPDES General Permit for Construction
Activities.
b. The refresher course test shall be open book and a passing score is not required
for certification. Upon completion of the test, the correct answers shall be
provided and discussed.
c. The refresher course may be taught using an alternative format (e.g. internet, CD
ROM, etc.) if the module is approved by Ecology.
Required Course Elements
1. Erosion and Sedimentation Impacts
a. Examples/Case studies
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-49
2. Erosion and Sedimentation Processes
a. Definitions
b. Types of erosion
c. Sedimentation
i. Basic settling concepts
ii. Problems with clays/turbidity
3. Factors Influencing Erosion Potential
a. Soil
b. Vegetation
c. Topography
d. Climate
4. Regulatory Requirements
a. NPDES - Construction Stormwater General Permit
b. Local requirements and permits
c. Other regulatory requirements
5. Stormwater Pollution Prevention Plan (SWPPP)
a. SWPPP is a living document – should be revised as necessary
b. 12 Elements of a SWPPP; discuss suggested BMPs (with examples)
1. Mark Clearing Limits
2. Establish Construction Access
3. Control Flow Rates
4. Install Sediment Controls
5. Stabilize Soils
6. Protect Slopes
7. Protect Drain Inlets
8. Stabilize Channels and Outlets
9. Control Pollutants
10. Control De-watering
11. Maintain BMPs
12. Manage the Project
6. Monitoring/Reporting/Recordkeeping
a. Site inspections/visual monitoring
i. Disturbed areas
ii. BMPs
iii. Stormwater discharge points
b. Water quality sampling/analysis
i. Turbidity
ii. pH
c. Monitoring frequency
i. Set by NPDES permit
ii. Inactive sites - reduced frequency
4-50 Volume II – Construction Stormwater Pollution Prevention February 2005
d. Adaptive Management
i. When monitoring indicates problem, take appropriate action (e.g.
install/maintain BMPs)
ii. Document the corrective action(s) in SWPPP
e. Reporting
i. Inspection reports/checklists
ii. Discharge Monitoring Reports (DMR)
iii. Non-compliance notification
Instructor Qualifications
1. Instructors must be qualified to effectively teach the required course elements.
2. At a minimum, instructors must have:
a. Current certification as a Certified Professional in Erosion and Sediment Control
(CPESC), or
b. Completed a training program for teaching the required course elements, or
c. The academic credentials and instructional experience necessary for teaching the
required course elements.
3. Instructors must demonstrate competent instructional skills and knowledge of the
applicable subject matter.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-51
4.2 Runoff Conveyance and Treatment BMPs
BMP C200: Interceptor Dike and Swale
Purpose Provide a ridge of compacted soil, or a ridge with an upslope swale, at the
top or base of a disturbed slope or along the perimeter 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 construction site.
Conditions of Use Where the runoff from an exposed site or disturbed slope must be conveyed
to an erosion control facility which can safely convey the stormwater.
• Locate upslope of a construction site to prevent runoff from entering
disturbed area.
• When placed horizontally across a disturbed slope, it reduces the
amount and velocity of runoff flowing down the slope.
• Locate downslope to collect runoff from a disturbed area and direct it
to a sediment basin.
Design and
Installation
Specifications
• Dike and/or swale and channel must be stabilized with temporary or
permanent vegetation or other channel protection during construction.
• Channel requires a positive grade for drainage; steeper grades require
channel protection and check dams.
• Review construction for areas where overtopping may occur.
• Can be used at top of new fill before vegetation is established.
• May be used as a permanent diversion channel to carry the runoff.
• Sub-basin tributary area should be one acre or less.
• Design capacity for the peak flow from a 10-year, 24-hour storm,
assuming a Type 1A rainfall distribution, for temporary facilities.
Alternatively, use 1.6 times the 10-year, 1-hour flow indicated by an
approved continuous runoff model. For facilities that will also serve
on a permanent basis, consult the local government’s drainage
requirements.
Interceptor dikes shall meet the following criteria:
Top Width 2 feet minimum.
Height 1.5 feet minimum on berm.
Side Slope 2:1 or flatter.
Grade Depends on topography, however, dike system minimum is
0.5%, maximum is 1%.
Compaction Minimum of 90 percent ASTM D698 standard proctor.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-57
Horizontal Spacing of Interceptor 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
Stabilization depends on velocity and reach
Slopes <5% Seed and mulch applied within 5 days of dike
construction (see BMP C121, Mulching).
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.
• The upslope side of the dike shall provide positive drainage to the dike
outlet. No erosion shall occur at the outlet. Provide energy dissipation
measures as necessary. Sediment-laden runoff must be released
through a sediment trapping facility.
• Minimize construction traffic over temporary dikes. Use temporary
cross culverts for channel crossing.
Interceptor swales shall meet the following criteria:
Bottom Width 2 feet minimum; the bottom shall be level.
Depth 1-foot minimum.
Side Slope 2:1 or flatter.
Grade Maximum 5 percent, with positive drainage to a
suitable outlet (such as a sediment pond).
Stabilization Seed as per BMP C120, Temporary and Permanent
Seeding, or BMP C202, Channel Lining, 12 inches
thick of riprap pressed into the bank and extending
at least 8 inches vertical from the bottom.
• Inspect diversion dikes and interceptor swales once a week and after
every rainfall. Immediately remove sediment from the flow area.
• Damage caused by construction traffic or other activity must be
repaired before the end of each working day.
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 stabilize the channel to blend with the natural surface.
4-58 Volume II – Construction Stormwater Pollution Prevention February 2005
BMP C207: Check Dams
Purpose Construction of small dams across a swale or ditch reduces the velocity of
concentrated flow and dissipates energy at the check dam.
Conditions of Use Where temporary channels or permanent channels are not yet vegetated,
channel lining is infeasible, and velocity checks are required.
• Check dams may not be placed in streams unless approved by the State
Department of Fish and Wildlife. Check dams may not be placed in
wetlands without approval from a permitting agency.
• Check dams shall not be placed 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
Whatever material is used, the dam should form a triangle when viewed
from the side. This prevents undercutting as water flows over the face of
the dam rather than falling directly onto the ditch bottom.
Check dams in association with sumps work more effectively at slowing
flow and retaining sediment than just a check dam alone. A deep sump
should be provided immediately upstream of the check dam.
• In some cases, if carefully located and designed, check dams can
remain as permanent installations with very minor regrading. They
may be left as either spillways, in which case accumulated sediment
would be graded and seeded, or as check dams to prevent further
sediment from leaving the site.
• Check dams can 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.
• Check dams should be placed perpendicular to the flow of water.
• The maximum spacing between the dams shall be such that the toe of
the upstream dam is at the same elevation as the top of the downstream
dam.
• Keep the maximum height at 2 feet at the center of the dam.
• Keep the center of the check dam at least 12 inches lower than the
outer edges at natural ground elevation.
• Keep the side slopes of the check dam at 2:1 or flatter.
• 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.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-75
• Use filter fabric foundation under a rock or sand bag check dam. If a
blanket ditch liner is used, this is not necessary. A piece of organic or
synthetic blanket cut to fit will also work for this purpose.
• Rock check dams shall be constructed of appropriately sized rock.
The rock must be placed by hand or by mechanical means (no
dumping of rock to form dam) to achieve complete coverage of the
ditch or swale and to ensure that the center of the dam is lower than
the edges. The rock used must be large enough to stay in place given
the expected design flow through the channel.
• 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.
• Ensure that channel appurtenances, such as culvert entrances below
check dams, are not subject to damage or blockage from displaced
stones. Figure 4.13 depicts a typical rock check dam.
Maintenance
Standards
Check dams shall be monitored for performance and sediment
accumulation during and after each runoff producing rainfall. Sediment
shall be removed when it reaches one half the sump depth.
• Anticipate submergence and deposition above the check dam and
erosion from high flows around the edges of the dam.
• If significant erosion occurs between dams, install a protective riprap
liner in that portion of the channel.
4-76 Volume II – Construction Stormwater Pollution Prevention February 2005
Figure 4.13 – Check Dams Figure 4.13 – Check Dams
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-77
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-77
BMP C220: Storm Drain Inlet Protection
Purpose To prevent coarse sediment from entering drainage systems prior to
permanent stabilization of the disturbed area.
Conditions of Use Where storm drain inlets are to be made operational before permanent
stabilization of the disturbed drainage area. Protection should be provided
for all storm drain inlets downslope and within 500 feet of a disturbed or
construction area, unless the runoff that enters the catch basin will be
conveyed to a sediment pond or trap. Inlet protection may be used
anywhere to protect the drainage system. It is likely that the drainage
system will still require cleaning.
Table 4.9 lists several options for inlet protection. All of the methods for
storm drain inlet protection are prone to plugging and require a high
frequency of maintenance. Drainage areas should be limited to 1 acre or
less. Emergency overflows may be required where stormwater ponding
would cause a hazard. If an emergency overflow is provided, additional
end-of-pipe treatment may be required.
Table 4.9
Storm Drain Inlet Protetion
Type of Inlet
Protection
Emergency
Overflow
Applicable for
Paved/ Earthen
Surfaces Conditions of Use
Drop Inlet Protection
Excavated drop inlet
protection
Yes,
temporary
flooding will
occur
Earthen Applicable for heavy flows. Easy
to maintain. Large area
Requirement: 30’ X 30’/acre
Block and gravel drop
inlet protection
Yes Paved or Earthen Applicable for heavy concentrated
flows. Will not pond.
Gravel and wire drop
inlet protection
No 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 protection
with a wooden weir
Small capacity
overflow
Paved Used for sturdy, more compact
installation.
Block and gravel curb
inlet protection
Yes Paved Sturdy, but limited filtration.
Culvert Inlet Protection
Culvert inlet sediment
trap
18 month expected life.
4-82 Volume II – Construction Stormwater Pollution Prevention February 2005
Design and
Installation
Specifications
Excavated Drop Inlet Protection - An excavated impoundment around the
storm drain. Sediment settles out of the stormwater prior to entering the
storm drain.
• Depth 1-2 ft as measured from the crest of the inlet structure.
• Side Slopes of excavation no steeper than 2:1.
• Minimum volume of excavation 35 cubic yards.
• Shape basin to fit site with longest dimension oriented toward the
longest inflow area.
• Install provisions for draining to prevent standing water problems.
• Clear the area of all debris.
• Grade the approach to the inlet uniformly.
• Drill weep holes into the side of the inlet.
• Protect weep holes with screen wire and washed aggregate.
• Seal weep holes when removing structure and stabilizing area.
• It may be necessary to build a temporary dike to the down slope side
of the structure to prevent bypass flow.
Block and Gravel Filter - A barrier formed around the storm drain inlet
with standard concrete blocks and gravel. See Figure 4.14.
• Height 1 to 2 feet above inlet.
• Recess the first row 2 inches into the ground for stability.
• Support subsequent courses by placing a 2x4 through the block
opening.
• Do not use mortar.
• Lay some blocks in the bottom row on their side for dewatering the
pool.
• Place hardware cloth or comparable wire mesh with ½-inch openings
over all block openings.
• Place gravel just below the top of blocks on slopes of 2:1 or flatter.
• An alternative design is a gravel donut.
• Inlet slope of 3:1.
• Outlet slope of 2:1.
• 1-foot wide level stone area between the structure and the inlet.
• Inlet slope stones 3 inches in diameter or larger.
• Outlet slope use gravel ½- to ¾-inch at a minimum thickness of 1-foot.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-83
Figure 4.14 – Block and Gravel Filter
Gravel and Wire Mesh Filter - A gravel barrier placed over the top of the
inlet. This structure does not provide an overflow.
Ponding Height
Notes: 1. Drop inlet sediment barriers are to be used for small, nearly level drainage areas. (less than 5%) 2. Excavate a basin of sufficient size adjacent to the drop inlet.
3. The top of the structure (ponding height) must be well below the ground elevation downslope to prevent
runoff from bypassing the inlet. A temporary dike may be necessary on the dowslope side of the structure.
• Hardware cloth or comparable wire mesh with ½-inch openings.
• Coarse aggregate.
• Height 1-foot or more, 18 inches wider than inlet on all sides.
• Place wire mesh over the drop inlet so that the wire extends a
minimum of 1-foot beyond each side of the inlet structure.
• If more than one strip of mesh is necessary, overlap the strips.
• Place coarse aggregate over the wire mesh.
• The depth of the gravel should be at least 12 inches over the entire
inlet opening and extend at least 18 inches on all sides.
4-84 Volume II – Construction Stormwater Pollution Prevention February 2005
Catchbasin Filters - Inserts should be designed by the manufacturer for
use at construction sites. The limited sediment storage capacity increases
the amount of inspection and maintenance required, which may be daily
for heavy sediment loads. The maintenance requirements can be reduced
by combining a catchbasin 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.
• 5 cubic feet of storage.
• Dewatering provisions.
• High-flow bypass that will not clog under normal use at a construction
site.
• The catchbasin filter is inserted in the catchbasin just below the
grating.
Curb Inlet Protection with Wooden Weir – Barrier formed around a curb
inlet with a wooden frame and gravel.
• Wire mesh with ½-inch openings.
• Extra strength filter cloth.
• Construct a frame.
• Attach the wire and filter fabric to the frame.
• Pile coarse washed aggregate against wire/fabric.
• Place weight on frame anchors.
Block and Gravel Curb Inlet Protection – Barrier formed around an inlet
with concrete blocks and gravel. See Figure 4.14.
• Wire mesh with ½-inch openings.
• Place two concrete blocks on their sides abutting the curb at either side
of the inlet opening. These are spacer blocks.
• Place a 2x4 stud through the outer holes of each spacer block to align
the front blocks.
• Place blocks on their sides across the front of the inlet and abutting the
spacer blocks.
• Place wire mesh over the outside vertical face.
• Pile coarse aggregate against the wire to the top of the barrier.
Curb and Gutter Sediment Barrier – Sandbag or rock berm (riprap and
aggregate) 3 feet high and 3 feet wide in a horseshoe shape. See Figure
4.16.
• 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.
• Construct a horseshoe shaped sedimentation trap on the outside of the
berm sized to sediment trap standards for protecting a culvert inlet.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-85
Maintenance
Standards
• Catch basin filters should be inspected frequently, especially after
storm events. If the insert becomes clogged, it should be cleaned or
replaced.
• For systems using stone filters: If the stone filter becomes clogged
with sediment, the stones must be pulled away from the inlet and
cleaned or replaced. Since cleaning of gravel at a construction site
may be difficult, an alternative approach would be to use the clogged
stone as fill and put fresh stone around the inlet.
• 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.
4-86 Volume II – Construction Stormwater Pollution Prevention February 2005
Figure 4.15 – Block and Gravel Curb Inlet Protection
A
Plan View
Wire Screen orFilter Fabric Catch Basin
Curb Inlet
Concrete BlockPonding Height
Overflow
2x4 Wood Stud(100x50 Timber Stud)
Concrete BlockWire Screen orFilter Fabric
Curb Inlet
¾" Drain Gravel(20mm)
¾" Drain Gravel(20mm)Section A - A
Back of Curb Concrete Block
2x4 Wood Stud
Catch BasinBack of Sidewalk
NOTES:1. Use block and gravel type sediment barrier when curb inlet is located in gently sloping street segment, where water can pond and allow sediment to separate from runoff.2. Barrier shall allow for overflow from severe storm event.3. Inspect barriers and remove sediment after each storm event. Sediment and gravel must be removed from the traveled way immediately.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-87
Figure 4.16 – Curb and Gutter Barrier Figure 4.16 – Curb and Gutter Barrier
4-88 Volume II – Construction Stormwater Pollution Prevention February 2005
4-88 Volume II – Construction Stormwater Pollution Prevention February 2005
BMP C233: Silt Fence
Purpose Use of a 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. See Figure 4.19 for
details on silt fence construction.
Conditions of Use Silt fence may be used downslope of all disturbed areas.
• Silt fence is not intended to treat concentrated flows, nor is it intended
to treat substantial amounts of overland flow. Any concentrated flows
must be conveyed through the drainage system to a sediment pond.
The only circumstance in which overland flow can be treated solely by
a silt fence, rather than by a sediment pond, is when the area draining
to the fence is one acre or less and flow rates are less than 0.5 cfs.
• Silt fences should not be constructed in streams or used in V-shaped
ditches. They are not an adequate method of silt control for anything
deeper than sheet or overland flow.
Figure 4.19 – Silt Fence
Design and
Installation
Specifications
• Drainage area of 1 acre or less or in combination with sediment basin
in a larger site.
• Maximum slope steepness (normal (perpendicular) to fence line) 1:1.
• Maximum sheet or overland flow path length to the fence of 100 feet.
• No flows greater than 0.5 cfs.
• The geotextile used shall meet 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 4.10):
4-94 Volume II – Construction Stormwater Pollution Prevention February 2005
Table 4.10
Geotextile Standards
Polymeric Mesh AOS
(ASTM D4751)
0.60 mm maximum for slit film wovens (#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
• Standard strength fabrics shall be supported with wire mesh, chicken
wire, 2-inch x 2-inch wire, safety fence, or jute mesh to increase the
strength of the fabric. Silt fence materials are available that have
synthetic mesh backing attached.
• Filter fabric 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.
• 100 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 local regulations.
• Standard Notes for construction plans and specifications follow. Refer
to Figure 4.19 for standard silt fence details.
The contractor shall install and maintain temporary silt fences at the
locations shown in the Plans. The silt fences shall be constructed in
the areas of clearing, grading, or drainage prior to starting those
activities. A silt fence shall not be considered temporary if the silt
fence must function beyond the life of the contract. The silt fence
shall prevent soil carried by runoff water from going beneath, through,
or over the top of the silt fence, but shall allow the water to pass
through the fence.
The minimum height of the top of silt fence shall be 2 feet and the
maximum height shall be 2½ feet above the original ground surface.
The geotextile shall be sewn together at the point of manufacture, or at
an approved location as determined by the Engineer, to form geotextile
lengths as required. All sewn seams shall be located at a support post.
Alternatively, two sections of silt fence can be overlapped, provided
the Contractor can demonstrate, to the satisfaction of the Engineer, that
the overlap is long enough and that the adjacent fence sections are
close enough together to prevent silt laden water from escaping
through the fence at the overlap.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-95
The geotextile shall be attached on the up-slope side of the posts and
support system with staples, wire, or in accordance with the
manufacturer's recommendations. The geotextile shall be attached to
the posts in a manner that reduces the potential for geotextile tearing at
the staples, wire, or other connection device. Silt fence back-up
support for the geotextile in the form of a wire or plastic mesh is
dependent on the properties of the geotextile selected for use. If wire
or plastic back-up mesh is used, the mesh shall be fastened securely to
the up-slope of the posts with the geotextile being up-slope of the
mesh back-up support.
The geotextile at the bottom of the fence shall be buried in a trench to
a minimum depth of 4 inches below the ground surface. The trench
shall be backfilled and the soil tamped in place over the buried portion
of the geotextile, such that no flow can pass beneath the fence and
scouring can not occur. When wire or polymeric back-up support
mesh is used, the wire or polymeric mesh shall extend into the trench a
minimum of 3 inches.
The fence posts shall be placed or driven a minimum of 18 inches. A
minimum depth of 12 inches is allowed if topsoil or other soft
subgrade soil is not present and a minimum depth of 18 inches cannot
be reached. Fence post depths shall be increased by 6 inches if the
fence is located on slopes of 3:1 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 prevent
overturning of the fence due to sediment loading.
Silt fences shall be located on contour as much as possible, except at
the ends of the fence, 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.
If the fence must cross contours, with the exception of the ends of the
fence, gravel check dams placed perpendicular to the back of the fence
shall be used to minimize concentrated flow and erosion along the
back of the fence. The gravel check dams shall be approximately 1-
foot deep at the back of the fence. It 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. The gravel check dams
shall consist of crushed surfacing base course, gravel backfill for
walls, or shoulder ballast. The gravel check dams shall be located
every 10 feet along the fence where the fence must cross contours.
The slope of the fence line where contours must be crossed shall not
be steeper than 3:1.
Wood, steel or equivalent posts shall be used. Wood posts shall have
minimum dimensions of 2 inches by 2 inches by 3 feet minimum
length, and shall be free of defects such as knots, splits, or gouges.
4-96 Volume II – Construction Stormwater Pollution Prevention February 2005
Steel posts shall consist of either size No. 6 rebar or larger, ASTM A
120 steel pipe with a minimum diameter of 1-inch, U, T, L, or C shape
steel posts with a minimum weight of 1.35 lbs./ft. or other steel posts
having equivalent strength and bending resistance to the post sizes
listed. The spacing of the support posts shall be a maximum of 6 feet.
Fence back-up 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 ultraviolet radiation as the geotextile it supports.
• Silt fence installation using the slicing method specification details
follow. Refer to Figure 4.20 for slicing method details.
The base of both end posts must be at least 2 to 4 inches above the top
of the silt fence 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.
Install posts 3 to 4 feet apart in critical retention areas and 6 to 7 feet
apart in standard applications.
Install posts 24 inches deep on the downstream side of the silt fence,
and as close as possible to the fabric, enabling posts to support the
fabric from upstream water pressure.
Install posts with the nipples facing away from the silt fence fabric.
Attach the 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. In
addition, each tie should be positioned to hang on a post nipple when
tightening to prevent sagging.
Wrap approximately 6 inches of fabric around the end posts and secure
with 3 ties.
No more than 24 inches of a 36-inch fabric is allowed above ground
level.
The rope lock system must be used in all ditch check applications.
The installation should be checked and corrected for any deviation
before compaction. Use a flat-bladed shovel to tuck fabric deeper into
the ground if necessary.
Compaction is vitally important for effective results. Compact the soil
immediately next to the silt fence 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.
February 2005 Volume II – Construction Stormwater Pollution Prevention 4-97
Maintenance
Standards
• Any damage shall be repaired immediately.
• If concentrated flows are evident uphill of the fence, they must be
intercepted and conveyed to a sediment pond.
• It is important to check the uphill side of the 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 or remove the trapped sediment.
• Sediment deposits shall either be removed when the deposit reaches
approximately one-third the height of the silt fence, or a second silt
fence shall be installed.
• If the filter fabric (geotextile) has deteriorated due to ultraviolet
breakdown, it shall be replaced.
Figure 4.20 – Silt Fence Installation by Slicing Method
4-98 Volume II – Construction Stormwater Pollution Prevention February 2005
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BMP C241: Temporary Sediment Pond
Purpose Sediment ponds remove sediment from runoff originating from disturbed
areas of the 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 Prior to leaving a construction site, stormwater runoff must pass through a
sediment pond or other appropriate sediment removal best management
practice.
A sediment pond shall be used where the contributing drainage area is 3
acres or more. Ponds must be used in conjunction with erosion control
practices to reduce the amount of sediment flowing into the basin.
Design and
Installation
Specifications
• Sediment basins must be installed only on sites where failure of the
structure would not result in loss of life, damage to homes or
buildings, or interruption of use or service of public roads or utilities.
Also, sediment traps and ponds are attractive to children and can be
very dangerous. Compliance with local ordinances regarding health
and safety must be addressed. If fencing of the pond is required, the
type of fence and its location shall be shown on the ESC plan.
• Structures having a maximum storage capacity at the top of the dam of
10 acre-ft (435,600 ft3) or more are subject to the Washington Dam
Safety Regulations (Chapter 173-175 WAC).
• See Figures 4.2.18, 4.2.19, and 4.2.20 for details.
• If permanent runoff control facilities are part of the project, they
should be used for sediment retention. The surface area requirements
of the sediment basin must be met. This may require temporarily
enlarging the permanent basin to comply with the surface area
requirements. The permanent control structure must be temporarily
replaced with a control structure that only allows water to leave the
pond from the surface or by pumping. The permanent control structure
must be installed after the site is fully stabilized. .
• Use of infiltration facilities for sedimentation basins during
construction tends to clog the soils and reduce their capacity to
infiltrate. If infiltration facilities are to be used, the sides and bottom of
the facility must only be rough excavated to a minimum of 2 feet
above final grade. Final grading of the infiltration facility shall occur
only when all contributing drainage areas are fully stabilized. The
infiltration pretreatment facility should be fully constructed and used
with the sedimentation basin to help prevent clogging.
• Determining Pond Geometry
Obtain the discharge from the hydrologic calculations of the peak flow
for the 2-year runoff event (Q2). The 10-year peak flow shall be used if
Volume II – Construction Stormwater Pollution Prevention - December 2014
4-106
the project size, expected timing and duration of construction, or
downstream conditions warrant a higher level of protection. If no
hydrologic analysis is required, the Rational Method may be used.
Determine the required surface area at the top of the riser pipe with the
equation:
SA = 2 x Q2/0.00096 or
2080 square feet per cfs of inflow
See BMP C240 for more information on the derivation of the surface
area calculation.
The basic geometry of the pond can now be determined using the
following design criteria:
• Required surface area SA (from Step 2 above) at top of riser.
• Minimum 3.5-foot depth from top of riser to bottom of pond.
• 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.
• One foot of freeboard between the top of the riser and the crest of the
emergency spillway.
• Flat bottom.
• Minimum 1-foot deep spillway.
• Length-to-width ratio between 3:1 and 6:1.
• Sizing of Discharge Mechanisms.
The outlet for the basin consists of a combination of principal and
emergency spillways. These outlets 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 spill-
way is not feasible, the principal 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. The
runoff calculations should be based 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 contained in this
standard will result in some reduction in the peak rate of runoff.
However, the riser outlet design will not adequately control the basin
discharge to the predevelopment discharge limitations as stated in
Minimum Requirement #7: Flow Control. However, if the basin for a
permanent stormwater detention pond is used for a temporary
Volume II – Construction Stormwater Pollution Prevention - December 2014
4-107
sedimentation basin, the control structure for the permanent pond can
be used to maintain predevelopment discharge limitations. The size of
the basin, the expected life of the construction project, the anticipated
downstream effects and the anticipated weather conditions during
construction, should be considered to determine the need of additional
discharge control. See Figure 4.2.21 for riser inflow curves.
Figure 4.2.18 – Sediment Pond Plan View
Figure 4.2.19 – Sediment Pond Cross Section
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Perforated polyethylenedrainage tubing, diametermin. 2" larger thandewatering orifice.Tubing shall comply with ASTM F667 and AASHTO M294
Polyethylene cap Provide adequatestrapping
Dewatering orifice, schedule,40 steel stub min.Diameter as per calculations
Alternatively, metal stakesand wire may be used toprevent flotation
2X riser dia. Min.
Concrete base
Corrugatedmetal riser
Watertightcoupling
18" min.
6" min.
Tack weld
3.5" min.
Figure 4.2.20 – Sediment Pond Riser Detail
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Figure 4.2.21 – Riser Inflow Curves
Volume II – Construction Stormwater Pollution Prevention - December 2014
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Principal Spillway: Determine the required diameter for the principal
spillway (riser pipe). The diameter shall be the minimum necessary to pass
the site’s 15-minute, 10-year flowrate. If using the Western Washington
Hydrology Model (WWHM), Version 2 or 3, design flow is the 10-year (1
hour) flow for the developed (unmitigated) site, multiplied by a factor of
1.6. Use Figure 4.2.21 to determine this diameter (h = 1-foot). Note: A
permanent control structure may be used instead of a temporary riser.
Emergency Overflow Spillway: Determine the required size and design
of the emergency overflow spillway for the developed 100-year peak flow
using the method contained in Volume III.
Dewatering Orifice: Determine the 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:
5.0
5.0
3600x6.0
)2(
Tg
hAAs
o =
where Ao = orifice area (square feet)
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 required surface area to the required diameter D of the orifice:
o
o AADx54.13x24==π
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.
• Additional Design Specifications
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 cells. The divider shall be at least one-
half the height of the riser and a minimum of one foot below the top of
the riser. Wire-backed, 2- to 3-foot high, extra strength filter fabric
supported by treated 4"x4"s can be used as a divider. Alternatively,
staked straw bales wrapped with filter fabric (geotextile) may be used.
If the pond is more than 6 feet deep, a different mechanism must be
proposed. A riprap embankment is one acceptable method of
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4-111
separation for deeper ponds. Other designs that satisfy the intent of this
provision are allowed as long as the divider is permeable, structurally
sound, and designed to prevent erosion under or around the barrier.
To aid in determining sediment depth, one-foot intervals shall be
prominently marked on the riser.
If an embankment of more than 6 feet is proposed, the pond must
comply with the criteria contained in Volume III regarding dam safety
for detention BMPs.
• The most common structural failure of sedimentation basins 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
granular soil with a head of sufficient magnitude to cause soil grains to
lose contact and capability for support.
The most critical construction sequences to prevent piping will be:
1. Tight connections between riser and barrel and other pipe
connections.
2. Adequate anchoring of riser.
3. Proper soil compaction of the embankment and riser footing.
4. Proper construction of anti-seep devices.
Maintenance
Standards
• Sediment shall be removed from the pond when it reaches 1–foot in
depth.
• Any damage to the pond embankments or slopes shall be repaired.
BMP C250: Construction Stormwater Chemical Treatment
Purpose This BMP applies when using stormwater chemicals in batch treatment or
flow-through treatment.
Turbidity is difficult to control once fine particles are suspended in
stormwater runoff from a construction site. Sedimentation ponds are
effective at removing larger particulate matter by gravity settling, but are
ineffective at removing smaller particulates such as clay and fine silt.
Traditional erosion and sediment control BMPs may not be adequate to
ensure compliance with the water quality standards for turbidity in
receiving water.
Chemical treatment can reliably provide exceptional reductions of
turbidity and associated pollutants. Chemical treatment may be required to
meet turbidity stormwater discharge requirements, especially when
construction is to proceed through the wet season.
Conditions of Use Formal written approval from Ecology is required for the use of chemical
treatment regardless of site size. The Local Permitting Authority may also
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require review and approval. When approved, the chemical treatment
systems must be included in the Construction Stormwater Pollution
Prevention Plan (SWPPP).
Design and
Installation
Specifications
See Appendix II-B for background information on chemical treatment.
Criteria for Chemical Treatment Product Use: Chemically treated
stormwater discharged from construction sites must be nontoxic to aquatic
organisms. The Chemical Technology Assessment Protocol (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:
http://www.ecy.wa.gov/programs/wq/stormwater/newtech/technologies.ht
ml.
Treatment System Design Considerations: The design and operation of
a chemical treatment system should take into consideration the factors that
determine optimum, cost-effective performance. It is important to
recognize the following:
• Only Ecology approved chemicals may be used and must follow
approved dose rate.
• The pH of the stormwater must be in the proper range for the polymers
to be effective, which is typically 6.5 to 8.5
• The coagulant must be mixed rapidly into the water to ensure proper
dispersion.
• A flocculation step is important to increase the rate of settling, to
produce the lowest turbidity, and to keep the dosage rate as low as
possible.
• Too little energy input into the water during the flocculation phase
results in flocs that are too small and/or insufficiently dense. Too much
energy can rapidly destroy floc as it is formed.
• Care must be taken in the design of the withdrawal system to minimize
outflow velocities and to prevent floc discharge. Discharge from a
batch treatment system should be directed through a physical filter
such as a vegetated swale that would catch any unintended floc
discharge. Currently, flow-through systems always discharge through
the chemically enhanced sand filtration system.
• System discharge rates must take into account downstream
conveyance integrity.
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Polymer Batch Treatment Process Description:
A batch chemical treatment system consists of the stormwater collection
system (either temporary diversion or the permanent site drainage system),
a storage pond, pumps, a chemical feed system, treatment cells, and
interconnecting piping.
The batch treatment system shall use a minimum of two lined treatment
cells in addition to an untreated stormwater storage pond. Multiple
treatment cells allow for clarification of treated water while other cells are
being filled or emptied. Treatment cells may be ponds or tanks. Ponds
with constructed earthen embankments greater than six feet high or which
impound more than 10 acre-feet require special engineering analyses. The
Ecology Dam Safety Section has specific design criteria for dams in
Washington State (see
http://www.ecy.wa.gov/programs/wr/dams/GuidanceDocs.html ).
Stormwater is collected at interception point(s) on the site and is diverted
by gravity or by pumping to an untreated stormwater storage pond or other
untreated stormwater holding area. The stormwater is stored until
treatment occurs. It is important that the holding pond be large enough to
provide adequate storage.
The first step in the treatment sequence is to check the pH of the
stormwater in the untreated stormwater storage pond. The pH is adjusted
by the application of carbon dioxide or a base until the stormwater in the
storage pond is within the desired pH range, 6.5 to 8.5. When used, carbon
dioxide is added immediately downstream of the transfer pump. Typically
sodium bicarbonate (baking soda) is used as a base, although other bases
may be used. When needed, base is added directly to the untreated
stormwater storage pond. The stormwater is recirculated with the
treatment pump to provide mixing in the storage pond. Initial pH
adjustments should be based on daily bench tests. Further pH adjustments
can be made at any point in the process.
Once the stormwater is within the desired pH range (dependant on
polymer being used), the stormwater is pumped from the untreated
stormwater storage pond to a treatment cell as polymer is added. The
polymer is added upstream of the pump to facilitate rapid mixing.
After polymer addition, the water is kept in a lined treatment cell for
clarification of the sediment-floc. In a batch mode process, clarification
typically takes from 30 minutes to several hours. Prior to discharge
samples are withdrawn for analysis of pH, flocculent chemical
concentration, and turbidity. If both are acceptable, the treated water is
discharged.
Several configurations have been developed to withdraw treated water
from the treatment cell. The original configuration is a device that
withdraws the treated water from just beneath the water surface using a
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float with adjustable struts that prevent the float from settling on the cell
bottom. This reduces the possibility of picking up sediment-floc from the
bottom of the pond. The struts are usually set at a minimum clearance of
about 12 inches; that is, the float will come within 12 inches of the bottom
of the cell. Other systems have used vertical guides or cables which
constrain the float, allowing it to drift up and down with the water level.
More recent designs have an H-shaped array of pipes, set on the
horizontal.
This scheme provides for withdrawal from four points rather than one.
This configuration reduces the likelihood of sucking settled solids from
the bottom. It also reduces the tendency for a vortex to form. Inlet
diffusers, a long floating or fixed pipe with many small holes in it, are also
an option.
Safety is a primary concern. Design should consider the hazards
associated with operations, such as sampling. Facilities should be designed
to reduce slip hazards and drowning. Tanks and ponds should have life
rings, ladders, or steps extending from the bottom to the top.
Polymer Batch Treatment Process Description:
At a minimum, a flow-through chemical treatment system consists of the
stormwater collection system (either temporary diversion or the permanent
site drainage system), an untreated stormwater storage pond, and the
chemically enhanced sand filtration system.
Stormwater is collected at interception point(s) on the site and is diverted
by gravity or by pumping to an untreated stormwater storage pond or other
untreated stormwater holding area. The stormwater is stored until
treatment occurs. It is important that the holding pond be large enough to
provide adequate storage.
Stormwater is then pumped from the untreated stormwater storage pond to
the chemically enhanced sand filtration system where polymer is added.
Adjustments to pH may be necessary before chemical addition. The sand
filtration system continually monitors the stormwater for turbidity and pH.
If the discharge water is ever out of an acceptable range for turbidity or
pH, the water is recycled to the untreated stormwater pond where it can be
retreated.
For batch treatment and flow-through treatment, the following equipment
should be located in a lockable shed:
• The chemical injector.
• Secondary containment for acid, caustic, buffering compound, and
treatment chemical.
• Emergency shower and eyewash.
• Monitoring equipment which consists of a pH meter and a
turbidimeter.
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System Sizing:
Certain sites are required to implement flow control for the developed
sites. These sites must also control stormwater release rates during
construction. Generally, these are sites that discharge stormwater directly,
or indirectly, through a conveyance system, into a fresh water. System
sizing is dependent on flow control requirements.
Sizing Criteria for Batch Treatment Systems for Flow Control
Exempt Water Bodies:
The total volume of the untreated stormwater storage pond and treatment
ponds or tanks must be large enough to treat stormwater that is produced
during multiple day storm events. It is recommended that at a minimum
the untreated stormwater storage pond be sized to hold 1.5 times the
runoff volume of the 10-year, 24-hour storm event. Bypass should be
provided around the chemical treatment system to accommodate extreme
storm events. Runoff volume shall be calculated using the methods
presented in Volume 3, Chapter 2. Worst-case land cover conditions (i.e.,
producing the most runoff) should be used for analyses (in most cases, this
would be the land cover conditions just prior to final landscaping).
Primary settling should be encouraged in the untreated stormwater storage
pond. A forebay with access for maintenance may be beneficial.
There are two opposing considerations in sizing the treatment cells. A
larger cell is able to treat a larger volume of water each time a batch is
processed. However, the larger the cell the longer the time required to
empty the cell. A larger cell may also be less effective at flocculation and
therefore require a longer settling time. The simplest approach to sizing
the treatment cell is to multiply the allowable discharge flow rate times the
desired drawdown time. A 4-hour drawdown time allows one batch per
cell per 8-hour work period, given 1 hour of flocculation followed by two
hours of settling.
If the discharge is directly to a flow control exempt receiving water listed
in Appendix I-E of Volume I or to an infiltration system, there is no
discharge flow limit.
Ponds sized for flow control water bodies must at a minimum meet the
sizing criteria for flow control exempt waters.
Sizing Criteria for Flow-Through Treatment Systems for Flow
Control Exempt Water Bodies:
When sizing storage ponds or tanks for flow-through systems for flow
control exempt water bodies, the treatment system capacity should be a
factor. The untreated stormwater storage pond or tank should be sized to
hold 1.5 times the runoff volume of the 10-year, 24-hour storm event
minus the treatment system flowrate for an 8-hour period. For a chitosan-
enhanced sand filtration system, the treatment system flowrate should be
sized using a hydraulic loading rate between 6-8 gpm/ft². Other hydraulic
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loading rates may be more appropriate for other systems. Bypass should
be provided around the chemical treatment system to accommodate
extreme storms. Runoff volume shall be calculated using the methods
presented in Volume 3, Chapter 2. Worst-case land cover conditions (i.e.,
producing the most runoff) should be used for analyses (in most cases, this
would be the land cover conditions just prior to final landscaping).
Sizing Criteria for Flow Control Water Bodies:
Sites that must implement flow control for the developed site condition
must also control stormwater release rates during construction.
Construction site stormwater discharges shall not exceed the discharge
durations of the pre-developed condition for the range of pre-developed
discharge rates from ½ of the 2-year flow through the 10-year flow as
predicted by an approved continuous runoff model. The pre-developed
condition to be matched shall be the land cover condition immediately
prior to the development project. This restriction on release rates can
affect the size of the storage pond and treatment cells.
The following is how WWHM can be used to determine the release rates
from the chemical treatment systems:
1. Determine the pre-developed flow durations to be matched by entering
the existing land use area under the “Pre-developed” scenario in
WWHM. The default flow range is from ½ of the 2-year flow through
the 10-year flow.
2. Enter the post developed land use area in the “Developed
Unmitigated” scenario in WWHM.
3. Copy the land use information from the “Developed Unmitigated” to
“Developed Mitigated” scenario.
4. While in the “Developed Mitigated” scenario, add a pond element
under the basin element containing the post-developed land use areas.
This pond element represents information on the available untreated
stormwater storage and discharge from the chemical treatment system.
In cases where the discharge from the chemical treatment system is
controlled by a pump, a stage/storage/discharge (SSD) table
representing the pond must be generated outside WWHM and
imported into WWHM. WWHM can route the runoff from the post-
developed condition through this SSD table (the pond) and determine
compliance with the flow duration standard. This would be an iterative
design procedure where if the initial SSD table proved to be
inadequate, the designer would have to modify the SSD table outside
WWHM and re-import in WWHM and route the runoff through it
again. The iteration will continue until a pond that complies with the
flow duration standard is correctly sized.
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Notes on SSD table characteristics:
• The pump discharge rate would likely be initially set at just below
½ of the 2-year flow from the pre-developed condition. As runoff
coming into the untreated stormwater storage pond increases and
the available untreated stormwater storage volume gets used up, it
would be necessary to increase the pump discharge rate above ½ of
the 2-year. The increase(s) above ½ of the 2-year must be such that
they provide some relief to the untreated stormwater storage needs
but at the same time will not cause violations of the flow duration
standard at the higher flows. The final design SSD table will
identify the appropriate pumping rates and the corresponding stage
and storages.
• When building such a flow control system, the design must ensure
that any automatic adjustments to the pumping rates will be as a
result of changes to the available storage in accordance with the
final design SSD table.
5. It should be noted that the above procedures would be used to meet the
flow control requirements. The chemical treatment system must be
able to meet the runoff treatment requirements. It is likely that the
discharge flow rate of ½ of the 2-year or more may exceed the
treatment capacity of the system. If that is the case, the untreated
stormwater discharge rate(s) (i.e., influent to the treatment system)
must be reduced to allow proper treatment. Any reduction in the flows
would likely result in the need for a larger untreated stormwater
storage volume.
If the discharge is to a municipal storm drainage system, the allowable
discharge rate may be limited by the capacity of the public system. It may
be necessary to clean the municipal storm drainage system prior to the
start of the discharge to prevent scouring solids from the drainage system.
If the municipal storm drainage system discharges to a water body not on
the flow control exempt list, the project site is subject to flow control
requirements. Obtain permission from the owner of the collection system
before discharging to it.
If system design does not allow you to discharge at the slower rates as
described above and if the site has a retention or detention pond that will
serve the planned development, the discharge from the treatment system may
be directed to the permanent retention/detention pond to comply with the flow
control requirement. In this case, the untreated stormwater storage pond and
treatment system will be sized according to the sizing criteria for flow-
through treatment systems for flow control exempt water bodies described
earlier except all discharge (water passing through the treatment system and
stormwater bypassing the treatment system) will be directed into the
permanent retention/detention pond. If site constraints make locating the
untreated stormwater storage pond difficult, the permanent
retention/detention pond may be divided to serve as the untreated stormwater
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storage pond and the post-treatment flow control pond. A berm or barrier
must be used in this case so the untreated water does not mix with the treated
water. Both untreated stormwater storage requirements, and adequate post-
treatment flow control must be achieved. The post-treatment flow control
pond’s revised dimensions must be entered into the WWHM and the WWHM
must be run to confirm compliance with the flow control requirement.
Maintenance
Standards
Monitoring: At a minimum, the following monitoring shall be conducted.
Test results shall be recorded on a daily log kept on site. Additional
testing may be required by the NPDES permit based on site conditions.
Operational Monitoring:
• Total volume treated and discharged.
• Flow must be continuously monitored and recorded at not greater than
15-minute intervals.
• Type and amount of chemical used for pH adjustment.
• Amount of polymer used for treatment.
• Settling time.
Compliance Monitoring:
• Influent and effluent pH, flocculent chemical concentration, and
turbidity must be continuously monitored and recorded at not greater
than 15-minute intervals. pH and turbidity of the receiving water.
Biomonitoring:
Treated stormwater must be non-toxic to aquatic organisms. Treated
stormwater must be tested for aquatic toxicity or residual chemicals.
Frequency of biomonitoring will be determined by Ecology.
Residual chemical tests must be approved by Ecology prior to their use.
If testing treated stormwater for aquatic toxicity, you must test for acute
(lethal) toxicity. Bioassays shall be conducted by a laboratory accredited
by Ecology, unless otherwise approved by Ecology. Acute toxicity tests
shall be conducted per the CTAPE protocol.
Discharge Compliance: Prior to discharge, treated stormwater must
be sampled and tested for compliance with pH, flocculent chemical
concentration, and turbidity limits. These limits may be established by
the Construction Stormwater General Permit or a site-specific discharge
permit. Sampling and testing for other pollutants may also be necessary at
some sites. pH must be within the range of 6.5 to 8.5 standard units and
not cause a change in the pH of the receiving water of more than 0.2
standard units. Treated stormwater samples and measurements shall be
taken from the discharge pipe or another location representative of the
nature of the treated stormwater discharge. Samples used for determining
compliance with the water quality standards in the receiving water shall
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not be taken from the treatment pond prior to decanting. Compliance with
the water quality standards is determined in the receiving water.
Operator Training: Each contractor who intends to use chemical
treatment shall be trained by an experienced contractor. Each site using
chemical treatment must have an operator trained and certified by an
organization approved by Ecology.
Standard BMPs: Surface stabilization BMPs should be implemented on
site to prevent significant erosion. All sites shall use a truck wheel wash to
prevent tracking of sediment off site.
Sediment Removal and Disposal:
• Sediment shall be removed from the storage or treatment cells as
necessary. Typically, sediment removal is required at least once during
a wet season and at the decommissioning of the cells. Sediment
remaining in the cells between batches may enhance the settling
process and reduce the required chemical dosage.
• Sediment that is known to be non-toxic may be incorporated into the
site away from drainages.
BMP C251: Construction Stormwater Filtration
Purpose Filtration removes sediment from runoff originating from disturbed areas
of the site.
Background Information:
Filtration with sand media has been used for over a century to treat water
and wastewater. The use of sand filtration for treatment of stormwater has
developed recently, generally to treat runoff from streets, parking lots, and
residential areas. The application of filtration to construction stormwater
treatment is currently under development.
Conditions of Use Traditional BMPs used to control soil erosion and sediment loss from sites
under development may not be adequate to ensure compliance with the
water quality standard for turbidity in the receiving water. Filtration may
be used in conjunction with gravity settling to remove sediment as small
as fine silt (0.5 µm). The reduction in turbidity will be dependent on the
particle size distribution of the sediment in the stormwater. In some
circumstances, sedimentation and filtration may achieve compliance with
the water quality standard for turbidity.
The use of construction stormwater filtration does not require approval
from Ecology as long as treatment chemicals are not used. Filtration in
conjunction with polymer treatment requires testing under the Chemical
Technology Assessment Protocol – Ecology (CTAPE) before it can be
initiated. Approval from the appropriate regional Ecology office must be
obtained at each site where polymers use is proposed prior to use. For
more guidance on stormwater chemical treatment see BMP C250.
10
APPENDIX C
Geotechnical Engineer’s Report C-1
1015 East 4th Avenue, Olympia, Washington 98506
Phone: 360.754.2128 Fax: 360.754.9299
July 12, 2019
C & E Developments LLC
PO Box 2983
Yelm, Washington 98597
Attention: Casey Peterson
Report
Geotechnical and Stormwater Investigation
Wyndstone Development
Proposed Multi-Family Residential
15025 Tahoma Boulevard SE
Yelm, Washington
Project No. 1142-001-01
INTRODUCTION
Insight Geologic is pleased to present our report of subsurface conditions at the location of your
proposed Wyndstone multi-family residential development to be located at 15025 Tahoma Boulevard
SE in Yelm, Washington. The location of the site is shown relative to surrounding physical features in
the Vicinity Map, Figure 1. The site of the proposed project consists of a single parcel of property
(Thurston County Tax Parcel No. 21724420300), comprising approximately 4.3 acres.
The project will include four, multi-family, multi-story residential buildings with appurtenant parking and
drive areas. Stormwater runoff from roads and parking areas is to be infiltrated to the subsurface in
the northern portion of the property.
SCOPE OF SERVICES
The objective of our services was to evaluate subsurface conditions on the property as a basis for
evaluating suitability of the soils for the proposed building and parking areas, as well as evaluating the
soils for stormwater infiltration. Our specific scope of services included the following tasks:
Stormwater Investigation
1. Provided for the location of subsurface utilities on the site. We conducted this task by notifying
the “One Call” system.
2. Conducted a site reconnaissance to evaluate and mark proposed boring locations at the site and
for truck-mounted drilling rig access.
3. Drilled two (2) borings in the location of the proposed stormwater disposal structure at the site
using a truck-mounted drilling rig.
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4. Installed one (1), 2-inch diameter monitoring well, constructed of PVC casing. The well was
finished inside a locking steel cover installed flush with the surrounding grade.
5. Collected soil samples continuously during drilling to the full depth of the borings.
6. Maintained logs of the soils encountered in the boreholes and provided well construction details.
Soils were described in general accordance with the Unified Soil Classification System and
presented on the field logs.
7. Conducted an evaluation of stormwater infiltration rates using the detailed method outlined in
Ecology’s 2014 Stormwater Management Manual, as adopted by the City of Yelm, and provide a
design infiltration rate for stormwater infiltration.
Geotechnical Investigation
8. Excavated a series of six (6) exploratory test pits across the project site using a small, track-
mounted excavator. The test pits were excavated to depths of between approximately 6 to 8 feet
below ground surface (bgs) across the site.
9. Collected representative soil samples from the test pits for possible laboratory analysis.
10. Logged the soils exposed in the test pits in general accordance with ASTM D2487-06.
11. Provided for laboratory testing of seven (7) soil samples for gradation analyses to evaluate bearing
capacity and for stormwater infiltration calculations.
12. Prepared a report summarizing our field activities including our recommendations for site
preparation and grading, bearing capacity, seismic class, temporary and final cut slopes, earth
pressures, and suitability of the on-site soils for use as fill.
FINDINGS
Surface Conditions
The project site is a rectangular shaped parcel situated at an elevation of approximately 340 to 350
feet above mean sea level (MSL) and is currently occupied by a single-family residence. The property
is bounded by Tahoma Boulevard SE to the north, Durant Street SE to the west, and residential
properties to the south and east. The site gently slopes down to the north with an elevation drop of
10 feet across the site. The subject site is vegetated with grasses, scotch broom, and isolated stands
of low growing trees and other shrubs.
Geology
Based on our review of available published geologic maps, Vashon age glacial recessional outwash
gravel deposits underlie the project site. This material is described as poorly-sorted gravel and sand.
This material was deposited by outwash rivers during the waning stages of the most recent glacial
period in the Puget Sound region and is not glacially consolidated.
Subsurface Explorations
We explored subsurface conditions at the site on June 10 and June 14, 2019 by excavating six test
pits and advancing two borings in the locations as shown on the Site Plan, Figure 2. The test pits
were excavated by Insight Geologic using a track-mounted excavator. The exploratory borings were
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completed by Holocene Drilling using a truck-mounted hollow stem auger drill rig. A geologist from
Insight Geologic monitored the explorations and maintained a log of the conditions encountered. The
test pits were completed to depths of 6 to 8 feet bgs, and the borings were completed to depths of
between 23 and 36.5 feet bgs. The soils were visually classified in general accordance with the system
described in ASTM D2487-06. A copy of the explorations is contained in Attachment A.
Soil Conditions
The explorations were generally consistent across the site. Underlying approximately 6 inches of sod,
we generally encountered between 1.5 to 2 feet of dark brown, poorly- to well-graded gravel and sand
with cobbles and varying levels of silt and organics (GP-GM, GP), in a loose and moist condition.
Underlying the dark brown unit, we encountered brown poorly- to well-graded gravels with cobbles
and varying percentages of sand (GP, GW) to poorly graded sands with gravels and cobbles and
varying percentages of silt (SP, SP-SM), in a loose to very dense and moist to wet condition to the
base of the explorations. In general, soils increased in compaction with depth.
The soils encountered are consistent with Nisqually loamy fine sand and Spanaway gravelly sandy
loam, which are mapped for the area. In general, the Nisqually loamy fine sand is mapped along the
north quarter of the site, while the Spanaway gravelly sandy loam is mapped on the remainder of the
property. These soils are generally formed from sandy and gravely glacial outwash and generally has
restrictive layers occurring greater than 7 feet below grade. Percolation is generally high, with rates
between 1.98 and 5.95 inches per hour, according to the U.S. Department of Agriculture Soil Survey.
Groundwater Conditions
Groundwater was encountered in boring MW-1 at a depth of 32 feet bgs. Groundwater was not
encountered in any of the remaining explorations completed on-site. The explorations were completed
during the summer season at a time that generally correlates to a lower groundwater elevation. In
addition, no evidence of high groundwater was encountered within the explorations at the site.
Laboratory Testing
We selected seven soil samples for gradation analyses in general accordance with ASTM D422 to
define soil class and obtain parameters for stormwater infiltration calculations. Our laboratory test
results are provided in Attachment B.
STORMWATER INFILTRATION
We completed a stormwater infiltration rate evaluation in general accordance with the Washington
State Department of Ecology Stormwater Manual for Western Washington (2014 Manual) as adopted
by the City of Yelm. For the purposes of this evaluation, we selected Method 3 “Soil Grain Size
Analysis Method”. The 2014 Manual utilizes the relationship between the D10, D60, and D90 results of
the ASTM grain-size distribution analyses, along with site specific correction factors to estimate long-
term design infiltration rates of each infiltration facility.
Based on our gradation analyses, we estimate that the long-term design infiltration rate (Fdesign) for the
proposed stormwater infiltration is between 1.6 and 20 inches per hour, after applying the appropriate
correction factors. The range of infiltration rate is the result of varying percentages of fines in the soil
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profile. Our calculations assume that the stormwater infiltration will occur at a depth of at least 3 feet
bgs or below the upper gravel with sand and silt unit. Changes to these infiltration rates are possible
depending on the depth to groundwater during winter months. For the purposes of stormwater
infiltration on this project, we recommend using an infiltration rate of 2.9 inches per hour for the pond
area and 5 inches per hour for roof downspouts in the central portion of the site.
Table 1. Design Infiltration Rates – Detailed Method
Exploration Unit Depth Range
(feet) D10 Value D60 Value D90 Value
Long Term Design
Infiltration Rate
(Inches per hour)
TP-2 GW 3.0 – 8.0 7.9 44 130 20
TP-5 SP 2.0 – 8.0 0.31 3.2 51 1.6
MW-1 GP 25.0 – 26.5 0.35 14 30
2.9 MW-1 GW 30.0 – 31.5 0.26 8.5 18
B-1 SP-SM 10.0 – 11.5 0.14 2.1 25
SEISMIC DESIGN CONSIDERATIONS
General
We understand that seismic design will likely be performed using the 2015 IBC standards. The
following parameters may be used in computing seismic base shear forces:
Table 2. 2015 IBC Seismic Design Parameters
Spectral Response Accel. at Short Periods (SS) = 1.25
Spectral Response Accel. at 1 Second Periods (S1) = 0.50
Site Class = D
Site Coefficient (FA) = 1.0
Site Coefficient (FV) = 1.5
A full report for the seismic design parameters is presented in Attachment C.
Ground Rupture
Because of the location of the site with respect to the nearest known active crustal faults, and the
presence of a relatively thick layer of glacial outwash deposits, it is our opinion that the risk of ground
rupture at the site due to surface faulting is low.
Soil Liquefaction
Liquefaction refers to a condition where vibration or shaking of the ground, usually from earthquake
forces, results in the development of excess pore water pressures in saturated soils, and a subsequent
loss of stiffness in the soil occurs. Liquefaction also causes a temporary reduction of soil shear
strength and bearing capacity, which can cause settlement of the ground surface above the liquefied
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soil layers. In general, soils that are most susceptible to liquefaction include saturated, loose to
medium dense, clean to silty sands and non-plastic silts within 50 feet of ground surface.
Based on our review of the Liquefaction Susceptibility Map of Thurston County (Palmer, 2004), the
project site is identified to have a very low potential risk for soil liquefaction. Based on our experience
with detailed seismic studies in the Yelm area, including areas that are mapped within the same
recessional outwash soil deposits as the project site, we concur with the reviewed map. It is our
opinion that there is a low risk for soil liquefaction at the site.
Seismic Compression
Seismic compression is defined as the accrual of contractive volumetric strains in unsaturated soils
during strong shaking from earthquakes (Stewart et al., 2004). Loose to medium dense clean sands
and non-plastic silts are particularly prone to seismic compression settlement. Seismic compression
settlement is most prevalent on slopes, but it can also occur on flat ground. It is our opinion that the
upper 15 feet of the soil profile at the site has a moderate risk for seismic compression settlement.
Seismic Settlement Discussion
Based on the materials encountered in our explorations, it is our preliminary opinion that seismic
settlements (liquefaction-induced plus seismic compression) could potentially total a few inches at the
site as the result of an IBC design level earthquake. We are available upon request to perform deep
subsurface explorations and detailed seismic settlement estimates during the design phase.
Seismic Slope Instability
The maximum inclination of the site is approximately 2 percent and we did not observe signs of slope
instability during our site work. In our opinion, there is a very low risk of seismic slope instability at the
project site under current conditions.
Lateral Spreading
Lateral spreading involves the lateral displacement of surficial blocks of non-liquefied soil when an
underlying soil layer liquefies. Lateral spreading generally develops in areas where sloping ground or
large grade changes are present. Based on our limited understanding of the subsurface conditions at
the site, it is our opinion that there is a low risk for the development of lateral spreading as a result of
an IBC design level earthquake.
CONCLUSIONS AND RECOMMENDATIONS
General
Based on the results of our subsurface explorations and engineering analyses, it is our opinion that
the proposed development is feasible from a geotechnical standpoint. We recommend that the
proposed structures be supported on shallow concrete foundations that are designed using an
allowable soil bearing capacity of 2,500 pounds per square foot (psf).
The soils encountered in our explorations are typically in a loose condition near ground surface. To
limit the potential for structure settlement, we recommend that shallow foundations and slabs-on-grade
be established on a minimum 1-foot thick layer of structural fill. Depending on final grading plans and
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the time of year earthwork is performed; it could be practical to reuse the on-site soils as structural fill
under the foundations/slabs.
Stormwater infiltration at the site is feasible. We propose a design infiltration rate of 2.9 inches per
hour for the stormwater infiltration systems, based on the assumption that stormwater infiltration will
occur within the clean gravels and sands below a depth of about 3 feet bgs. This value is based on
an idealized soil column located in the area of the proposed stormwater infiltration trench on the north
side of the site. It may be possible to increase the infiltration rate with additional testing such as a
Pilot Infiltration Test in the location of the proposed infiltration facility.
Alternatively, based on the U.S. Department of Agriculture Soil Survey map, areas of increased
infiltration may be present within the Spanaway gravelly sandy loam mapped on the southern portions
of the site. Additional evaluation of this area at depth would be required for a more detailed analysis.
Earthwork
General
We anticipate that site development earthwork will include removing the existing vegetation, stripping
sod/topsoil materials, preparing subgrades, excavating for utility trenches, and placing and compacting
structural fill. We expect that the majority of site grading can be accomplished with conventional
earthmoving equipment in proper working order.
Our explorations did not encounter appreciable amounts of debris or unsuitable soils associated with
past site development. Still, it is possible that concrete slabs, abandoned utility lines or other
development features could be encountered during construction. The contractor should be prepared
to deal with these conditions.
Clearing and Stripping
Clearing and stripping should consist of removing surface and subsurface deleterious materials
including sod/topsoil, trees, brush, debris and other unsuitable loose/soft or organic materials.
Stripping and clearing should extend at least 5 feet beyond all structures and areas to receive
structural fill.
We estimate that a stripping depth of about 0.5 feet will be required to remove the sod encountered in
several of our explorations. Deeper stripping depths may be required if additional unsuitable soils are
exposed during stripping operations. We recommend that trees be removed by overturning so that
the majority of roots are also removed. Depressions created by tree or stump removal should be
backfilled with structural fill and properly compacted.
Subgrade Preparation
After stripping and excavating to the proposed subgrade elevation, and before placing structural fill or
foundation concrete, the exposed subgrade should be thoroughly compacted to a firm and unyielding
condition. The exposed subgrade should then be proof-rolled using loaded, rubber-tired heavy
equipment. We recommend that Insight Geologic be retained to observe the proof-rolling prior to
placement of structural fill or foundation concrete. Areas of limited access that cannot be proof-rolled
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can be evaluated using a steel probe rod. If soft or otherwise unsuitable areas are revealed during
proof-rolling or probing, that cannot be compacted to a stable and uniformly firm condition, we
generally recommend that: 1) the subgrade soils be scarified (e.g., with a ripper or farmer’s disc),
aerated and recompacted; or 2) the unsuitable soils be overexcavated and replaced with structural fill.
Temporary Excavations and Groundwater Handling
Excavations deeper than 4 feet should be shored or laid back at a stable slope if workers are required
to enter. Shoring and temporary slope inclinations must conform to the provisions of Title 296
Washington Administrative Code (WAC), Part N, “Excavation, Trenching and Shoring.” Regardless of
the soil type encountered in the excavation, shoring, trench boxes or sloped sidewalls were required
under the Washington Industrial Safety and Health Act (WISHA). The contract documents should
specify that the contractor is responsible for selecting excavation and dewatering methods, monitoring
the excavations for safety and providing shoring, as required, to protect personnel and structures.
In general, temporary cut slopes should be inclined no steeper than about 1.5H:1V (horizontal:
vertical). This guideline assumes that all surface loads are kept at a minimum distance of at least one-
half the depth of the cut away from the top of the slope, and that significant seepage is not present on
the slope face. Flatter cut slopes were necessary where significant seepage occurs or if large voids
are created during excavation. Some sloughing and raveling of cut slopes should be expected.
Temporary covering with heavy plastic sheeting should be used to protect slopes during periods of
wet weather.
We anticipate that if perched groundwater is encountered during construction can be handled
adequately with sumps, pumps, and/or diversion ditches. Groundwater handling needs will generally
be lower during the late summer and early fall months. We recommend that the contractor performing
the work be made responsible for controlling and collecting groundwater encountered during
construction.
Permanent Slopes
We do not anticipate that permanent slopes will be utilized for the proposed project. If permanent
slopes are necessary, we recommend the slopes be constructed at a maximum inclination of 2H:1V.
Where 2H:1V permanent slopes are not feasible, protective facings and/or retaining structures should
be considered.
To achieve uniform compaction, we recommend that fill slopes be overbuilt and subsequently cut back
to expose well-compacted fill. Fill placement on slopes should be benched into the slope face and
include keyways. The configuration of the bench and keyway depends on the equipment being used.
Bench excavations should be level and extend into the slope face. We recommend that a vertical cut
of about 3 feet be maintained for benched excavations. Keyways should be about 1-1/2 times the
width of the equipment used for grading or compaction.
Erosion Control
We anticipate that erosion control measures such as silt fences, straw bales and sand bags will
generally be adequate during development. Temporary erosion control should be provided during
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construction activities and until permanent erosion control measures are functional. Surface water
runoff should be properly contained and channeled using drainage ditches, berms, swales, and
tightlines, and should not discharge onto sloped areas. Any disturbed sloped areas should be
protected with a temporary covering until new vegetation can take effect. Jute or coconut fiber matting,
excelsior matting or clear plastic sheeting is suitable for this purpose. Graded or disturbed slopes
should be tracked in-place with the equipment running perpendicular to the slope contours so that the
track marks provide a texture to help resist erosion. Ultimately, erosion control measures should be
in accordance with local regulations and should be clearly described on project plans.
Wet Weather Earthwork
Some of the near surface soils contain up to about 7 percent fines. When the moisture content of the
soil is more than a few percent above the optimum moisture content, the soil will become unstable and
it may become difficult or impossible to meet the required compaction criteria. Disturbance of near
surface soils should be expected if earthwork is completed during periods of wet weather.
The wet weather season in this area generally begins in October and continues through May.
However, periods of wet weather may occur during any month of the year. If wet weather earthwork
is unavoidable, we recommend that:
The ground surface is sloped so that surface water is collected and directed away from the work
area to an approved collection/dispersion point.
Earthwork activities not take place during periods of heavy precipitation.
Slopes with exposed soil be covered with plastic sheeting or otherwise protected from erosion.
Measures are taken to prevent on-site soil and soil stockpiles from becoming wet or unstable.
Sealing the surficial soil by rolling with a smooth-drum roller prior to periods of precipitation should
reduce the extent that the soil becomes wet or unstable.
Construction traffic is restricted to specific areas of the site, preferably areas that are surfaced with
materials not susceptible to wet weather disturbance.
A minimum 1-foot thick layer of 4- to 6-inch quarry spalls is used in high traffic areas of the site to
protect the subgrade soil from disturbance.
Contingencies are included in the project schedule and budget to allow for the above elements.
Structural Fill Materials
General
Material used for structural fill should be free of debris, organic material and rock fragments larger
than 3 inches. The workability of material for use as structural fill will depend on the gradation and
moisture content of the soil. As the amount of fines increases, soil becomes increasingly more
sensitive to small changes in moisture content and adequate compaction becomes more difficult or
impossible to achieve.
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On-Site Soil
We anticipate that the majority of the on-site soils encountered during construction will consist of
gravels, cobbles and sands, located at or near the surface of the site. It is our opinion, that this material
is a suitable source for structural fill during a significant portion of the year. On-site materials used as
structural fill should be free of roots, organic matter and other deleterious materials and particles larger
than 3 inches in diameter. Significant quantities of material greater than 3 inches in diameter were
observed during our site explorations. This material will cause significand difficulties in soil grading
and compaction efforts. We recommend that the material greater than 3 inches in diameter be
screened and removed or crushed for reuse on-site.
Select Granular Fill
Select granular fill should consist of imported, well-graded sand and gravel or crushed rock with a
maximum particle size of 3 inches and less than 5 percent passing a U.S. Standard No. 200 sieve
based on the minus ¾-inch fraction. Organic matter, debris or other deleterious material should not
be present. In our experience, “gravel borrow” as described in Section 9-03.14(1) of the 2018 WSDOT
Standard Specifications is typically a suitable source for select granular fill during periods of wet
weather, provided that the percent passing a U.S. Standard No. 200 sieve is less than 5 percent based
on the minus ¾-inch fraction.
Structural Fill Placement and Compaction
General
Structural fill should be placed on an approved subgrade that consists of uniformly firm and unyielding
inorganic native soils or compacted structural fill. Structural fill should be compacted at a moisture
content near optimum. The optimum moisture content varies with the soil gradation and should be
evaluated during construction.
Structural fill should be placed in uniform, horizontal lifts and uniformly densified with vibratory
compaction equipment. The maximum lift thickness will vary depending on the material and
compaction equipment used, but should generally not exceed the loose thicknesses provided on Table
3. Structural fill materials should be compacted in accordance with the compaction criteria provided
in Table 4.
Table 3. Recommended Uncompacted Lift Thickness
Compaction
Equipment
Recommended Uncompacted Fill Thickness
(inches)
Granular Materials
Maximum Particle Size
1 1/2 inch
Granular Materials Maximum Particle Size >
1 1/2 inch
Hand Tools (Plate Compactors
and Jumping Jacks) 4 – 8 Not Recommended
Rubber-tire Equipment 10 – 12 6 – 8
Light Roller 10 – 12 8 – 10
Heavy Roller 12 – 18 12 – 16
Hoe Pack Equipment 18 – 24 12 – 16
Note: The above table is intended to serve as a guideline and should not be included in the project specifications.
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Table 4. Recommended Compaction Criteria in Structural Fill Zones
Fill Type
Percent Maximum Dry Density Determined by
ASTM Test Method D 1557 at ±3% of Optimum Moisture
0 to 2 Feet Below
Subgrade
> 2 Feet Below
Subgrade Pipe Zone
Imported or On-site Granular,
Maximum Particle Size < 1-1/4-inch 95 95 -----
Imported or On-site Granular,
Maximum Particle Size >1-1/4-inch N/A (Proof-roll) N/A (Proof-roll) -----
Trench Backfill1 95 92 90
Note: 1Trench backfill above the pipe zone in nonstructural areas should be compacted to at least 85 percent.
Shallow Foundation Support
General
We recommend that the proposed structures be founded on continuous wall or isolated column
footings, bearing on a minimum 1-foot thick overexcavation and replacement with compacted
structural fill where underlying soils are not able to be compacted as structural fill. The structural fill
zone should extend to a horizontal distance equal to the overexcavation depth on each side of the
footing. The actual overexcavation depth will vary, depending on the conditions encountered.
We recommend that a representative from Insight Geologic observe the foundation surfaces before
overexcavation, and before placing structural fill in overexcavations. This representative should
confirm that adequate bearing surfaces have been prepared and that the soil conditions are as
anticipated. Unsuitable foundation bearing soils should be recompacted or removed and replaced
with compacted structural fill, as recommended by the geotechnical engineer.
Bearing Capacity and Footing Dimensions
We recommend an allowable soil bearing pressure of 2,500 psf for shallow foundations that are
supported as recommended. This allowable bearing pressure applies to long-term dead and live loads
exclusive of the weight of the footing and any overlying backfill. The allowable soil bearing pressure
can be increased by one-third when considering total loads, including transient loads such as those
induced by wind and seismic forces.
We recommend a minimum width of 18 inches for continuous wall footings and 2 feet for isolated
column footings. For settlement considerations, we have assumed a maximum width of 4 feet for
continuous wall footings and 6 feet for isolated column footings.
Perimeter footings should be embedded at least 12 inches below the lowest adjacent grade where the
ground is flat. Interior footings should be embedded a minimum of 6 inches below the nearest adjacent
grade.
Settlement
We estimate that total settlement of footings that are designed and constructed as recommended
should be less than 1 inch. We estimate that differential settlements should be ½ inch or less between
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comparably loaded isolated footings or along 50 feet of continuous footing. We anticipate that the
settlement will occur essentially as loads are applied during construction.
Lateral Load Resistance
Lateral loads on shallow foundation elements may be resisted by passive resistance on the sides of
footings and by friction on the base of footings. Passive resistance may be estimated using an
equivalent fluid density of 303 pounds per cubic foot (pcf), assuming that the footings are backfilled
with structural fill. Frictional resistance may be estimated using 0.25 for the coefficient of base friction.
The lateral resistance values provided above incorporate a factor of safety of 1.5. The passive earth
pressure and friction components can be combined, provided that the passive component does not
exceed two-thirds of the total. The top foot of soil should be neglected when calculating passive
resistance, unless the foundation perimeter area is covered by a slab-on-grade or pavement.
Slabs-On-Grade
Slabs-on-grade should be established on a minimum 1-foot thick section of structural fill extending to
an approved bearing surface. A modulus of vertical subgrade reaction (subgrade modulus) can be
used to design slabs-on-grade. The subgrade modulus varies based on the dimensions of the slab
and the magnitude of applied loads on the slab surface; slabs with larger dimensions and loads are
influenced by soils to a greater depth. We recommend a modulus value of 300 pounds per cubic inch
(pci) for design of on-grade floor slabs with floor loads up to 500 psf. We are available to provide
alternate subgrade modulus recommendations during design, based on specific loading information.
We recommend that slabs-on-grade in interior spaces be underlain by a minimum 4-inch thick capillary
break layer to reduce the potential for moisture migration into the slab. The capillary break material
should consist of a well-graded sand and gravel or crushed rock containing less than 5 percent fines
based on the fraction passing the ¾-inch sieve. The 4-inch thick capillary break layer can be included
when calculating the minimum 1-foot thick structural fill section beneath the slab. If dry slabs are
required (e.g., where adhesives are used to anchor carpet or tile to the slab), a waterproofing liner
should be placed below the slab to act as a vapor barrier.
Subsurface Drainage
It is our opinion that foundation footing drains and underslab drains are likely unnecessary for the
proposed structures. The majority of subsurface site soils are well draining and it is unlikely that
subsurface drains would produce water. The soils are suitable for roof runoff drywells and should be
classified as Group A for the purposes of design.
Conventional Retaining Walls
General
We do not anticipate that retaining walls will be utilized for the proposed project. We should be
contacted during the design phase to review retaining wall plans and provide supplemental
recommendations, if needed.
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Drainage
Positive drainage is imperative behind any retaining structure. This can be accomplished by using a
zone of free-draining material behind the wall with perforated pipes to collect water seepage. The
drainage material should consist of coarse sand and gravel containing less than 5 percent fines based
on the fraction of material passing the ¾-inch sieve. The wall drainage zone should extend horizontally
at least 12 inches from the back of the wall. If a stacked block wall is constructed, we recommend
that a barrier such as a non-woven geotextile filter fabric be placed against the back of the wall to
prevent loss of the drainage material through the wall joints.
A perforated smooth-walled rigid PVC pipe, having a minimum diameter of 4 inches, should be placed
at the bottom of the drainage zone along the entire length of the wall. Drainpipes should discharge to
a tightline leading to an appropriate collection and disposal system. An adequate number of cleanouts
should be incorporated into the design of the drains in order to provide access for regular maintenance.
Roof downspouts, perimeter drains or other types of drainage systems should not be connected to
retaining wall drain systems.
Design Parameters
We recommend an active lateral earth pressure of 37 pcf (equivalent fluid density) for a level backfill
condition. This assumes that the top of the wall is not structurally restrained and is free to rotate. For
restrained walls that are fixed against rotation (at-rest condition), an equivalent fluid density of 56 pcf
can be used for the level backfill condition. For seismic conditions, we recommend a uniform lateral
pressure of 14H psf (where H is the height of the wall) be added to the lateral pressures. This seismic
pressure assumes a peak ground acceleration of 0.32 g. Note that if the retaining system is designed
as a braced system but is expected to yield a small amount during a seismic event, the active earth
pressure condition may be assumed and combined with the seismic surcharge.
The recommended earth pressure values do not include the effects of surcharges from surface loads
or structures. If vehicles were operated within one-half the height of the wall, a traffic surcharge should
be added to the wall pressure. The traffic surcharge can be approximated by the equivalent weight of
an additional 2 feet of backfill behind the wall. Other surcharge loads, such as construction equipment,
staging areas and stockpiled fill, should be considered on a case-by-case basis.
DOCUMENT REVIEW AND CONSTRUCTION OBSERVATION
We recommend that we be retained to review the portions of the plans and specifications that pertain
to earthwork construction and stormwater infiltration. We recommend that monitoring, testing and
consultation be performed during construction to confirm that the conditions encountered are
consistent with our explorations and our stated design assumptions. Insight Geologic would be
pleased to provide these services upon request.
REFERENCES
International Code Council, International Building Code, 2015.
Seismic Compression of As-compacted Fill Soils with Variable Levels of Fines Content and Fines
Plasticity, Department of Civil and Environmental Engineering, University of California, Los
Angeles, July 2004.
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Washington State Department of Transportation (WSDOT), Standard Specifications for Road, Bridge
and Municipal Construction Manual, 2018.
Washington State Department of Ecology (WSDOE), Stormwater Management Manual of Western
Washington, 2014.
LIMITATIONS
We have prepared this geotechnical and stormwater investigation report for the exclusive use of C &
E Developments LLC and their authorized agents, for the proposed development located at 15025
Tahoma Boulevard SE in Yelm, Washington.
Within the limitations of scope, schedule and budget, our services have been executed in accordance
with generally accepted practices in the field of geotechnical engineering in this area at the time this
report was prepared. No warranty or other conditions, expressed or implied, should be understood.
Please refer to Attachment D titled “Report Limitations and Guidelines for Use” for additional
information pertaining to use of this report.
__________________________
We appreciate the opportunity to be of service to you on this project. Please contact us if you have
questions or require additional information.
Respectfully Submitted,
Insight Geologic, Inc.
William E. Halbert, L.E.G., L.HG.
Principal
Attachments
Insight Geologic, Inc.
FIGURES
Insight Geologic, Inc.
ATTACHMENT A
EXPLORATION LOGS
Insight Geologic, Inc.
ATTACHMENT B
LABORATORY ANALYSES RESULTS
Job Name:Wyndstone Sample Location:TP-2
Job Number:1142-001-01 Sample Name:TP-2 0.5'-3.0'
Date Tested:7/1/19 Depth:0.5 - 3 Feet
Tested By:Kevin Vandehey
4.3%
Percent Percent by
Sieve Size Passing Size Fraction Weight
3.0 in. (75.0) 100.0 Coarse Gravel 68.7
1.5 in. (37.5) 51.9 Fine Gravel 9.7
3/4 in. (19.0) 31.3
3/8 in. (9.5-mm) 24.9 Coarse Sand 2.8
No. 4 (4.75-mm) 21.6 Medium Sand 6.9
No. 10 (2.00-mm) 18.8 Fine Sand 7.4
No. 20 (.850-mm) 15.9
No. 40 (.425-mm) 11.9 Fines 4.5
No. 60 (.250-mm) 8.6 Total 100.0
No. 100 (.150-mm) 6.1
No. 200 (.075-mm) 4.5
LL - -
PL - -
Pl - -
D10 0.31
D30 17.00
D60 41.00
D90 65.00
Cc 22.74
Cu 132.26
ASTM Classification
Group Name:Poorly Graded Gravel with Sand
Symbol:GP
Gradation Analysis Summary Data
Moisture Content (%)
Job Name:Wyndstone Sample Location:TP-2
Job Number:1142-001-01 Sample Name:TP-2 3.0'-8.0'
Date Tested:7/1/19 Depth:3 - 8 Feet
Tested By:Kevin Vandehey
1.2%
Percent Percent by
Sieve Size Passing Size Fraction Weight
3.0 in. (75.0) 64.7 Coarse Gravel 72.7
1.5 in. (37.5) 57.3 Fine Gravel 21.2
3/4 in. (19.0) 27.3
3/8 in. (9.5-mm) 12.4 Coarse Sand 3.2
No. 4 (4.75-mm) 6.0 Medium Sand 1.8
No. 10 (2.00-mm) 2.8 Fine Sand 0.7
No. 20 (.850-mm) 1.7
No. 40 (.425-mm) 0.9 Fines 0.2
No. 60 (.250-mm) 0.5 Total 100.0
No. 100 (.150-mm) 0.3
No. 200 (.075-mm) 0.2
LL - -
PL - -
Pl - -
D10 7.90
D30 20.50
D60 44.00
D90 130.00
Cc 1.21
Cu 5.57
ASTM Classification
Group Name:Well Graded Gravel
Symbol:GW
Gradation Analysis Summary Data
Moisture Content (%)
Job Name:Wyndstone Sample Location:TP-5
Job Number:1142-001-01 Sample Name:TP-5 0.5'-2.0'
Date Tested:7/1/19 Depth:0.5 - 2 Feet
Tested By:Kevin Vandehey
7.0%
Percent Percent by
Sieve Size Passing Size Fraction Weight
3.0 in. (75.0) 100.0 Coarse Gravel 47.3
1.5 in. (37.5) 67.9 Fine Gravel 15.5
3/4 in. (19.0) 52.7
3/8 in. (9.5-mm) 42.9 Coarse Sand 4.9
No. 4 (4.75-mm) 37.2 Medium Sand 15.6
No. 10 (2.00-mm) 32.3 Fine Sand 9.5
No. 20 (.850-mm) 26.6
No. 40 (.425-mm) 16.7 Fines 7.2
No. 60 (.250-mm) 11.5 Total 100.0
No. 100 (.150-mm) 9.2
No. 200 (.075-mm) 7.2
LL - -
PL - -
Pl - -
D10 0.18
D30 1.40
D60 28.00
D90 60.00
Cc 0.39
Cu 155.56
ASTM Classification
Group Name:Poorly Graded Gravel with Sand and Silt
Symbol:GP-GM
Gradation Analysis Summary Data
Moisture Content (%)
Job Name:Wyndstone Sample Location:TP-5
Job Number:1142-001-01 Sample Name:TP-5 2.0'-8.0'
Date Tested:7/1/19 Depth:2 - 8 Feet
Tested By:Kevin Vandehey
4.9%
Percent Percent by
Sieve Size Passing Size Fraction Weight
3.0 in. (75.0) 100.0 Coarse Gravel 31.3
1.5 in. (37.5) 81.7 Fine Gravel 7.3
3/4 in. (19.0) 68.7
3/8 in. (9.5-mm) 64.5 Coarse Sand 3.7
No. 4 (4.75-mm) 61.4 Medium Sand 39.1
No. 10 (2.00-mm) 57.7 Fine Sand 17.1
No. 20 (.850-mm) 48.6
No. 40 (.425-mm) 18.5 Fines 1.5
No. 60 (.250-mm) 5.2 Total 100.0
No. 100 (.150-mm) 2.6
No. 200 (.075-mm) 1.5
LL - -
PL - -
Pl - -
D10 0.31
D30 0.55
D60 3.20
D90 51.00
Cc 0.30
Cu 10.32
ASTM Classification
Group Name:Poorly Graded Sand with Gravel
Symbol:SP
Moisture Content (%)
Gradation Analysis Summary Data
Job Name:Wyndstone Sample Location:MW-1
Job Number:1142-001-01 Sample Name:MW-1 25.0'-26.5'
Date Tested:7/1/19 Depth:25 - 26.5 Feet
Tested By:Kevin Vandehey
4.5%
Percent Percent by
Sieve Size Passing Size Fraction Weight
3.0 in. (75.0) 100.0 Coarse Gravel 30.9
1.5 in. (37.5) 100.0 Fine Gravel 25.5
3/4 in. (19.0) 69.1
3/8 in. (9.5-mm) 53.5 Coarse Sand 11.3
No. 4 (4.75-mm) 43.6 Medium Sand 20.4
No. 10 (2.00-mm) 32.3 Fine Sand 8.6
No. 20 (.850-mm) 20.3
No. 40 (.425-mm) 11.9 Fines 3.3
No. 60 (.250-mm) 7.6 Total 100.0
No. 100 (.150-mm) 5.3
No. 200 (.075-mm) 3.3
LL - -
PL - -
Pl - -
D10 0.35
D30 1.70
D60 14.00
D90 30.00
Cc 0.59
Cu 40.00
ASTM Classification
Group Name:Poorly Graded Gravel with Sand
Symbol:GP
Gradation Analysis Summary Data
Moisture Content (%)
Job Name:Wyndstone Sample Location:MW-1
Job Number:1142-001-01 Sample Name:MW-1 30.0'-31.5'
Date Tested:7/1/19 Depth:30 - 31.5 Feet
Tested By:Kevin Vandehey
6.7%
Percent Percent by
Sieve Size Passing Size Fraction Weight
3.0 in. (75.0) 100.0 Coarse Gravel 7.9
1.5 in. (37.5) 100.0 Fine Gravel 47.8
3/4 in. (19.0) 92.1
3/8 in. (9.5-mm) 64.1 Coarse Sand 13.2
No. 4 (4.75-mm) 44.3 Medium Sand 17.9
No. 10 (2.00-mm) 31.1 Fine Sand 9.3
No. 20 (.850-mm) 19.9
No. 40 (.425-mm) 13.2 Fines 3.9
No. 60 (.250-mm) 9.4 Total 100.0
No. 100 (.150-mm) 6.5
No. 200 (.075-mm) 3.9
LL - -
PL - -
Pl - -
D10 0.26
D30 1.80
D60 8.50
D90 18.00
Cc 1.47
Cu 32.69
ASTM Classification
Group Name:Well Graded Gravel with Sand
Symbol:GW
Moisture Content (%)
Gradation Analysis Summary Data
Job Name:Wyndstone Sample Location: B-1
Job Number:1142-001-01 Sample Name: B-1 10.0'-11.5'
Date Tested:7/1/19 Depth:10 - 11.5 Feet
Tested By:Kevin Vandehey
3.9%
Percent Percent by
Sieve Size Passing Size Fraction Weight
3.0 in. (75.0)100.0 Coarse Gravel 15.0
1.5 in. (37.5)100.0 Fine Gravel 12.7
3/4 in. (19.0)85.0
3/8 in. (9.5-mm) 77.6 Coarse Sand 13.7
No. 4 (4.75-mm) 72.3 Medium Sand 31.1
No. 10 (2.00-mm) 58.7 Fine Sand 22.3
No. 20 (.850-mm) 42.5
No. 40 (.425-mm) 27.6 Fines 5.3
No. 60 (.250-mm) 17.2 Total 100.0
No. 100 (.150-mm) 10.6
No. 200 (.075-mm) 5.3
LL - -
PL - -
Pl - -
D10 0.14
D30 0.47
D60 2.10
D90 25.00
Cc 0.75
Cu 15.00
ASTM Classification
Group Name:Poorly Graded Sand with Gravel and Silt
Symbol:SP-SM
Moisture Content (%)
Gradation Analysis Summary Data
01020304050607080901000.0010.010.11101001000Percent Passing by Weight Grain Size in MillimetersU.S. Standard Sieve SizeTP-2 0.5'-3.0'TP-2 3.0'-8.0'TP-5 0.5'-2.0'TP-5 2.0'-8.0'COBBLESGRAVELSILT OR CLAYSANDCOARSEMEDIUMFINECOARSEFINE3" 1.5" 3/4" 3/8" #4 #10 #20 #40 #60 #100 #200Graph 1Gradation Analysis ResultsWYNDSTONEYELM, WASHINGTON
01020304050607080901000.0010.010.11101001000Percent Passing by Weight Grain Size in MillimetersU.S. Standard Sieve SizeMW-1 25.0'-26.5'MW-1 30.0'-31.5'B-1 10.0'-11.5'COBBLESGRAVELSILT OR CLAYSANDCOARSEMEDIUMFINECOARSEFINE3" 1.5" 3/4" 3/8" #4 #10 #20 #40 #60 #100 #200Graph 2Gradation Analysis ResultsWYNDSTONEYELM, WASHINGTON
Insight Geologic, Inc.
ATTACHMENT C
SIESMIC DESIGN PARAMETERS
Hazards by Location
1 of 2
Search Information
Coordinates:46.94455144795768, -122.62151451110839
Elevation:350 ft
Timestamp:2019-07-10T17:29:07.126Z
Hazard Type:Seismic
Reference Document:IBC-2015
Risk Category:IV
Site Class:D
MCER Horizontal Response Spectrum Design Horizontal Response Spectrum
Basic Parameters
Name Value Description
SS 1.251 MCER ground motion (period=0.2s)
S1 0.499 MCER ground motion (period=1.0s)
SMS 1.251 Site-modified spectral acceleration value
SM1 0.749 Site-modified spectral acceleration value
SDS 0.834 Numeric seismic design value at 0.2s SA
SD1 0.5 Numeric seismic design value at 1.0s SA
0 5 10 15 Period (s)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
Sa(g)
0 5 10 15 Period (s)
0.00
0.20
0.40
0.60
0.80
Sa(g)
2 of 2
Insight Geologic, Inc.
ATTACHMENT D
REPORT LIMITATIONS AND GUIDELINES FOR USE
Insight Geologic, Inc. Limitations
ATTACHMENT D
REPORT LIMITATIONS AND GUIDELINES FOR USE1
This attachment provides information to help you manage your risks with respect to the use of this
report.
GEOTECHNICAL SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES, PERSONS
AND PROJECTS
This report has been prepared for the exclusive use of C & E Developments LLC (Client) and their
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A GEOTECHNICAL ENGINEERING OR GEOLOGIC REPORT IS BASED ON A UNIQUE SET
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Insight Geologic, Inc. considered a number of unique, project-specific factors when establishing the
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not prepared for you,
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not prepared for the specific site explored, or
completed before important project changes were made.
For example, changes that can affect the applicability of this report include those that affect:
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elevation, configuration, location, orientation or weight of the proposed structure;
composition of the design team; or
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If important changes are made after the date of this report, Insight Geologic should be given the
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1 Developed based on material provided by ASFE, Professional Firms Practicing in the Geosciences; www.asfe.org .
Insight Geologic, Inc. Limitations
SUBSURFACE CONDITIONS CAN CHANGE
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GEOTECHNICAL ENGINEERING REPORT RECOMMENDATIONS ARE NOT FINAL
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Insight Geologic, Inc. Limitations
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Some owners and design professionals believe they can make contractors liable for unanticipated
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READ THESE PROVISIONS CLOSELY
Some clients, design professionals and contractors may not recognize that the geoscience practices
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INTERCHANGED
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geotechnical or geologic concerns regarding a specific project.
TO: Casey Peterson
FROM: William Halbert, L.E.G., L.Hg.
DATE: July 26, 2019
PROJECT: 1142-001-02 Wyndstone Residential
SUBJECT: Supplemental Infiltration Rate Evaluation
At the request of Peterson Brothers LLC, we have conducted a supplemental evaluation for the
proposed stormwater infiltration at the Wyndstone multi-family residential development to be located
15025 Tahoma Boulevard SE in Yelm, Washington.
Our previous investigations and evaluation of design stormwater infiltration rates for the project, using
the “Detailed Approach” as described in the Department of Ecology’s 2014 Stormwater Management
Manual for Western Washington (2014 Manual), as adopted by the City of Yelm, produced artificially
low infiltration rates for the site based on similar sites in the area in similar soils. It was decided that
we also run a full-scale Pilot Infiltration Test (PIT) as a more realistic method of determining the
infiltration rate of the soil. On July 24, 2019, we completed two stormwater infiltration rate evaluations
in general accordance with the 2014 Manual consisting of full-scale PITs. The PITs were performed
at the north and south side of the site at a depth of 5 feet below ground surface.
For the PITs, a 10-foot by 10-foot area was excavated to a depth of about 5 feet below ground surface.
The PIT located on the north side of the site was located within the area of the proposed stormwater
infiltration gallery. A second PIT was excavated on the south side of the site for comparison purposes.
The base of the excavations correlated to the approximate elevation of the base of the proposed
stormwater infiltration gallery. The soils exposed in the base of the excavations consisted of fine to
course gravel and cobbles with sand and trace silt, which was consistent with our previous
observations.
Water was added to the excavations using a water tuck provided by Peterson Brothers LLC to saturate
the underlying soils. Datalogging pressure transducers were placed in the bottom of the excavations
to provide a constant record of the water level during the PITs.
Despite adding approximately 4,000 gallons of water to PIT-1 at the maximum rate available to the
water truck, we were unable to develop standing water in the base of the PIT excavation. Water levels
were able to be maintained in in PIT-2 until the water truck was drained and then the excavation
drained in approximately 15 minutes. The water levels over time for PIT-1 and PIT-2 are shown in
Figure 1 and Figure 2, below. The initial infiltration rate was calculated using the fall of the water level
in inches over time.
MEMORANDUM
1015 East 4th Avenue
Olympia, Washington 98506
Telephone: (360) 754-2128
Fax: (360) 754-9299
Wyndstone Supplemental
July 26, 2019
Page 2
Figure 1.
PIT-1 Hydrograph
Figure 2.
PIT-2 Hydrograph
0
0.1
0.2
0.3
0.4
0.5
8:38 AM 8:45 AM 8:52 AM 9:00 AM 9:07 AMWater Depth (Feet)Time
0
0.5
1
1.5
2
2.5
9:36 AM 9:50 AM 10:04 AM 10:19 AM 10:33 AM 10:48 AM 11:02 AM 11:16 AMWater Depth (Feet)Time
Wyndstone Supplemental
July 26, 2019
Page 3
Based on the “Simple Approach” as described in the 2014 Manual, we then applied the appropriate
correction factors to the initial infiltration rates which generated a design infiltration rate of between
132 and 104 inches per hour. However, as the site has a contributing area of larger than 1 acre the
2014 Manual recommends the use of the “Detailed Approach” to determine the design infiltration rate.
Using the additional site-specific correction factors and depth to groundwater utilized in the Detailed
Approach, the design infiltration rate is between 12.2 and 8.3 inches per hour. Based on the gravel
and cobbly nature of the site and that the depth to groundwater is greater than 30 feet below ground
surface, it is our opinion that the reduction in infiltration rate generated by the Detailed Approach is
overly conservative as groundwater mounding is unlikely to develop in the gravel soils at the site. As
a result, we have generated a discretionary correction factor of 0.4 that takes into account the
corrections presented on the Detailed Approach while reducing the correction that is based on
potential mounding effects of the groundwater table. Correction values are shown in Table 1, below.
Our final design infiltration rate based on these revised correction values are between 32 and 21
inches per hour. Please note that this design infiltration rate is based on current site conditions and
may be adjusted depending on significant increases in groundwater elevations during the winter
groundwater monitoring period.
Table 1.
Design Infiltration Rate Calculation
PIT
Initial
Infiltration
Rate (in./hr.)
Testing
Methodology
Correction
Factor
Site
Variability
Correction
Factor
Plugging
Correction
Factor
Discretionary
Correction
Factor
Design
Infiltration
Rate (in./hr.)
PIT-1 132.8 0.75 0.9 0.9 0.4 32.2
PIT-2 86 0.75 0.9 0.9 0.4 20.9
We trust this meets your current requirements. Please contact us if you have questions regarding our
testing.