Sno Co BMPs

The 1979 [SCC 24.10.010; effective date July 1, 1979) and the 1998 [SCC 24.10.040(1); effective date September 19, 1998] versions of the drainage ordinances for Snohomish County stipulated that a Snohomish County Drainage Manual would be developed and adopted. The Director of Public Works formally adopted the 1992 Washington DOE Stormwater Manual, along with a Snohomish County Addendum, as the Snohomish County Drainage Manual (Addendum to Chapter I-1; Introduction, page 1) on September 21, 1998.

The Addendum stipulates that “... In designing and constructing drainage facilities and preparing drainage plans, a person will encounter three different documents: Snohomish County Code (SCC) Title 24, County Engineering Design and Development Standards (EDDS), and the Drainage Manual.” That Introduction section of the Addendum further stipulates that Title 24 contains the basic legal requirements for planning and construction while EDDS and the Drainage Manual contain the design standards.

The approved Best Management Practices (BMPs) for temporary erosion and sediment control (TESC) are contained in Chapter II-5 of the 1992 DOE Stormwater Manual. By adopting the DOE Stormwater Manual as the Snohomish County Manual, the design criteria specified for BMPs in the DOE Manual become the approved design criteria for Snohomish County approved BMPs.

Use of BMPs that are not contained in the DOE Manual require the approval of the Director of Public Works. This is also the pathway for a new technique and for new technologies to become approved TESC BMPs.

This pamphlet contains information about the design criteria and maintenance requirements for the approved TESC BMPs. This pamphlet was prepared because the DOE Stormwater Manual is an imposing document. Reference to the location within the DOE Stormwater Manual is provided for each of the BMPs so that the information contained herein may be compared to the legal document.

Please note that any proposed deviation from the specified design standards requires a Waiver or Modification at Request of Applicant [SCC 24.30.035(2)]. SCC 24.30.035(2) stipulates that, in order for a waiver or modification to be approved, at least one of the following criteria must be met:

· Compliance with the requirement will not substantially achieve the intended purpose because of unusual site conditions;

· Compliance with the requirement will impose an undue hardship on the applicant that does not generally occur when the requirement is met on other sites, and the hardship is due to special circumstances on the site such as topography, location or surroundings, and is not the result of the applicant’s own actions;

· The requirement is not technically feasible;

· The requirement will cause or poses a significant threat of harm to public health, safety or welfare; the environment; or public or private property; which harm outweighs the requirement’s benefit;

· An emergency situation necessitates approval of the waiver or modification;

· No reasonable use of the property is possible unless the waiver or modification is approved; or

· An alternative technology or approach provides a better way to meet or exceed the protections afforded by this title, provided that the director may condition final approval and acceptance of the alternative upon proof of successful operation after construction.

Natural Vegetation and Buffer Zones (Page II-5-14-19; 115, DOE Manual) Top

Function: To stabilize the soil by the plants and plant roots, to break-up the flow of water, and to filter the flow of water through the plants, blades of living grass and the dead grass/leaf litter.

Use:

Preserving natural vegetation should be a component of the site design whenever possible.

Buffers for streams and wetlands are established. (NOTE: Recommended buffers differ from SCC Title 32.10 and Administrative rules for threatened species. SCC 32.10 supercedes the DOE Manual)

Design Criteria

· Natural vegetation can be preserved as individual trees or in natural clumps.

· Maples, cottonwoods and willows have water-seeking roots and should not be used near sewer lines, infiltration areas, filter fields, and drain fields.

· Lowering the natural ground level can seriously damage trees and shrubs. To increase the chances for survival, it is best to limit grade changes and other soil disturbances to areas outside the dripline of the plant.

· Changing the natural ground level will alter grades which affect the plant’s ability to obtain the necessary air, water, and minerals. trees can tolerate fill of 6 inches or less. For shrubs and other plants, the fill should be less. A tile system protects a tree from a raised grade. [A tile system is figured in the DOE Manual]

Temporary Vegetative Cover (Page II-5-2, DOE Manual) Top

Function: To provide temporary soil stabilization where permanent cover is not appropriate but the area needs to be stabilized for months.

Temporary seeding is essential to preserve the integrity of earthen structures used to control sediment, such as dikes, diversions, and the banks and dams of sediment basins.

Design Criteria:

· Application of seed mix may be by hand, by mechanical means and by hydroseeding.

· If the ground slopes >10% for over 100 feet (plan view), mulch or matting is also required.

· Planting should be done:

April 1 to June 30 no additional requirements

July 1 to August 31 irrigation may be required

September 1 to October 31 no additional requirements

November 1 to March 31 mulching shall be required

· Before seeding, install all needed surface control measures, such as gradient terraces, interceptor swales and sediment basins.

· A minimum of 2-4 inches of tilled topsoil is required

Permanent Vegetative Cover (Pages II-5-20; II-5-24, DOE Manual) Top

Function: To establish permanent vegetative cover of grasses, legumes, shrubs and trees as rapidly as possible to prevent soil erosion by wind or water.

SEEDING

Use: (seeding)

· Graded, final graded or cleared areas where permanent vegetative cover is needed to stabilize the soil.

· Areas that will not be brought to final grade for a year or more.

· Vegetation-lined channels.

· Retention or detention ponds, as required.

Design Criteria: (seeding)

· Vegetation cannot be expected to supply an erosion control cover and prevent slippage on a soil that is not stable due to its texture, structure, water movement, or excessive slope.

· Seeding should be done immediately after final shaping, except during the period of November 1 through March 1, when the site should be protected by mulching or plastic covering until the next seeding period.

· Permanent vegetation may be in the form a grass-type growth by seeding or sodding. Establishing this cover may require the use of supplemental materials, such as mulch or jute netting.

SOD

Use: (sodding)

· to provide IMMEDIATE vegetative cover

· waterways carrying intermittent flow

Design Criteria: (sodding)

· apply 2-4 inches of topsoil and add sufficient lime to reach a soil pH value of 6.5 (based upon soil tests).

· till the soil/lime cover into the soil to a soil-depth of 1-2 inches

· smooth the ground and apply the sod

Ground Cover – Flat (Page II-5-5, DOE Manual) Top

Straw, Wood Fiber (blown Cellulose), Bark Chips

Function: To provide immediate protection to exposed soils during the period of shore construction delays or over winter months.

Use:

· any area with a slope < 2:1 (horizontal to vertical); slopes <50%

· all exposed soils without seeding, with temporary seeding or with permanent seeding.

Design Criteria:

· Apply mulch to a minimum depth of 2 inches AFTER NATURAL COMPACTION.

· Mulches that are made of plant materials are very similar in effectiveness of controlling run-off.

· Straw can be windblown and MUST BE anchored down (netting, crimping, disking, rolling into soil, spray with tackifier)

· Wood chips require the application of 12-pounds of nitrogen per ton of mulch.

· On slopes > 6%, wood and bark chips tend to wash downslope and are not preferred BMPs for those sloping areas.

· usually used in conjunction with seeding.

Ground Cover – Slopes (Page II-5-5, DOE Manual) Top

Matting, Netting, and Erosion Blankets

Function: To provide immediate protection to exposed soils.

Use:

· To stabilize soils in areas sloping > 2:1; >50% slope.

· To stabilize soils in waterways, drainage conveyance and drainage facilities.

Design Criteria:

· Except for Excelsior Blankets, netting shall be underlain with mulch.

· Firm, continuous contact between netting and the mulch is required.

· The netting must be stapled and/or anchored with No. 11 gauge wire staples at least 6 inches long and with a minimum overlap of 3 inches.

NOTE: Slopes greater than 50% slope, that is 2(H) : 1(V), are required to be designed by a licensed engineer.

Plastic Sheeting (Page II-5-12, DOE Manual) Top

Function: To provide immediate, temporary erosion protection to slopes and disturbed areas that cannot be covered by mulching, in particular during the specified seeding periods. Clear plastic is also used to protect disturbed areas which must be covered during short periods of inactivity to meet November 1 to March 31 cover requirements.

Use:

· disturbed areas that require immediate erosion protection

· areas seeded during the time period of November 1 to March 31

CAUTION: Use of plastic sheeting causes additional stormwater problems

· velocity of run-off is increased

· time-of-concentration is decreased

Design Criteria:

· Clear plastic sheeting shall have a minimum thickness of 6 mil

· The plastic sheeting shall meet the requirements of WSDOT/APWA Section 9-14.5.

· Covering shall be installed and maintained tightly in place using sandbags or tires with a maximum spacing of 10 feet

· All seams shall have a 1-2 foot overlap

· All seams shall be taped or weighted for the full length

· When plastic sheeting is used on slopes that are not seeded, the covering shall remain in-place until the next seeding period.

· Sheeting should be toed at the top of the slope in order to prevent surface flow beneath the plastic.

Recommendations:

· stockpile stabilization – provide 15’ perimeter mulch and water containment BMP (brush barrier, silt fence, or straw bale barrier)

· > 30% slope stabilization - provide interceptor dike or swale at the base of the slope. If the water is clean, direct to a level spreader. If the water requires treatment, direct to a sediment trap or pond.

Construction Entrances (Page II-5-30, DOE Manual) Top

Function: To reduce the amount of mud, dirt, rocks, etc. transported onto public roads by motor vehicles or runoff by constructing a stabilized pad of rock spalls at entrances to construction sites and washing of tires during egress.

Use:

· whenever traffic will be leaving a construction site and moving directly onto a public road or other paved area.

Design Criteria:

· Material should be quarry spalls, 4-8 inches in size

· The rock pad shall be at least 12 inches thick and 100 feet in length for sites >1 acre

· The rock pad shall be at least 12 inches thick and 50 feet in length for sites <1 acre

· Filter fabric shall be laid beneath the rock pad

· Width shall be the full width of the vehicle ingress and egress area (minimum 20 feet)

· Additional rock should be added periodically to maintain proper function of the pads

· Tire washing should be done before the vehicle enters a paved street. Washing should be done on an area covered with crushed rack and the wash water should be drained to a sediment retention facility such as a sediment trap or basin.

· The volume of wash water produced by tire washing should be included when calculating the sediment trap or basin size

Check Dams (Page II-5-58, DOE Manual) Top

Function: To reduce the velocity of concentrated flows, reducing erosion of the swale or ditch, and to slow water velocity to allow retention of sediments.

Use:

· DO NOT USE STRAW BALE BARRIERS AS CHECK DAMS.

· Do not use straw bale barriers in concentrated water flows.

· Where temporary or permanent channels are not yet vegetated, channel lining is infeasible, and, therefore, velocity checks are required.

· In small open channels which drain 10 acres or less.

· NO CHECK DAMS MAY BE PLACED IN STREAMS unless approved by State Department of Fisheries.

Design Criteria:

· Check dams shall be used for water conveyance channels that slope >4%

· May be constructed of stone, logs or pea gravel filled sandbags.

· A deep sump is required immediately upstream from the check dam. Side slopes of the sump shall be a maximum of 2H:1V, and shall be a minimum of 12 inches deep.

· The center elevation of the checkdam shall be 6 inches less than the edge elevations.

· Rock shall be placed by hand or mechanical means (NO DUMPING) to insure that the swale is completely covered and the center of the dam is lower than the edges. Rocks shall be large enough to stay in place given the expected design flow through the channel. Side slopes shall be a maximum of 2H:1V.

· Log check dams shall be constructed of 4-6 inch diameter logs, driven vertically into the swale. All logs shall be embedded into the soil at least 18 inches. A log cross-piece at the elevation of the center of the dam shall be imbedded behind the vertical logs. In swales with a slope < 4%, the check dams shall be removed when the grass has matured sufficiently to protect the swale,

· Sediment shall be removed when sediment depth reaches 1/2 the sump depth.

· Check dams shall be placed so the center overflow from the down-gradient check dam is the same elevation as the base of the adjacent up-gradient check dam.

Interceptor Dikes (Page II-5-54, DOE Manual) Top

Function: A ridge of compacted soil with vegetative lining intended to intercept storm run-off and direct the run-off to a stabilized outlet.

Use:

· To reduce the volume and velocity of run-off from exposed or disturbed slopes.

· When interceptor dikes are placed above a disturbed slope, it reduces the volume of water reaching the disturbed area.

· When interceptor dikes are placed horizontally across a disturbed slope, it reduces the velocity of run-off flowing down the slope by reducing the distance that the run-off can flow directly downhill

Design Criteria:

· Minimum of 2 feet top width

· Minimum height of 18 inches, measured from the upslope toe

· Maximum side slope of 2:1. flatter preferred

· Minimum compaction of dike is 90% ASTM D698 standard proctor

· The dike shall have a grade between 0.5 and 1.0%. Thus, dikes must be located on contour.

· Horizontal spacing: slope < 5% 300 feet apart

slope 5-10% 200 feet apart

slope 10-40% 100 feet apart

slope >40% not allowed

· Stabilization: slope <5% apply seed and mulch within 5 days of dike construction

slope 5-40% stabilization should be done immediately using either sod or rip-rap

· The upslope side of the dike shall provide positive drainage to the dike outlet.

· Provide energy dissipation measures as necessary at the outlet to insure that no erosion shall occur.

· Sediment-laden run-off shall be released through a sediment trapping facility.

Interceptor Swales (Page II-5-54, DOE Manual) Top

Function: A swale with vegetative lining intended to intercept storm run-off and direct the run-off to a stabilized outlet.

Use:

· To reduce the volume and velocity of run-off from exposed or disturbed slopes.

· When interceptor dikes are placed above a disturbed slope, it reduces the volume of water reaching the disturbed area.

Design Criteria:

· Minimum bottom width of 2 feet

· Minimum depth is 1 foot

· Side slopes shall be 2H:1V or flatter

· Maximum bottom slope of the swale shall be 5% with positive drainage to a suitable outlet

· Horizontal spacing: slope < 5% 300 feet apart

slope 5-10% 200 feet apart

slope 10-40% 100 feet apart

slope >40% not allowed

· Stabilization: Seed as per temporary vegetative cover and provide sufficient time for the seed to sprout and for the vegetation to become fully established;

or Place sod or riprap on the bottom and on the side slope to a minimum height of 8 inches vertical from the bottom. If riprap is used, a minimum of 12 inches shall be applied to the bottom and pressed into the bank.

· The outlet shall be a level spreader or riprap to stabilize the outlet. If necessary to reduce turbidity, the outlet shall be directed to a sedimentation pond.

Outlet Protection - Riprap (Page III-2-22, DOE Manual) Top

Function: To prevent scour at stormwater outlets; to slow the velocity of concentrated stormwater flow or to stabilize slopes with seepage problems

Use:

· Applicable to the outlets of all pipes, interceptor swale outlets, and channel sections where the velocity of flow at the design capacity of the outlet will exceed the permissible velocity of the receiving channel or area; at soil-water interfaces where water turbulence and water velocity are such that erosion may occur.

Design Criteria:

· discharge thickness width length

0 – 5 fps 12 inches pipe diameter + 6 feet 8 feet or 4x pipe diameter

6 – 10 fps 12 inches pipe diameter + 6 feet 12 feet or 3X pipe

OR 3X pipe diameter, diameter, whichever is

whichever is greater greater

11 – 20 fps GABION as required as required

required,

12 in. min.

> 20fps Engineered Energy Dissipator Required

Pipe Slope Drains (Page II-5-35, DOE Manual) Top

Function: To carry concentrated water down steep slopes without causing erosion or saturation of the soils.

Use:

· to convey runoff downslope without causing erosion

Design Criteria:

· Capacity for temporary drains shall be sufficient to carry a 10-yr, 24-h peak flow.

· Capacity for permanent drains shall be sufficient to carry a 25-yr, 24-h flow.

· Maximum drainage area per pipe is 10 acres.

· For areas greater than 10 acres, either install more than one pipe or install a rock-lined channel.

· The entrance of 12 inch culverts and larger shall be a standard flared end section, with a minimum 6-inch metal toe plate to prevent runoff from undercutting the pipe inlet.

· Pipe slope at the entrance shall be at least 3%.

· Soil around and under the pipe and entrance section shall be thoroughly compacted to prevent undercutting

· The flared inlet section shall be securely connected to the slope drain and have watertight connecting bands.

· Slope drain sections shall be securely fastened together and have gasketed watertight fittings, and be securely anchored to the soil.

· Interceptor dikes shall be used to direct runoff into a slope drain.

· The height of the dike shall be at least 1-foot higher at all points than the top of the inlet pipe

· the area below the pipe must be stabilized with a riprap apron.

· If the pipe slope drain is conveying sediment-laden water, direct all flows into the sediment trapping facility.

Triangular Silt Dike (not in DOE Manual) Top

Additional Installation Specifications: (DOE 1999 Draft Manual, Volume II, page 98)

Function: May be used for temporary check dams, for perimeter protection, for temporary soil stockpile protection, for drop inlet protection, or as a temporary interceptor dike.

Use:

· May be used for temporary check dams, for perimeter protection, for temporary soil stockpile protection, for drop inlet protection, or as a temporary interceptor dike.

· May be used in place of straw bales for temporary check dams in ditches of any dimension.

· May be used on soil or pavement with adhesive or staples.

Design Criteria:

· Construction is of urethane foam sewn into a woven geosynthetic fabric.

· Triangular in shape, 10-14 inches high in the center with a 20-28 inch base.

· A 2-foot apron extends beyond both sides of the triangle along its standard section of 7 feet. One end of the apron is open to form a sleeve, so that additional sections can be added.

· Install with ends curved up in order to prevent water from flowing around the ends.

· The fabric flaps and the check dam are attached to the ground with 11 gauge wire, 200-300 mm long (8-12 inches long)

· Check dams should be placed in ditches perpendicular to the flow of water

· Sediment shall be removed when it reaches one half the height of the dam.

Brush Barrier (Page II-5-74, DOE Manual) Top

Function: To intercept and retain sediment from limited disturbed areas.

Use:

· Downslope from disturbed areas < 1/4 acre and that are subject to sheet and rill erosion

· Shall be used in conjunction with a sediment trap or a pond.

Design Criteria:

· Height: 3 feet minimum to 5 feet maximum

· Width: 5 feet at base minimum to 15 feet maximum

· Filter fabric entrenched in 6x6 inch trough along the uphill side and then draped over the brush pile. Native backfill is allowed if compacted.

· Anchor the downhill edge of the brush barrier and filter fabric with twine --- twine to be fastened to the filter fabric and fastened to stakes at the downstream edge of the brush berm.

Gravel Filter Berm (Page II-5-85, DOE Manual) Top

Function: To retain sediment from traffic areas.

Use:

· Primarily used to prevent sediment from entering traffic areas on the construction site. Place these structures across the roads and access routes. NOTE: These structures look like a broad speed-bump in the road.

Design Criteria:

· Berm material shall be 3/4 to 3 inch size, washed and well-graded gravel or crush rock, with less than 5% fines

· Spacing slopes < 5% 300 feet apart

slopes 5-10% 200 feet apart

slopes > 10% 100 feet apart

· Minimum height 1 foot

· Maximum side slopes shall be 3H:1V

· To achieve 3H:1V slope and 1 foot height, minimum base dimension shall be 6 feet.

· Extend laterally beyond the road surface.

Filter Fabric / Silt Fencing (Page II-5-74, DOE Manual) Top

Function: to remove larger particles by filtration and to retain water for sediment settlement

Site Suitability:

· To test for site suitability for filter fabric fencing, perform the following test:

· obtain a dry soil sample that has the soil and clumps broken.

· sieve through 2 mm mesh (US Standard Sieve # 10), capturing the material passing through the mesh and discarding everything captured by the mesh.

· weigh the soil sample

· pass the weighed sample through a 70 mm sieve (US Standard Sieve No. 200).

· the site is not suitable for filter fabric installation if > 85% of the sample passes through the 70 mm sieve.

· Use AOS 200 fabric for glacial and outwash soils (50-85% of the sample passes through an US Standard Sieve No. 200).

· Use AOS 100 fabric for most other C-type soils (<50% of the sample passes through an US Standard Sieve No. 200).

What Is Removed By Filter Fabric Fence

sediment size % reduction by weight

Total Suspended Solids 86

Organics 74

Phosphates 74

Heavy Metals 71

Turbidity 3

(Fabric Size = AOS 100 ; Horner et al. 1990 a, 1990b)

Use:

· downslope of all disturbed areas with sheetflow

Design Criteria

· May use alone on basins with water collection area < 1 acre, otherwise use only in combination with a sediment basin

· Maximum slope steepness perpendicular to the fence = 100% (1:1 horizontal to vertical or a 100% slope)

· Maximum overland water flow path length to the fence < 100 feet

· Shall not be placed in areas with maximum design storm resulting in concentrated flows >0.5 cfs

jargon:

AOS – apparent opening size. The larger the AOS number, the smaller the opening. The reason for this is that the AOS number has it’s basis in the number of meshes per inch; the greater the number of meshes per inch the smaller the actual size of the mesh.

Straw Bale Barrier (Page II-5-79, DOE Manual) Top

Function: To intercept and detain small amounts of sediment from disturbed areas of limited extent.

Use:

· On slight slopes that are subject to sheet and rill erosion

· As interceptor barrier when drainage area is less than 1/4 acre per 100 feet of barrier length, the maximum slope length is 100 feet, and the maximum slope gradient is less than 50%

· In minor swales and/or ditches where the maximum contributing drainage area is < 2 acres

· Where effectiveness is required for less than 3 months

SHEET FLOW APPLICATIONS

Design Criteria:

· Bales shall be placed in a single row, lengthwise on the contour, with the ends of the bales tightly abutting one another

· The barrier shall be entrenched to a depth of 4 inches and backfilled on the downslope side to a depth of 4 inches above grade.

· Each bale shall be securely fastened with at least 2 stakes or re-bars driven through the bales.

· The gaps between the bales shall be chinked with straw.

CHANNEL FLOW APPLICATIONS

Design Criteria:

· Bales shall be placed in a single row, lengthwise, oriented perpendicular to the contour, with the ends tightly abutting

· The barrier shall extend to such a length that the bottom of the end bales are higher in elevation that the top of the lowest middle bale in order to insure that sediment laden water will flow over the barrier and not around it.

Temporary Sediment Traps/Ponds (Page II-5-93-102, DOE Manual) Top

Function: To collect and store sediment from sites cleared and/or graded during construction. Traps are to be used on sites less that 3 acres when the design life of the facility is less than 6-months. Ponds are for larger sites and have a longer life expectancy.

Use:

· To be used in conjunction with interceptor dikes and swales.

· The contributing drainage area should be divided into separate drainage areas:

If the drainage area is <3 acres then a sediment trap is allowed

If the drainage area is > 3 acres but < 10 acres, then a pond is allowed

If the drainage area is > 10 acres, separate basins must be established

· Sediment traps and ponds are effective in removing sediment down to about medium sized silt. Sediment traps and ponds will not remove finer particles from the water. Additional BMPs are required to remove finer particles.

Design Criteria:

Sediment Traps:

· construction: may be constructed by excavation or by building a berm

· berm thickness: 4’ minimum top dimension, with 3H:1V side slopes, to the bottom of the sediment trap

· surface dimensions: aspect ratio 3L:1W, typical

· volume: designed to hold 2-yr, 24-h storm

· total depth: 3.5 feet deep from top of spillway to bottom of the trap (live storage = 2 feet; sediment sump = 1.5 feet)

· outlet vertical profile as a spillway through the berm.

· Berm to be native soil or equivalent to the top of the sediment sump. The live storage vertical profile of the dike should be :

lower 12” 0.75-1.50 “ washed gravel

next 12” 2.0 – 4.0” washed gravel

freeboard 12” minimum between top of spillway and top of berm

outlet width 6’ minimum

· perimeter fence REQUIRED – 6-feet tall with locking gate

· maintenance required remove sediment when accumulation = 12”

Sediment Ponds:

· construction: may be constructed by partial excavation or by constructing a compacted embankment

· inflow into the pond: one or more

· outflow from the pond: controlled stormwater release, with spillway serving as the emergency overflow.

· riser pipe: shall be solid with two 1-inch diameter dewatering holes located at the top of the sediment storage volume on opposite sides of the riser pipe.

· riser diameter: open on top; rebar trash rack required on riser pipes > 18”

· outlet pipe: shall be sealed with two anti-seep collars. Anti-seep collars to be located approximately at equal intervals through the berm, in order to prevent leakage.

· outlet protection: dissipater pad required at the outlet

· baffles: required component of the sediment pond, in order to reduce turbulence

· berm thickness: 6’ minimum top dimension.

· side slopes: maximum 3H:1V internal side slope and 2H:1V external side slope.

· depth profile: dead storage shall be 2-3 feet (2’ minimum to 3’ maximum)

· live storage: live storage shall be 2-4 feet (2’ minimum; 4’ maximum)

· spillway height: 1 foot above the design storm level

· berm height: 1 foot above the spillway height

· volume: designed to hold 10-yr, 24-h storm

Copyright © 2002 ECO-3