5.2 Small Parcel Erosion and Sediment Control Guidelines · Small Parcel Erosion and Sediment Control Guidelines Greater Vancouver Sewerage & Drainage District Liquid Waste Management

5-2 Best Management Practices Guide for StormwaterAppendix H: Construction Site Erosion and Sediment Control Guide

Small Parcel Erosion and Sediment Control Guidelines

Greater Vancouver Sewerage & Drainage DistrictLiquid Waste Management Plan-Stormwater Management

5.2 Small Parcel Erosion and Sediment Control Guidelines

The Small Parcel Erosion and Sediment Control Guidelines contained in this section aresuitable for the following types of development:

(a) individual, detached, single family residences and duplexes;(b) creation or addition of less than 465 square metres (5,000 square feet) of

impervious surface area; and(c) land disturbing activities of less than 0.4 hectare (one acre).

Objective: The Objective of this strategy is to address the cumulative effect of sedimentcoming from a large number of small sites.

The Small Parcel BMPs found in Section 6.9 in this manual can be used to develop a planor strategy for Small Parcel Erosion and Sediment Control (SPESC). The BMP controlmeasures are meant to be temporary in nature to deal with erosion and sediment generatedduring the construction phase only. Local governments may choose to apply additionalpermanent, site-specific stormwater controls to Small Parcels.

A checklist similar to that found in Table 5-1 at the end of Section 5.2 together withSmall Parcel Guidelines #1 through #4 can be used to develop a strategy or plan forSPESC. These elements can be adapted as necessary to suit the individual needs of alocal government.

The nature and content of a SPESC plan or strategy will depend upon the exact nature ofthe project. It may contain some elements of the Large Parcel ESC approach (see Section5.3) as determined by the local government.

If a formal SPESC plan is to be required, the plan should describe how Small ParcelGuidelines will be met, and should contain a plot plan which includes:

• a vicinity map to clearly locate property;• an accurate location of the structure and its access;• all applicable setback requirements;• location of all applicable ESC BMPs; and• existing site features or water quality sensitive areas, if appropriate.

The checklist found in Table 5-1 can also be used.

Small Parcel Guideline #1 – Construction Access Route

• Construction vehicle access should be, whenever possible, limited to one route. Access points should be stabilized with quarry spall or crushed rock to minimize

Best Management Practices Guide for Stormwater 5-3Appendix H: Construction Site Erosion and Sediment Control GuideSmall Parcel Erosion and Sediment Control Guidelines


the tracking of sediment onto public roads.

Supplemental Guidelines: If sediment is inadvertently transported onto public roads,roads should be cleaned thoroughly at the end of the day by shovelling or sweeping. Street washing should only be done after the bulk of the sediment has been removed bysweeping.

Small Parcel Guideline #2 – Stabilization of Denuded Areas

• Soil stabilization. All exposed and unworked soils should be stabilized bysuitable application of BMPs, including but not limited to sod or other vegetation,plastic covering, mulching, or application of ground base on areas to be paved. All BMPs should be selected, designed, and maintained in accordance with anappropriate manual. From October 1 through April 30, no soils should remainexposed for more than 2 days. From May 1 through September 30, no soilsshould remain exposed for more than 7 days.

Supplemental Guidelines: BMPs should be selected which are appropriate for the time ofthe year and anticipated duration of use.

Small Parcel Guideline #3 – Protection of Adjacent Properties

• Adjacent properties should be protected from sediment deposition by appropriateuse of vegetative buffer strips, sediment barriers or filters, dikes or mulching, orby a combination of these measures and other appropriate BMPs.

Small Parcel Guideline #4 – Maintenance

• All erosion and sediment control BMPs should be regularly inspected andmaintained to ensure continued performance of their intended function.


Small Parcel Erosion and Sediment Control Guidelines


Table 5.1Checklist For Small Parcel Guidelines(Adapted From The City Of Redmond)

• Provide the name, address, and phone number of owner or contact person.• Designate north arrow, lot number, address, date and street name fronting structure.• Show all proposed structure and any existing structures on the site.• Locate and size all streams, swales or drainage channels on or within 7.6 m of the site which may

involve or affect the drainage of the site to be developed. Indicate all existing stormwater pipes.• Indicate the direction and location of surface water runoff entering the site from all adjacent property.• Indicate what type of system(s) will be used to convey runoff away from the proposed structures.• Phase clearing so that only those areas that are actively being worked are uncovered. From October 1

through April 30, no soils should remain exposed for more than 2 days. From May 1 throughSeptember 30, no soils should remain exposed for more than 7 days.

• Indicate where excavated basement soil stockpiles (if applicable) will be located. The stockpileshould be covered until the stockpile is either used or removed. Piles should be situated so thatsediment does not run into the street or adjoining yards.

• Backfill basement walls, where applicable, as soon as possible and rough grade the lot.• Remove excess soil from the site as soon as possible after backfilling.• Stabilize denuded areas of the site by mulching, seeding, planting, or sodding.• Adjacent properties should be protected from sediment deposition by appropriate use of vegetative

buffer strips, sediment barriers or filters, dikes or mulching, or by a combination of these measuresand other appropriate BMPs.

• If a lot has a soil bank higher than the curb, the trench or berm should be installed moving the bankmore than 1 metre behind the curb. This will reduce the occurrence of gully and rill erosion whileproviding a storage and settling area for stormwater.

• Construction vehicle access shall be, whenever possible, limited to one route. Indicate the location ofthe construction entrance. Apply gravel or crushed rock to the driveway area and restrict truck trafficto this one route. Driveway paving can be installed directly over the gravel. This measure requiresperiodic inspection and maintenance including washing, top-dressing with additional stone, reworkingand compaction.

• Provide for periodic street cleaning to remove any sediment that may have been tracked out. Sediment should be removed by shovelling or sweeping and carefully removed to a suitable disposalarea where it will not be re-eroded.

• Inspect all erosion and sediment control BMPs regularly especially after any large storm. Maintenance, including removal and proper disposal of sediment should be done as necessary(generally when one-half or more of the total capacity of the system is filled with sediment).

For further information on small site BMPs, please see Section 6.9.

Best Management Practices Guide for Stormwater 5-5Appendix H: Construction Site Erosion and Sediment Control GuideStep-by-Step Procedure for Large Parcel Erosion and Sediment Control


5.3 Large Parcel Erosion and Sediment Control Guidelines

The Large Parcel Erosion and Sediment Control Guidelines contained in this section aresuitable for all new development and redevelopment that include land disturbingactivities over an area 0.4 ha and greater. Land disturbing activities are defined as anyactivity that results in a change in the existing soil cover (both vegetative andnonvegetative) and/or the existing topography. Land disturbing activities includeconstruction, clearing, grading, filling and excavation.

Objective: To control erosion and prevent sediment from leaving the site.

Large Parcel Erosion and Sediment Control (LPESC) should provide for the interceptionand treatment of all potential silt-laden runoff that could occur during clearing, grading,construction and site stabilization. Measures should be provided to assure that no silt-laden runoff leaves the site during construction and stabilization. The BMPs contained inSection 4 and 6 of this appendix are applicable to Large Parcels.

A LPESC plan or strategy should contain sufficient information to satisfy the localgovernment that the problems of erosion and sedimentation have been adequatelyaddressed for a proposed project. The length and complexity of the LPESC strategy willbe commensurate with the size of the project, the severity of site conditions, and thepotential for off-site damage.

In general, LPESC strategies for constructing a few homes in a small subdivision do notneed to be as complex for large shopping center developments or large subdivisions. Also, strategies for projects undertaken on flat terrain will generally be less complicatedthan those for projects constructed on steep slopes where the erosion potential is higher. The greatest level of planning and detail should be evident for projects which are large insize, directly adjacent to flowing streams, other sensitive areas, or high value propertieswhere damage may be particularly costly or detrimental to the environment.

If a formal LPESC plan is to be required, the plan should include a written portion(narrative) and a site plan. The narrative is a written statement which explains andjustifies the erosion and sediment control decisions made for a particular project. Thenarrative contains concise information concerning existing site conditions, constructionschedules, and other pertinent items which are not contained in a typical site plan. Thenarrative is also important to the construction superintendent, inspector or other personnelwho may be responsible to see that the plan is implemented properly. It provides a singlereport which describes where and when the various erosion and sediment control BMPsshould be installed.

The site plan for a formal LPESC plan (if required) should include the following:


Step-by-Step Procedure for Large Parcel Erosion and Sediment Control


• location of clearing limits and easements, setbacks, water quality sensitive areasand their buffers;

• locations and descriptions of all erosion and sediment control measures for eachphase of construction; and

• cross sections of fill or excavations.

The primary elements that determine the adequacy of a LPESC plan or strategy are LargeParcel Guidelines #1 through #14 below. As a guide to format, the site planners and planreviewers can use the checklist contained in Section 5.4.7. The step-by-step procedureoutlined in this section can be used for the development of all plans or strategies.

Large Parcel Guideline #1 – Stabilization and Sediment Trapping

• All exposed and unworked soils should be stabilized by suitable application ofBMPs. From October 1 to April 30, no soils should remain unstabilized for morethan 2 days. From May 1 to September 30, no soils should remain unstabilizedfor more than 7 days. Prior to leaving the site, stormwater runoff should passthrough a sediment pond or sediment trap, or other appropriate BMPs.

Supplemental Guidelines: This guideline applies both to soils not yet at final grade andsoils at final grade. The type of stabilization BMP used may be different depending onthe length of time that the soil is to remain unworked.

Soil stabilization refers to BMPs which protect soil from the erosive forces of raindropimpact and flowing water. Applicable practices include vegetative establishment,mulching, plastic covering, and the early application of gravel base on areas to be paved. Soil stabilization measures should be selected to be appropriate for the time of year, siteconditions and estimated duration of use. Soil stockpiles should be stabilized orprotected with sediment trapping measures to prevent soil loss.

These requirements are especially important in areas adjacent to streams, wetlands orother sensitive or critical areas.

Large Parcel Guideline #2 – Delineate Clearing and Easement Limits

• In the field, mark clearing limits and/or any easements, setbacks, sensitive/criticalareas and their buffers, trees and drainage courses.

Large Parcel Guideline #3 – Protection of Adjacent Properties

• Properties adjacent to the project site should be protected from sedimentdeposition.

Supplemental Guidelines: This may be accomplished by preserving a well-vegetated



buffer strip around the lower perimeter of the land disturbance, by installing perimetercontrols such as sediment barriers, filters or dikes, or sediment basins, or by acombination of such measures.

Vegetated buffer strips may be used alone where runoff in sheet flow is expected. Bufferstrips should be at least 7.6 m in width. If at any time it is found that a vegetated bufferstrip alone is ineffective in stopping sediment movement onto adjacent property,additional perimeter controls should be provided.

Large Parcel Guideline #4 – Timing and Stabilization of Sediment Trapping Measures

• Sediment ponds and traps, perimeter dikes, sediment barriers, and other BMPsintended to trap sediment on-site should be constructed as a first step in grading. These BMPs should be functional before land disturbing activities take place. Earthen structures such as dams, dikes, and diversions should be seeded andmulched according to the timing indicated in Large Parcel Guideline #1.

Large Parcel Guideline #5 – Cut and Fill Slopes

• Cut and fill slopes should be designed and constructed in a manner that willminimize erosion. In addition, slopes should be stabilized in accordance withLarge Parcel Guideline #1.

Supplemental Guidelines: Consideration should be given to the length and steepness ofthe slope, the soil type, upslope drainage area, ground water conditions, and otherapplicable factors. Slopes which are found to be eroding excessively within two years ofconstruction should be provided with additional slope stability measures until theproblem is corrected.

1. Roughened soil surfaces are preferred to smooth surfaces on slopes (see BMPEC6 in Section 6.6).

2. Interceptors (see BMP EC10 in Section 6.6) should be constructed at the top oflong steep slopes which have significant drainage areas above the slope. Diversions or terraces may also be used to reduce slope length.

3. Concentrated stormwater should not be allowed to flow down cut or fill slopesunless contained within an adequate temporary or permanent channel, or pipeslope drain (see BMP EC4 in Section 6.6).

4. Wherever a slope face crosses a water seepage plane which endangers the stabilityof the slope, adequate drainage or other protection should be provided (BMPsEC5 and EC13 in Section 6.6).




Large Parcel Guideline #6 – Controlling Off-site Erosion

• Properties and waterways downstream from development sites should beprotected from erosion due to increases in the volume, velocity, and peak flowrate of stormwater runoff from the project site.

Large Parcel Guideline #7 – Stabilization of Temporary Conveyance Channels andOutlets

• All temporary on-site conveyance channels should be designed, constructed andstabilized to prevent erosion from the expected velocity of flow from the designstorm for the developed condition. Stabilization adequate to prevent erosion ofoutlets, adjacent streambanks, slopes and downstream reaches should be providedat the outlets of all conveyance systems.

Large Parcel Guideline #8 – Storm Drain Inlet Protection

• All storm drain inlets made operable during construction should be protected sothat stormwater runoff does not enter the conveyance system without first beingfiltered or otherwise treated to remove sediment.

Large Parcel Guideline #9 – Underground Utility Construction

• The construction of underground utility lines should be subject to the followingcriteria:

(i) Where feasible, no more than 150 m of trench should be opened at onetime.

(ii) Where consistent with safety and space considerations, excavated materialshould be placed on the uphill side of trenches.

(iii) Trench dewatering devices should discharge into a sediment trap orsediment pond.

Large Parcel Guideline #10 – Construction Access Route

• Wherever construction vehicle access routes intersect paved roads, provisionsshould be made to minimize the transport of sediment (mud) onto the paved road. If sediment is transported onto a road surface, the roads should be cleanedthoroughly at the end of each day. Sediment should be removed from roads byshoveling or sweeping and be transported to a controlled sediment disposal area. Street washing should be allowed only after sediment is removed in this manner.



Large Parcel Guideline #11 – Removal of Temporary BMPs

• All temporary erosion and sediment control BMPs should be removed within 30days after final site stabilization is achieved or after the temporary BMPs are nolonger needed. Trapped sediment should be removed or stabilized on site. Disturbed soil areas resulting from removal should be permanently stabilized.

Large Parcel Guideline #12 – Dewatering Construction Site

• Dewatering devices should discharge into a sediment trap or sediment pond.

Erosion and Sediment Control Guideline #13 – Control of Pollutants Other ThanSediment on Construction Sites

• All pollutants other than sediment that occur on-site during construction should behandled and disposed of in a manner that does not cause contamination ofstormwater.

Large Parcel Guideline #14 – Maintenance

• All temporary and permanent erosion and sediment control BMPs should bemaintained and repaired as needed to assure continued performance of theirintended function. All maintenance and repair should be conducted in accordancewith an appropriate manual.

5.3.1 BMP Guidelines

Sections 4 and 6 of this manual contain guidelines and design criteria for specific BMPs. Wherever any of these BMPs are to be employed on a site, the specific title and numberof the BMP should be clearly referenced in the narrative and marked on the appropriatesite plan or map. By referencing this manual properly (or other locally adopted manuals),the site planner can reduce the need for detailed drawings and lengthy descriptions of theESC practices.

Modifications to standard practices or new innovative conservation practices may also beemployed, but such practices (Experimental BMPs) should be thoroughly described anddetailed before they are used.

5.3.2 General Principles in Selecting BMPs for Large Parcel Erosion and Sediment Control

• Prevention of pollutant release is superior to pollutant capture later. Select sourcecontrol BMPs as a first step.

• Selection of BMPs should depend on site characteristics and the constructionplan.




• The proper first step is a site drainage analysis. Determine where runoff willenter, cross and exit the site.

• Flowing water has a tendency to concentrate in channels instead of flowing assheet flow.

• Determine whether subsurface water is a factor.• Divert runoff from exposed areas wherever possible.• Existing vegetation is the most effective erosion control.• Limit and phase clearing.• Use materials found on the site wherever possible.• Incorporate natural drainage features whenever possible, using adequate buffers

and protecting areas where flow enters the drainage system.• Keep structures simple.• Minimize slope length and steepness.• Keep runoff velocities low.• Reduce the tracking of sediment off-site.• Select and install controls that can be maintained.• Select appropriate BMPs from Section 4 of this manual for the control of

pollutants not associated with sediment.

5.3.3 Standard Practice Coding System

Site planners can use the standard numbering and coding system for BMPs contained inthis manual. Table 3.2 at the end of Section 3 lists each practice with its designatednumber, symbol, and code. Use of this coding system will result in increased uniformityand simplify BMP plans or strategies for reviewers, job superintendents, and inspectors. Since the BMPs in Section 4 are not site-specific, they have not been given codes orsymbols.

5.3.4 Comprehensive Site Planning

Erosion and sediment control planning should be an integral part of the site planningprocess, not just an afterthought. The potential for soil erosion should be a significantconsideration when deciding upon the layout of buildings, parking lots, roads, and otherfacilities. Adverse environmental impacts and costly erosion and sediment controlmeasures can be minimized if the site design can be adapted to existing site conditionsand good conservation principles are used. Additionally, if thought is given to the designof temporary erosion control devices, they may be able to be converted into permanentfacilities as well.



5.4 Step-By-Step Procedure for Large Parcel Erosion andSediment Control

The five basic steps in Large Parcel Erosion and Sediment Control are summarizedbelow. This procedure can be modified to suit the needs of individual municipalities asrequired.

Step 1 - Data Collection

A. TopographyB. DrainageC. SoilsD. Ground CoverE. Adjacent AreasF. Existing DevelopmentG. On and Off-Site Utilities

Step 2 - Data Analysis

A. Topography - Slope gradients, lengthsB. Drainage - Outline existing natural and manmade drainage patternsC. Soils - Erodibility (K) factors, permeabilityD. Ground Cover - Trees, grassy areas, sensitive or endangered vegetationE. Adjacent Areas - Streams, roads, buildings, etc.

Step 3 - Site Plan Development

A. Fit development to terrainB. Locate construction in the least critical areasC. Utilize cluster development whenever possibleD. Minimize paved areasE. Utilize the natural drainage system and natural drainage locations

whenever possible

Step 4 - Plan for Erosion and Sediment Control

A. Determine limits of clearing and gradingB. Divide the site into drainage areas

• Consider each area separatelyC. Select erosion and sediment control BMPs, emphasizing source control

and vegetative BMPs.• Vegetative, especially buffers, preservation of natural vegetation

and flagging




• Structural• Management measures

D. Plan for stormwater management

Step 5 - Include BMPs for the Control of Pollutants Other Than Sediment

A. Review Section 4 in this manual.B. Select appropriate BMPs based on the practices which will be used on-site.

Step 6 - Plan Preparation (if required)

A. NarrativeB. Site Plan

5.4.1 Step 1 - Data Collection

Inventory the existing site conditions to gather information which will help develop themost effective erosion and sediment control plan. The information obtained should beplotted on a map and explained in the narrative portion of the plan (if required).

A. Topography - A small-scale topographic map of the site should be prepared toshow the existing contour elevations at intervals of from 0.3 m to 1.5 mdepending upon the slope of the terrain.

B. Drainage Patterns - All existing drainage swales and patterns on the site should belocated and clearly marked on the topographic map including all existingunderground storm drain pipe systems.

C. Soils - Major soil type(s) on the site should be determined and shown on thetopographic map. Soils information can be obtained from a soil survey if one hasbeen published for the area. Soils information should be plotted directly onto themap or an overlay of the same scale for ease of interpretation.

D. Ground Cover - The existing vegetation on the site should be shown. Suchfeatures as tree clusters, grassy areas, and unique or sensitive vegetation should beshown on the map. Unique vegetation may include existing trees above a givendiameter. Local requirements regarding tree preservation should be investigated. In addition, existing denuded or exposed soil areas should be indicated.

E. Adjacent Areas - Areas adjacent to the site should be delineated on thetopographic map. Such features as streams, roads, lakes, wetlands, and woodedareas, etc., should be shown. These features should receive special attentionduring the construction project because of the potential for off-site damage.



F. Existing Development - Existing buildings and facilities (if any) on-site oradjacent to the site should be shown on the topographic map.

G. On and Off-Site Utilities - Identify all utility corridor locations, roadways,associated clearing limits and BMPs for all on-site and off-site utilityconstruction.

5.4.2 Step 2 - Data Analysis

When all of the data in Step 1 are considered together, a picture of the site potentials andlimitations should begin to emerge. Determination should be made to determine thoseareas which have potential critical erosion hazards. The following are some importantpoints to consider in site analysis:

A. Topography - The primary topographic considerations are slope steepness andslope length. Because of the effect of runoff, the longer and steeper the slope, thegreater the erosion potential. When the percent of slope has been determined,areas of similar steepness should be outlined. Slope gradients can be grouped intothree general ranges of soil erodibility:

0-7% - Low erosion hazard7-15% - Moderate erosion hazard>15% - High erosion hazard

Within these slope gradient ranges, the greater the slope length, the greater theerosion hazard. Therefore, in determining potential critical areas the plannershould be aware of excessively long slopes. As a general rule, the erosion hazardwill become critical if slope lengths exceed the following values:

0-7% - 100 m7-15% - 50 m>15% - 25 m

These distances may be shorter in areas with highly erodible soils.

B. Natural Drainage - Natural drainage patterns which consist of overland flow,swales and depressions, and natural watercourses, should be identified in order toplan around critical areas where water will concentrate. Where it is possible,natural drainageways and discharge locations should be used to convey runoffover and off the site to avoid the expense and problems of constructing anartificial drainage system. Man-made ditches and waterways will become part ofthe erosion problem if they are not properly stabilized. Care should also be takento be sure that increased runoff from the site will not erode or flood the existingnatural drainage system. Possible sites for stormwater retention and detention




should also be located at this point.

The site should also be checked for areas of saturated soil and/or areas whereground water may be encountered during construction. Construction in theseareas should be avoided where possible.

C. Soils - Such soils properties as flood hazard, natural drainage, depth to bedrock,depth to seasonal water table, permeability, shrink-swell potential, texture, anderodibility should exert a strong influence on land development decisions.

D. Ground Cover - Ground cover is the most important factor in terms of preventingerosion. Any existing vegetation which can be saved will prevent erosion betterthan any constructed BMP. Trees and other vegetation protect the soil andbeautify the site after construction. If the existing vegetation cannot be saved,consider such practices as staging construction, temporary seeding, or temporarymulching. Staging of construction involves stabilizing one part of the site beforedisturbing another. In this way, the entire site is not disturbed at once and thetime without ground cover is minimized. Temporary seeding and mulchinginvolve seeding or mulching areas which would otherwise lie open.

Buffers around water bodies or other sensitive areas should be delineated and theclearing limits flagged.

E. Adjacent Areas - An analysis of adjacent properties should focus on areasdownslope from the construction project. Water bodies which will receive directrunoff from the site are a major concern. The potential for sediment pollutionand/or downstream channel erosion and deposition should be considered andaddressed. The potential for sediment deposition on adjacent properties due tosheet and rill erosion should also be analyzed so that appropriate sediment trap-ping measures can be planned.

5.4.3 Step 3 - Site Plan Development

After analyzing the data and determining the site limitations, a site plan can be developed.Locate the buildings, roads, and parking lots and develop landscaping plans to exploit thestrengths and overcome the limitations of the site. The following are some points toconsider when making these decisions:

A. Fit the development to the terrain. The development of an area should be tailoredto the existing site conditions to avoid unnecessary land disturbance and minimizeerosion hazards and costs and other environmental impacts.



B. Confine construction activities to the least critical areas. Any land disturbance inhighly erodible areas will necessitate the installation of more costly controlmeasures.

C. Cluster buildings together. This minimizes the amount of disturbed area,concentrates utility lines and connections in one area, and provides more opennatural space. The cluster concept not only lessens the erodible area and theamount of impervious surface, it reduces runoff, and generally reducesdevelopment costs.

D. Minimize impervious areas. Keep paved areas such as parking lots and roads to aminimum. This goes hand in hand with cluster developments in eliminating theneed for duplicating parking areas, access roads, etc.

E. Utilize the natural drainage system. The natural drainage system and naturaldrainage locations of a site should be preserved instead of replaced with stormdrains or concrete channels. The potential for downstream damages due toincreased runoff can thus be minimized.

5.4.4 Step 4 - Erosion and Sediment Control Planning

When the layout of the site has been decided upon, a strategy to control erosion andsedimentation from the disturbed areas can be formulated.

The following general procedure is recommended for ESC control planning:

A. Determine limits of clearing and grading. Decide exactly which areas must bedisturbed in order to accommodate the proposed construction. Pay specialattention to critical areas. Show all limits of clearing for flagging in the field.

B. Divide the site into drainage areas. Determine how runoff will travel over the site.Consider how erosion and sedimentation can be controlled in each small drainagearea before looking at the entire site. Remember, it is easier to control erosionthan to contend with sediment after it has been carried downstream.

C. Select erosion and sediment control BMPs. Erosion and sediment control BMPscan be divided into three broad categories: cover practices, structural practices,and management measures. Cover practices, such as leaving buffer strips, seedingand mulching are the preferred BMPs and should be used first. Structuralpractices, such as sediment ponds and inlet protection should be implementedonly after cover practices are used as a first line of defense. Managementmeasures are construction management techniques such as staging constructionwhich, if properly utilized, can minimize the need for physical controls andpossibly reduce costs.




1. Cover Practices - Keep in mind that the first line of defense is to preventerosion. This is accomplished by protecting the soil surface from raindropimpact and overland flow of runoff using source control BMPs. The bestway to protect the soil surface is to preserve the existing ground cover. Where land disturbance is necessary, temporary seeding or mulching canbe used on areas which will be exposed.

Erosion and sediment control strategies should contain provisions forpermanent stabilization of disturbed areas. Selection of permanentvegetation should include the following considerations:

a. establishment requirements;b. adaptability to site conditions;c. aesthetics; andd. maintenance requirements.

2. Structural Practices - Structural practices are generally more costly andless efficient than are source controls. However, they are usuallynecessary since not all disturbed areas can be protected with vegetation. They are often used as a second or third line of defense in series with othervegetative or structural practices to capture sediment before it leaves thesite.

It is very important that structural practices be selected, designed, andconstructed according to the guidelines and specifications in Section 6 ofthis manual. Improper use or inadequate installation can create problemswhich are greater than the structure was designed to solve.

3. Management Measures - Good construction management is as importantas physical practices for erosion and sediment control, and there isgenerally little or no cost involved. Following are some managementconsiderations which can be employed.

a. Sequence construction so that no area remains exposed for anunnecessarily long period of time.

b. Temporary seeding should be done immediately after grading. c. When possible, avoid grading activities during November through

March since these months have the highest potential for erosiverainfall.

d. On large projects, stage the construction so that one area can bestabilized before another is disturbed.

e. Develop and carry out a regular maintenance schedule for erosionand sediment control practices.



f. Physically mark off limits of land disturbance on the site with tape,signs or other methods, so the workers can see areas to beprotected.

g. Make sure that all workers understand the major provisions of theerosion and sediment control measures being implemented at thesite.

h. Responsibility for implementing the erosion and sediment controlmeasures should be designated to one individual (preferably the jobsuperintendent or foreman).

D. Properties and waterways downstream from the development site should beprotected from erosion due to increases in volume, velocity and peak flow rate ofstormwater runoff.

5.4.5 Step 5 -Include BMPs for the Control of Pollutants Other than Sediment

A. Review Section 4 in this appendix - Section 4 provides information on commonconstruction practices which cause pollution other than erosion andsedimentation. These range from nutrient and pesticide control to disposal ofsolid and/or dangerous wastes.

B. Select appropriate BMPs based on the practices which will be used on-site -Based on the type of work to be done on-site, select the appropriate BMPs. Areaswhere equipment washing may occur or where contaminated soils may be locatedon the site also should be noted on the site plan (if required).

5.4.6 Step 6 – Develop Erosion and Sediment Control Strategy

All of the necessary planning work has been done in steps 1 through 5. The final stepconsists of consolidating the collected information and developing it into a specificerosion and sediment control plan or strategy for the project.

If a formal LPESC plan is to be required, the plan typically consists of two parts: anarrative and a site plan. The narrative verbally explains the problems and their solutionswith all necessary documentation. Justification should be provided for all solutions. Thesite plan is typically a series of maps or drawings pictorially explaining informationcontained in the narrative.

Following is a checklist of items which could be included in a narrative and on a siteplan. This checklist can be used by a site planner as a quick reference to determine if allthe major items are included in the erosion and sediment control measures at adevelopment site.




5.4.7 Checklist for Erosion and Sediment Control

Narrative

• Project description - Briefly describe the nature and purpose of the land disturbingactivity, and the amount of grading involved.

• Existing site conditions - A description of the existing topography, vegetation, anddrainage.

• Adjacent areas - A description of neighboring areas such as streams, lakes,residential areas, roads, etc., which might be affected by the land disturbance. Provide perimeter control of runoff on all necessary property boundaries.

• Soils - A brief description of the soils on the site giving such information as soilnames, mapping unit, erodibility, permeability, depth, texture, and soil structure.

• Critical areas - A description of areas on the site which have potential seriouserosion problems.

• Erosion and sediment control BMPs - A description of the BMPs which will beused to control erosion and sedimentation on the site. Specify the constructionsequence.

• Permanent stabilization - A brief description, including specifications, of how thesite will be stabilized after construction is completed.

• Stormwater management considerations - Will the development of the site resultin increased peak rates of runoff? Will this potentially result in channeldegradation downstream? If so, consideration should be given to stormwatercontrol structures on the site.

• Maintenance - A schedule of regular inspections and repair of erosion andsediment control structures should be set forth.

• Calculations - Any calculations made for the design of such items as sedimentponds, diversions, waterways, and calculations for runoff and stormwaterdetention basin design (if applicable).

• Non-ESC BMPs - Indicate which BMPs from Section 4 will be used on-site.

Site Plan

• Vicinity map - A small map locating the site in relation to the surrounding area. • Existing contours - Existing contours of the site should be shown on a map. • Existing vegetation - The existing tree lines, grassy areas, or unique vegetation

should be shown on a map. • Soils - The boundaries of the different soil types should be shown on a map. • Indicate north - The direction of north in relation to the site should be shown. • Critical erosion areas - Areas with potentially serious erosion problems should be

shown on a map. • Existing drainage patterns - The dividing lines and the direction of flow for the

different drainage areas should be shown on a map.



• Final contours - Changes to the existing contours should be shown on a map. Usea bold dashed line showing developed condition drainage divides.

• Limits of clearing and grading - Areas which are to be cleared and graded shouldbe outlined on a map.

• Cut and Fill Slopes - Show all cut and fill slopes, indicating top/bottom of slopecatch lines.

• Conveyance -(1) Designate locations for grass-lined swales, interceptor trenches, or ditches.(2) Show all drainage pipes, ditches, or cut-off trenches associated with

erosion/sediment control.(3) Provide all temporary pipe inverts or minimum slopes and cover.(4) Show grades, dimensions, location, and direction of flow in all ditches and

swales.(5) Provide details of bypassing off-site runoff around clearing

limits/disturbed areas and sediment pond/trap.(6) Indicate locations and outlets of any possible dewatering systems.

• Location of BMPs - The locations of the erosion and sediment control andstormwater management BMPs used on the site should be shown on a map. Inparticular, locate the construction entrance and detail. Specify length, width,thickness and rock size of the entrance.

• Sediment Control Facilities(1) Show all the locations of sediment trap(s)/pond(s) (if required) and all

associated pipes and structures.(2) Dimension pond berm widths and all inside and outside pond slopes.(3) Indicate the trap/pond storage and the depth, length, and width

dimensions.(4) Provide typical section views throughout pond and outlet structure.(5) Provide typical details of gravel cone and standpipe, and/or other filtering

devices.(6) Detail stabilization techniques for outlet/inlet.(7) Detail control/restrictor device location and details.(8) Specify mulch and/or recommended cover of berms and slopes.(9) Provide rock specifications and detail for rock check dam, if used.(10) Specify spacing for rock check dams as required for actual slopes on-site.(11) Provide front and side sections of typical rock check dams.(12) Indicate locations and provide details and specifications for silt fabric

fence (include installation detail).• Detailed drawings - Any structural practices used that are not referenced to this

manual or other local manuals should be explained and illustrated with detaileddrawings.

• Non-ESC BMPs - Indicate any equipment washdown areas, areas of contaminatedsoils or other BMPs used where there are site-specific requirements.

Best Management Practices Guide for Stormwater 6-1Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMPs for Erosion and Sediment Control


6.0 BEST MANAGEMENT PRACTICES FOR EROSIONAND SEDIMENT CONTROL

6.1 Introduction

Best Management Practices (BMPs) are defined as physical, structural and/or managerialpractices, that when used singly or in combination, prevent or reduce pollution of water. This chapter contains guidelines and design criteria for erosion and sediment controlBMPs.

6.2 Guidelines For Cover Practices

Design criteria for BMPs for erosion and sedimentation control can be broadly dividedinto two categories: cover practices (such as seeding and mulching) and structuralpractices (such as sediment ponds, filter fences, etc.) which require engineeringguidelines and design criteria. Cover practices are described in detail in Sections 6.3 and6.4. Structural ESC BMPs are dealt with in Sections 6.5 through 6.7 of this appendix.

Vegetative cover is the most important form of erosion control possible because itprevents or reduces erosion rather than attempting to trap sediment after soil has alreadyeroded. In addition, it adds to the aesthetic and functional value of a development.

Cover practices can be divided into temporary and permanent measures. Temporarymeasures are implemented to provide a quick cover to soils that are exposed for longerthan 2-7 days, or if an erosion problem already exists on the site during the developmentphase. They include the following:

• seeding;• mulching and matting; and• clear plastic covering.

Permanent measures are implemented both during and on completion of constructionactivities. They include the following:

• preserving natural vegetation;• buffer zones;• permanent seeding and planting; and• sodding.

GVRD GVRD

6-2 Best Management Practices Guide for StormwaterAppendix H: Construction Site Erosion and Sediment Control Guide for Stormwater Management

BMP TC1: Temporary Seeding of Stripped Areas


6.3 Temporary Cover Practices

6.3.1 BMP TC1: Temporary Seeding of Stripped Areas

Definition The establishment of a temporary vegetative cover on disturbed areas byseeding with appropriate rapidly growing annual plants.

Purpose

To provide temporary soil stabilization by planting grasses and legumes to areas whichwould remain bare for more than 7 days where permanent cover is not necessary orappropriate.

Conditions Where Practice Applies

• Permanent structures are to be installed or extensive re-grading of the area willoccur prior to the establishment of permanent vegetation.

• Areas which will not be subjected to heavy wear by construction traffic.• Areas sloping up to 10% for 30 m or less (where temporary seeding is the only

BMP used.

Advantages

• This is a relatively inexpensive form of erosion control but should only be used onsites awaiting permanent planting or grading. Those sites should have permanentmeasures used (see BMP PC3, Permanent Seeding and Planting).

• Vegetation will not only prevent erosion from occurring, but will also trapsediment in runoff from other parts of the site.

• Temporary seeding offers fairly rapid protection to exposed areas.

Disadvantages/Problems

• Temporary seeding is only viable when there is a sufficient window in time forplants to grow and establish cover. During the establishment period the bare soilshould be protected with mulch (see BMP TC2) and/or clear plastic covering (seeBMP TC3).

• If sown on subsoil, growth will be poor unless heavily fertilized and limed. Because over-fertilization can cause pollution of stormwater runoff, otherpractices such as mulching (BMP TC2) alone may be more appropriate. Thepotential for over-fertilization is an even worse problem in or near aquaticsystems.

• Once seeded, areas cannot be used for heavy traffic.

Best Management Practices Guide for Stormwater 6-3Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP TC1: Temporary Seeding of Stripped Areas


• May require regular irrigation to flourish. Regular irrigation is not encouragedbecause of the expense and the potential for erosion in areas that are not regularlyinspected. The use of low maintenance native species should be encouraged, andplanting should be timed to minimize the need for irrigation.

Planning Considerations

Sheet erosion, caused by the impact of rain on bare soil, is the source of most fineparticles in sediment. To reduce this sediment load in runoff, the soil surface itselfshould be protected. The most efficient and economical means of controlling sheet andrill erosion is to establish vegetative cover. Annual plants which sprout rapidly andsurvive for only one growing season are suitable for establishing temporary vegetativecover. Temporary seeding is effective when combined with construction phasing so bareareas of the site are minimized at all times.

Temporary seeding may prevent costly maintenance operations on other erosion controlsystems. For example, sediment basin clean-outs will be reduced if the drainage area ofthe basin is seeded where grading and construction are not taking place. Perimeter dikeswill be more effective if not choked with sediment.

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

Proper seedbed preparation and the use of quality seed are important in this practice justas in permanent seeding. Failure to carefully follow sound agronomic recommendationswill often result in an inadequate stand of vegetation that provides little or no erosioncontrol.

Design Criteria

• Time of Planting - Planting should preferably be done between April 1 andJune 30, and September 1 through October 31. If planting is done in the monthsof July and August, irrigation may be required. If planting is done betweenNovember 1 and March 31, mulching should be required immediately afterplanting. If seeding is done during the summer months, irrigation of some sortwill probably be necessary.

• Site Preparation - Before seeding, install needed surface runoff control measuressuch as gradient terraces, interceptor dike/swales, level spreaders, and sedimentbasins.

• Seedbed Preparation - The seedbed should be firm with a fairly fine surface. Perform all cultural operations across or at right angles to the slope. See BMPPC5 Topsoiling, and BMP EC6 Surface Roughening for more information onseedbed preparation. A minimum of 50 mm to 100 mm of tilled topsoil isrequired.


BMP TC1: Temporary Seeding of Stripped Areas


• Fertilization - as per suppliers recommendations. Developments adjacent to waterbodies must use non-phosphorus fertilizer.

• Seeding - seeding mixtures will vary depending on the exact location, soil type,slope, etc. Information on mixes may be obtained from local suppliers. However,approval to use any particular mix should be obtained from the local government.The following seed mix is supplied as guidance.

NameProportionsby Weight

PercentPurity

PercentGermination

Redtop (Agrostis alba) 10% 92 90Annual Rye (Lolium multiflorum) 40% 98 90Chewings Fescue (Festuca rubracommutata)

40% 97 80

White Dutch Clover (Trifoliumrepens)

10% 96 90

• "Hydro-seeding" applications with approved seed-mulch-fertilizer mixtures may also beused.

Maintenance

• Seeding should be supplied with adequate moisture. Supply water as needed,especially in abnormally hot or dry weather or on adverse sites. Water applicationrates should be controlled to prevent runoff.

• Re-seeding - Areas which fail to establish vegetative cover adequate to preventerosion shall be re-seeded as soon as such areas are identified.

• All temporary erosion and sediment control measures should be removed within30 days after final site stabilization is achieved or after the temporary BMPs are nolonger needed. Trapped sediment should be removed or stabilized on site. Disturbed soil areas resulting from removal should be permanently stabilized.

Best Management Practices Guide for Stormwater 6-5Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP TC2: Mulching and Matting


6.3.2 BMP TC2. Mulching and Matting

Definition Application of plant residues or other suitable materials to the soil surface.

Purpose

To provide immediate protection to exposed soils during the period of short constructiondelays, or over winter months through the application of plant residues, or other suitablematerials, to exposed soil areas.

Mulches also enhance plant establishment by conserving moisture and moderating soiltemperatures. Mulch helps hold fertilizer, seed, and topsoil in place in the presence ofwind, rain, and runoff and maintains moisture near the soil surface.


• In areas which have been seeded either for temporary or permanent cover,mulching should immediately follow seeding.

• Areas which cannot be seeded because of the season, or are otherwise unfavorablefor plant growth.

• Areas which have been seeded as specified in Temporary Seeding (BMP TC1). • In an area of greater than 2:1 slope, mulching should immediately follow seeding.

Advantages

• Mulching offers instant protection to exposed areas.• Mulches conserve moisture and reduce the need for irrigation. • Neither mulching nor matting require removal; seeds can grow through them

unlike plastic coverings.


• Care must be taken to apply mulch at the specified thickness, and on steep slopesmulch must be supplemented with netting.

• Thick mulches can reduce the soil temperature, delaying seed germination.• Mulches such as straw, which are often applied to areas after grading must then be

removed and either composted or landfilled. Straw is hollow, so it can actuallydraw water into the ground below it if the straw is at an angle.


BMP TC2: Mulching and Matting



Mulches are applied to the soil surface to conserve a desirable soil property or topromote plant growth. A surface mulch is one of the most effective means ofcontrolling runoff and erosion on disturbed land (see Figure 6.1 for a comparison ofpollutant loading reductions for various mulches).

Figure 6.1: Mean TSS and Overall Pollutant Loading Reductionsof Slope Treatments Relative to Controls, from (1)

Mulches can increase the infiltration rate of the soil, reduce soil moistureloss by evaporation, prevent crusting and sealing of the soil surface, modifysoil temperatures, and provide a suitable microclimate for seed germination.

Organic mulch materials, such as straw, wood chips, bark, and wood fiber, have beenfound to be the most effective.

A variety of nets and mats have been developed for erosion control in recent years, andthese are also used as mulches, particularly in critical areas such as waterways. They maybe used to hold other mulches to the soil surface.



The choice of materials for mulching will be based on the type of soil to be protected, siteconditions, season, and economics. It is especially important to mulch liberally in mid-summer and prior to winter, and on cut slopes and southern slope exposures. Table 6.1gives a comparison of costs based on 1988 figures.

Table 6.1Summary of Mulch and Mat Estimated Service Lives and Costs

1988 Base, adopted from (1)

Technique*Estimated Service

Life (months)

Estimated Cost(Cdn. $/ha served)(6 months service)

Straw (9.1 tonne/ha) 3 $12,000Straw (2.8 tonne/ha) 3 $9,400Straw (9.1 tonne/ha), manure-mulched, fertilized, seeded

6 $9,000

Jute mat 6 $13,900Excelsior 6 $13,500Woven straw blanket 6 $15,400Synthetic fiber blanket 6 $12,400Wood fiber mulch (2.8 tonne/ha)fertilized, seeded

6 $4,900

Wood fiber mulch (2.8 tonne/ha)with tackifier (475 L/ha), fertilized,seeded

6 $7,100

Wood fiber mulch (2.8 tonne/ha)with tackifier (850 L/ha), fertilized,seeded

6 $7,900

Wood fiber mulch (2.8 tonne/ha)with tackifier (1,140 L/ha),fertilized, seeded

6 $8,600

Chemical agent 6 $7,900Plastic sheeting 6 $8,600Designed sedimentation pond > 6 < $15,800Non-designed pond > 6 < $28,100

* The estimated cost of seeding where it was used is based on hydro-seeding (approximately Cdn. $1,875/ha).

Organic Mulches

Straw - Straw is the mulch most commonly used in conjunction with seeding. Its use isrecommended where immediate protection is desired and preferably where the need forprotection will be less than 3 months. The straw should come from wheat or oats, andmay be spread by hand or machine. If the straw is not clean, weed growth can occur.




Straw can be windblown and must be anchored down. Common anchoring methods areas follows:

1. crimping, disking, rolling or punching into the soil;2. covering with netting;3. spraying with a chemical or fiber binder (tackifier); and4. keeping moist. Natural precipitation can often provide sufficient moisture.(2)

Corn Stalks - These should be shredded into 100 mm to 150 mm lengths. Stalksdecompose slowly and are resistant to windblow.

Wood Chips - Suitable for areas that will not be closely mowed, and around ornamentalplantings. Chips decompose slowly and do not require tacking. They must be treatedwith 6 kg nitrogen per tonne to prevent nutrient deficiency in plants. Chips can be a veryinexpensive mulch if they are obtained from trees cleared on the site. However, bothwood and bark chips tend to wash down slopes of more than 6 percent and createproblems by clogging inlet grates etc. and are therefore not preferred for use in thoseareas.

Bark Chips, Shredded Bark - By-products of timber processing. Used in landscapedplantings. Bark is also a suitable mulch for areas planted to grasses and not closelymowed; may be applied by hand or mechanically. Bark is not usually toxic to grasses orlegumes, and additional nitrogen fertilizer is not required.

Wood Fiber - Used in hydro-seeding operations, applied as part of the slurry. These shortcellulose fibers do not require tacking, although a tacking agent or soil binders aresometimes used with wood fiber. The longer the fiber length, the better the wood fiberwill work in erosion control. This form of mulch does not provide sufficient protection toerodible soils to be used alone during the severe heat of summer or for late fall seedings. Wood fiber hydro-seed slurries may be used to tack straw mulch. This combinationtreatment is well suited for steep slopes and critical areas, and severe climate conditions.

There are other organic materials which make excellent mulches but are only availablelocally or seasonally. Creative use of these materials can reduce costs.

Manure Mulches - Manure mulches should be well-aged and are not recommended foruse near waterbodies.

Chemical Mulches and Soil Binders

The use of synthetic, spray-on materials (except tacking agents used with hydro-seeding)is not recommended. A major problem with their use is the creation of impervioussurfaces and, possibly, adverse effects on water quality. Research has shown that theycan cause more erosion when used than does bare exposed soil.Nets and Mats - Used alone, netting does not retain soil moisture or modify soil



temperature. It stabilizes the soil surface while grasses are being established, and isuseful in grassed waterways and on slopes. Light netting may also be used to hold othermulches in place. Its relatively high cost makes it most suitable for small sites.

The most critical aspect of installing nets and mats is obtaining firm, continuous contactbetween the material and the soil. Without such contact, the material is useless anderosion occurs. It is important to use an adequate number of staples and to roll thematerial after laying it to ensure that the soil is protected.

Design Criteria

• Site Preparation - Same as Temporary Seeding.• Mulch Materials, Application Rates, and Specifications - See Table 6.2.• Erosion blankets (nets and mats) may be used on level areas, on slopes up to

50 percent, and in waterways. Where soil is highly erodible, nets shall only beused in connection with an organic mulch such as straw and wood fiber. Jute netsshall be heavy, uniform cloth woven of single jute yarn, which if 1 m to 1.2 mwide shall weigh an average of 0.6 kg/linear metre. It must be so applied that it isin complete contact with the soil. If it is not, erosion will occur beneath it. Netting shall be securely anchored to the soil with No. 11 gauge wire staples atleast 150 mm long, with an overlap of three inches.

• Excelsior blankets are considered protective mulches and may be used alone onerodible soils and during all times of year.

• See Figure 6.2 for orientation of netting and matting.

Maintenance

• Mulched areas should be checked periodically, especially following severe storms,when damaged areas of mulch or tie-down material should be repaired.

• All temporary erosion and sediment control measures shall be removed within30 days after final site stabilization is achieved or after the temporary BMPs areno longer needed. Trapped sediment shall be removed or stabilized on site. Disturbed soil areas resulting from removal shall be permanently stabilized.




Table 6.2Guide to Mulch Materials, Rates and Uses

Mulch Material QualityGuidelines Application Rates

Depth ofApplication Remarks1

Gravel, slag orcrushed stone

Washed, 19 mm– 38 mm size

74 m3/1000 m2 75 mm Excellent mulch for shortslopes and around woodyplants & ornamentals. Use where subject to foottraffic. Approx. 1,200kg/m3.

Hay or straw Air dried, freefrom unwantedseeds andcoarse material

370 kg/1000 m2 to 490kg/1000 m2 or

20 to 30 bales/1000 m2

Minimum of 50mm

Use where the mulchingeffect is to be maintainedfor >3 months. Is subjectto wind blowing unlesskept moist or tackeddown. Most common &widely used mulchingmaterial. Can be used incritical erosion area

Wood fibercellulose(partiallydigested woodfibers)

Dyed greenshould notcontain growthinhibitingfactors.

120 kg/1000 m2 to 145kg/1000 m2

If used on critical areas,double the normalapplication rate. Applyw/hydromulcher. No tie-down required. Packaged in 45 kg bags.

1 All mulches will provide some degree of (1) erosion control, (2) moisture conservation, (3) weed control, and (4) reduction of soil crusting.

References

(1) Horner, Richard R., Juno Guedry and Michael H. Kortenhof, Improving the CostEffectiveness of Highway Construction Site Erosion and Pollution Control,Washington State Dept. of Transportation, WA-RD 200.1, January, 1990.



Figure 6.2 Orientation of Netting and Matting


BMP TC3: Clear Plastic Covering


6.3.3 BMP TC3: Clear Plastic Covering

C o d e : S y m b o l:

Definition The covering with clear plastic sheeting of bare areas which need immediateprotection from erosion.

Purpose

To provide immediate temporary erosion protection to slopes and disturbed areas thatcannot be covered by mulching, in particular during the specified seeding periods or asotherwise required by the local government. Clear plastic is also used to protect disturbedareas which must be covered during short periods of inactivity to meet November 1-March 31 cover requirements. Because of many disadvantages clear plastic covering isthe least preferred covering BMP.


• Disturbed areas which require immediate erosion protection.• Areas seeded during the time period from November 1 to March 1. Note:

Plantings at this time require clear plastic covering for germination and protectionfrom heavy rains.

Advantages

• Clear plastic covering is a good method of protecting bare areas which needimmediate cover and for winter plantings.

• May be quickly and easily placed.


• There can be problems with vandals and maintenance.• The sheeting will result in rapid, 100% runoff which may cause serious erosion

problems and/or flooding at the base of slopes unless the runoff is properlyintercepted and safely conveyed by a collecting drain. This is strictly a temporarymeasure, so permanent stabilization is still required.

• It is relatively expensive.• The plastic may blow away if it is not adequately overlapped and anchored. • Ultraviolet and possibly visible light can cause some types of plastic to become

brittle and easily torn.• Plastic must be disposed of at a landfill; it is not easily degradable in the

environment.• If plastic is left on too long during the spring it can severely burn any vegetation

that has grown under it during cooler periods.

Best Management Practices Guide for Stormwater 6-13Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP TC3: Clear Plastic Covering


Design Criteria

• Clear plastic sheeting shall have a minimum thickness of 6 mil.• Covering shall be installed and maintained tightly in place by using sandbags or

tires on ropes with a maximum 3 m grid spacing in all directions. All seams shallbe taped or weighted down full length and there shall be at least a 300 mm to 600mm overlap of all seams. Seams should then be rolled and staked or tied.

• Covering shall be installed immediately on areas seeded between November 1 toMarch 1, and remain until vegetation is firmly established.

• When the covering is used on unseeded slopes, it shall be left in place until thenext seeding period.

• Sheeting should be toed in at the top of the slope to prevent surface flow beneaththe plastic.

• Sheeting should be removed as soon as is possible once vegetation is well grownto prevent burning the vegetation through the plastic sheeting, which acts as agreenhouse.

Maintenance

• Check regularly for rips and places where the plastic may be dislodged. Contactbetween the plastic and the ground should always be maintained. Any air bubblesfound should be removed immediately or the plastic may rip during the nextwindy period. Re-anchor or replace the plastic as necessary.

All temporary erosion and sediment control measures should be removed within30 days after final site stabilization is achieved or after the temporary BMPs areno longer needed. Trapped sediment should be removed or stabilized on site. Disturbed soil areas resulting from removal should be permanently stabilized.


BMP PC1: Preserving Natural Vegetation


6.4 Permanent Cover Practices

6.4.1 BMP PC1: Preserving Natural Vegetation


Definition Minimizing exposed soils and consequent erosion by clearing only whereconstruction will occur.

Purpose

To reduce erosion by preserving natural vegetation wherever practicable.

Condition Where Practice Applies

• Natural vegetation should be preserved on steep slopes, near perennial andintermittent watercourses or swales, and on building sites in wooded areas.

• As required by local governments.

Advantages

Preserving natural vegetation will:

• Help reduce soil erosion.• Beautify an area.• Save money on landscaping costs.• Provide areas for wildlife.• Possibly increase the value of the land.• Provide buffers and screens against noise. • Moderate temperature changes and provide shade and cover habitat for surface

waters and land. This is especially important where detention ponds drain tosalmonid-bearing streams. Increases in water temperature tend to lower thedissolved oxygen available for aquatic life.


• Saving individual trees can be difficult, and older trees may become a safetyhazard. Cottonwood and alder trees are especially prone to blowdown.


New development often takes place on tracts of forested land. In fact, building sites areoften selected because of the presence of mature trees. However, unless sufficient care istaken and planning done, in the interval between buying the property and completing

Best Management Practices Guide for Stormwater 6-15Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP PC1: Preserving Natural Vegetation


construction much of this resource is likely to be destroyed. The property owner isultimately responsible for protecting as many trees as possible, with their understory andgroundcover. This responsibility is usually exercised by agents--the planners, designersand contractors. It takes 20 to 30 years for newly planted trees to provide the benefits forwhich we value trees so highly.

Design Criteria

Natural vegetation can be preserved in natural clumps or as individual trees, shrubs andvines.

The preservation of individual plants is more difficult because equipment is generallyused to remove unwanted vegetation. The points to remember when attempting to saveindividual plants are:

• Is the plant worth saving? Consider the location, species, size, age, vigor, and thework involved. Local governments may also have bylaws to save naturalvegetation and trees.

• Is the tree or shrub a desirable plant? Is it shallow-rooted, do the roots seek water,or are insects and disease a problem? Shallow-rooted plants can cause problemsin the establishment of lawns or ornamental plants. Water-seeking roots canblock sewer and tile lines. Insects and diseases can make the plant undesirable. This is especially true with aphid on alder and maple.

• Old and/or large plants do not generally adapt to changes in environment asreadily as young plants of the same species. Usually, it is best to leave treeswhich are less than 40 years of age. Some of the hardwoods (Red alder, Cherry,etc.) mature at approximately 50 years of age. After maturity they rapidly declinein vigor. Conifers, after 40 years of age, may become a safety hazard due to thepossibility of breakage or blowdown, especially where construction has left only afew scattered trees in an area that was formerly dense woods. While old largetrees are sometimes desirable, the problem of later removal should be considered.Again, local governments may have requirements to preserve older, largerspecimen trees. It is expensive to cut a large tree and to remove the tree andstump from a developed area. Thinning some branches from trees can provideavenues for wind and hence lessen the "sail" effect.

• Clearly flag or mark areas around trees that are to be saved. It is preferable tokeep ground disturbance away from the trees at least as far out as the dripline.

Plants need protection from three kinds of injuries:

• Construction Equipment -- This injury can be above or below the ground level. Damage results from scarring, cutting of roots, and compaction of the soil. Suchinjuries can be prevented by roping or fencing a buffer zone around plants to besaved prior to construction (Figure 6.3).

• Grade Changes -- Changing the natural ground level will alter grades which affect




the plant's ability to obtain the necessary air, water, and minerals. Minor fillsusually do not cause problems although sensitivity between species does vary. Cedars are more sensitive. Trees can tolerate fill of 150 mm or less. For shrubsand other plants the fill should be less. When there are major changes in grade, itmay become necessary to supply air to the roots of plants. This can be done byplacing a layer of gravel and a tile system over the roots before the fill is made. Atile system protects a tree from a raised grade.

The tile system should be laid out on the original grade leading from a dry wellaround the tree trunk. The system should then be covered with small stones toallow air to circulate over the root area (see Figure 6.3).

Lowering the natural ground level can seriously damage trees and shrubs. Thehighest percentage of the plant roots are in the upper 300 mm of the soil and cutsof only 50 mm to 75 mm can cause serious injury. To protect the roots it may benecessary to terrace the immediate area around the plants to be saved. If roots areexposed, construction of retaining walls may be needed to keep the soil in place. Plants can also be preserved by leaving them on an undisturbed, gently slopingmound. To increase the chances for survival, it is best to limit grade changes andother soil disturbances to areas outside the dripline of the plant (Figure 6.3).

• Excavations -- Protect trees and other plants when excavating for tile, water, andsewer lines. Where possible, the trenches should be routed around trees and largeshrubs. When this is not possible, it is best to tunnel under them. This can bedone with hand tools or with power augers.

If it is not possible to route the trench around plants to be saved, then thefollowing should be observed:

- Cut as few roots as possible. When you have to cut -- cut clean. Paint cut rootends with a wood dressing like asphalt base paint.

- Backfill the trench as soon as possible.- Tunnel beneath root systems as close to the center of the main trunk as

possible to preserve most of the important feeder roots.

Some problems that can be encountered with a few specific trees are:

• Maple, Dogwood, Red alder, Western hemlock, Western red cedar and Douglasfir do not readily adjust to changes in environment and special care should betaken to protect these trees.

• The tipover hazard of Pacific silver fir is high while that of Western hemlock ismoderate. The danger of tipover increases where dense stands have been thinned.Other species (unless they are on shallow, wet soils under 20 inches deep) have alow tipover hazard.

• Cottonwoods, maples, and willows have water-seeking roots. These can cause

Best Management Practices Guide for Stormwater 6-17Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP PC1: Preserving Natural Vegetation


trouble in sewer lines and filter fields. On the other hand, they thrive in highmoisture conditions that other trees would succumb to.

• Thinning operations in pure or mixed stands of Grand fir, Pacific silver fir, Noblefir, Sitka spruce, Western red cedar, Western hemlock, Pacific dogwood, and Redalder can cause serious disease problems. Disease can become establishedthrough damaged limbs, trunks, roots, and freshly cut stumps. Diseased andweakened trees are also susceptible to insect attack.

Maintenance

• Inspect flagged areas regularly to make sure flagging has not been removed. Iftree roots have been exposed or injured, re-cover and/or seal them.




Figure 6.3 Preserving Natural Vegetation

Best Management Practices Guide for Stormwater 6-19Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP PC2: Buffer Zones


6.4.2 BMP PC2: Buffer Zones

BZC o d e : S y m b o l:

Definition and Purpose An undisturbed area or strip of natural vegetation or anestablished suitable planting that will provide a living filter to reduce soil erosion andrunoff velocities.


• Natural buffer zones are used along streams and other bodies of water that needprotection from erosion and sedimentation. Vegetative buffer zones can be usedto protect natural swales and can be incorporated into natural landscaping of anarea.

Advantages

• Buffer zones provide critical habitat adjacent to streams and wetlands, as well asassist in controlling erosion, especially on unstable steep slopes. Buffers alongstreams and other water bodies also provide wildlife corridors, a protected areawhere wildlife can move from one place to another.

• Act as a visibility and noise screen.


• Extensive buffers will increase development costs.

Design Criteria

• Preserving natural vegetation or plantings in clumps, blocks, or strips is generallythe easiest and most successful method.

• Leave all unstable steep slopes in natural vegetation.• Fence or flag clearing limits and keep all equipment and construction debris out of

the natural areas.• Keep all excavations outside the dripline of trees and shrubs.• Do not push debris or extra soil into the buffer zone area because it will cause

damage from burying and smothering.• Vegetative buffer zones for streams, lakes or other waterways should be a

minimum of 30 m wide on each side with increases subject to other on-sitesensitive conditions, existing vegetative conditions and erosion hazard potential.


BMP PC2: Buffer Zones


Maintenance

• Inspect the area frequently to make sure flagging remains in place and the arearemains undisturbed.

Best Management Practices Guide for Stormwater 6-21Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP PC3: Permanent Seeding and Planting


6.4.3 BMP PC3: Permanent Seeding and Planting

PS PSC o d e : S y m b o l:

Definition The establishment of perennial vegetative cover on disturbed areas.

Purpose

To establish permanent vegetation (such as grasses, legumes and trees and shrubs) asrapidly as possible to prevent soil erosion by wind or water, and to improve wildlifehabitat and site aesthetics.

To provide pollutant filtration (biofiltration) in vegetation-lined channels and to establishconstructed wetlands as required (see BMPs S8, S9, S10).


• Graded, final graded or cleared areas where permanent vegetative cover is neededto stabilize the soil.

• Areas which will not be brought to final grade for a year or more.• Vegetation-lined channels.• Retention or detention ponds as required.

Advantages

• Well established grass and ground covers can give an aesthetically pleasing,finished look to a development.

• Once established, the vegetation will serve to prevent erosion and retard thevelocity of runoff.


• Vegetation and mulch cannot prevent soil slippage and erosion if soil is notinherently stable.

• Coarse, high grasses that are not mowed can create a fire hazard in some locales. Very short mowed grass, however, provides less stability and sediment filteringcapacity.

• Grass planted to the edge of a watercourse may encourage fertilizing and mowingnear the water's edge and increase nutrient and pesticide contamination.

• May require regular irrigation to establish and maintain.


BMP PC3: Permanent Seeding and Planting



Vegetation controls erosion by reducing the velocity and the volume of overland flow andprotecting the bare soil surface from raindrop impact.

Areas which must be stabilized after the land has been disturbed require vegetative cover.The most common and economical means of establishing this cover is by seeding grassesand legumes.

Advantages of seeding over other means of establishing plants include the small initialestablishment cost, the wide variety of grasses and legumes available, low laborrequirement, and ease of establishment in difficult areas.

Consider the microclimate(s) within the development area. Low areas may be frostpockets and require hardier vegetation since cold air tends to sink and flow towards lowspots. South-facing slopes may be more difficult to re-vegetate because they tend to besunnier and drier.

Disadvantages which must be dealt with are the potential for erosion during theestablishment stage, a need to reseed areas that fail to establish, limited periods during theyear suitable for seeding, and a need for water and appropriate climatic conditions duringgermination.

There are so many variables in plant growth that an end product cannot be guaranteed. Much can be done in the planning stages to increase the chances for successful seeding. Selection of the right plant materials for the site, good seedbed preparation, timing, andconscientious maintenance are important. Whenever possible, native species of plantsshould be used for landscaping. These plants are already adapted to the locale andsurvivability should be higher than with exotic species.

Native species are also less likely to require irrigation, which can be a large maintenanceburden and is neither cost-effective nor ecologically sound.

If non-native plant species are used, they should be tolerant of a large range of growingconditions and be as low-maintenance as possible.

Design Criteria

• Vegetation cannot be expected to supply an erosion control cover and preventslippage 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 periodof November 1 through March 1, when the site should be protected by mulchingor plastic covering until the next seeding period.



• Permanent vegetation may be in the form of grass-type growth by seeding orsodding, or it may be trees or shrubs, or a combination of these. Establishing thiscover may require the use of supplemental materials, such as mulch or jute netting(see BMP TC2).

• Site Preparation: Install needed surface runoff control measures such as gradientterraces, berms, dikes, level spreaders, waterways, and sediment basins prior toseeding or planting.

• Seeding Grasses and Legumes: Seedbed Preparation -- If infertile or coarsetextured subsoil will be exposed during land shaping, it is best to stockpile topsoiland respread it over the finished slope at a minimum 50 mm to 150 mm depth androll it to provide a firm seedbed. If construction fills have left soil exposed with aloose, rough, or irregular surface, smooth with blade and roll. If cuts or construc-tion equipment have left a tightly compacted surface, break with chisel plow orother suitable implement. Perform all cultural operations across or at right anglesto the slope (contoured), such as with cat tracks on the final pass. The seedbedshould be firm with a fairly fine surface.

• Soil Amendments: Rates will depend on site characteristics and soil, but as aguide, apply lime at the rate of 490 kg per 1,000 m2. Apply actual nitrogen at therate of 4.9-9.8 kg per 1,000 m2, phosphoric acid at the rate of 7.3 kg per 1,000 m2,and potassium at the rate of 7.3 kg per 1,000 m2. Work in lime and other nutrientsto a depth of a minimum of 100 mm with suitable equipment. Scatteramendments uniformly and work into the soil during seedbed preparation.

• Seeding: Apply an appropriate mixture to the prepared seedbed at a rate of 136kg/ha. (Seed mixture may be varied by the local government to take account oflocal conditions).

Urban Application:

Name Portions by Weight Percent Purity GerminationKentucky BluegrassCreeping Red FescuePerennial Rye

30%40%30%

859885

809090

Rural Application:

Name Portions by Weight Percent Purity GerminationKentucky Bluegrass (Poa pratensis)Tall Fescue (Festuca arundincea)Perennial Rye (Lolium perenne)Chewings Fescue

15%40%30%15%

85959595

80909090


BMP PC3: Permanent Seeding and Planting


Cover the seed with topsoil or mulch no deeper than 13 mm. It is better to work topsoilinto the upper soil layer rather than spread a layer of it directly onto the top of the nativesoil.

"Hydro-seeding" applications with approved seed-mulch-fertilizer mixtures may also beused.

Wetlands Seed Mixtures: For newly created wetlands, a wetlands specialist shoulddesign plantings to provide the best chance of success. As a guide apply the followingmixture at a rate of 68 kg/ha, and/or additional tubers for cattail, bulrush, slough sedge, asrequired by the local government (see BMP S8 for more information on constructedwetlands).

Do not under any circumstances use introduced, invasive plants like reedcanarygrass (Phalaris arundinacea) or purple loosestrife (Lythrum salicaria). Usingplants such as these will cause many more problems than they will ever solve.

Name Portions by Weight Percent Purity GerminationRed Top (Agrostis alba)Birdsfoot Trefoil (Lotus corniculatus)Creeping Red Fescue

30%30%40%

929098

808090

Tree and Shrub Planting

Besides their erosion and sediment control values, trees and shrubs also provide naturalbeauty and wildlife benefits. When used for the latter, they are usually more effectivewhen planted in clumps or blocks. These procedures should be followed:

1. Trees and shrubs will do best in topsoil. If no topsoil is available, they can beestablished in subsoil with proper amendment. If trees and shrubs are to beplanted in subsoil, particular attention should be paid to amending the soil withgenerous amounts of organic matter. Mulches should also be used.

2. Good quality planting stock should be used. Normally one or two-year olddeciduous seedlings, and three or four-year old coniferous transplants, whenproperly produced and handled are adequate. Stock should be kept cool and moistfrom time of receipt and planted as soon as possible.

3. Competing vegetation, if significant, should be pulled out of the area where theplant or plants are to be placed.

Maintenance

Inspect seeded areas for failure and make necessary repairs and reseed immediately. Conduct or follow-up survey after one year and replace failed plants where necessary.



• If vegetative cover is inadequate to prevent rill erosion, overseed and fertilize inaccordance with soil test results.

• If a stand has less than 40% cover, reevaluate choice of plant materials andquantities of lime and fertilizer. Re-establish the stand following seedbedpreparation and seeding recommendations, omitting lime and fertilizer in theabsence of soil test results. If the season prevents resowing, mulch or jute nettingis an effective temporary cover.


BMP PC4: Sodding


6.4.4 BMP PC4: Sodding

SO SOC o d e : S y m b o l:

Definition Stabilizing fine-graded disturbed areas by establishing permanent grassstands with sod.

Purpose

To establish permanent turf for immediate erosion protection or to stabilize drainagewayswhere concentrated overland flow will occur.


• Disturbed areas which require immediate vegetative cover.• Waterways carrying intermittent flow, where immediate stabilization or aesthetics

are factors and other locations which are particularly suited to stabilization withsod.

Advantages

• Sod will give immediate protection.• Sod gives an immediate vegetative cover, which is both effective in checking

erosion and is aesthetically pleasing.• Good sod has a high density of growth which is superior in protection to a

recently seeded area. • Sod can be placed at any time of the year provided that soil moisture is adequate

and the ground is not frozen.


• Sod is expensive. • Sod is heavy and handling costs are high.• Good quality sod, free from weed species, may be difficult to obtain.• If laid in an unfavorable season, midsummer irrigation may be required. This also

applies to very droughty sandy soils.• Grass species in the sod may not be suitable for site conditions.• If mowing is required, do not use grass sod on slopes steeper than 3:1 (use

minimum maintenance ground covers). • If not anchored or drained properly, sod will "roll up" in grassed waterways.

Best Management Practices Guide for Stormwater 6-27Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP PC4: Sodding


Design Criteria

• Shape and smooth the surface to final grade in accordance with the approvedgrading plan.

• Use of topsoil shall be in accordance with the requirements of Topsoiling (BMPPC5).

• Add lime to reach a soil pH value of 6.5 (based on soil tests).• Fertilize according to a soil test or in the absence of a test use available nitrogen,

phosphorus and potash as prescribed for permanent seeding. Use fertilizers thatare not highly soluble.

• Work lime and fertilizer into the soil 25 mm to 50 mm deep and smooth thesurface.

• Lay strips of sod beginning at the lowest area to be sodded and perpendicular tothe direction of water flow. Wedge strips securely in place. Square the ends ofeach strip to provide for a close, tight fit. Stagger joints at least 300 mm. Stapleif on steep slopes.

• Roll the sodded area and irrigate.• When sodding is carried out in alternating strips, or other patterns, seed the areas

between the sod immediately after sodding.• Sod should be free of weeds and be of uniform thickness (Approx. 25 mm) and

should have a dense root mat for mechanical strength.

Maintenance

• Inspect sodded areas regularly, especially after large storm events. Re-tack, re-sod, or re-seed as necessary.


BMP PC5: Topsoiling


6.4.5 BMP PC5: Topsoiling

While not a permanent cover practice in itself, topsoiling has been included in this sectionbecause it is an integral component of preparing permanent cover to those areas wherethere is an unsuitable soil surface for plant growth. Use of in-situ or imported topsoil isalways preferable to planting in subsoil.

Definition Preserving and using topsoil to enhance final site stabilization withvegetation.

Purpose

To provide a suitable growth medium for final site stabilization with vegetation.


• Preservation or importation of topsoil is determined to be the most effectivemethod of providing a suitable growth medium, and the slopes are less than 2:1.

• Applicable to those areas with highly dense or impermeable soils or areas whereplanting is to be done in subsoil, where mulch and fertilizer alone would notprovide a suitable growth medium.

Advantages

• Topsoil stockpiling ensures that a good growth medium will be available forestablishing plant cover on graded areas. It has a high organic matter content andfriable consistency, water holding capacity and nutrient content.

• The stockpiles can be used as noise and view baffles during construction.


• Stripping, stockpiling, and reapplying topsoil, or importing topsoil may notalways be cost-effective. It may also create an erosion problem if improperlysecured.

• Unless carefully located, storage banks of topsoil may also obstruct site operationsand therefore require double handling.

• Topsoiling can delay seeding or sodding operations, increasing exposure time ofdenuded areas.

• Most topsoil contains some weed seeds.

Best Management Practices Guide for Stormwater 6-29Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP PC5: Topsoiling



Topsoil is the surface layer of the soil profile, generally characterized as being darker thanthe subsoil due to the presence of organic matter. It is the major zone of rootdevelopment, carrying much of the nutrients available to plants, and supplying a largeshare of the water used by plants.

Topsoiling is strongly recommended where ornamental plants or high-maintenance turfwill be grown. Topsoiling is a required procedure when establishing vegetation onshallow soils, and soils of critically low pH (high acid) levels.

If topsoiling is to be done, the following items should be considered:

1. Whether an adequate volume of topsoil exists on the site. Topsoil should bespread at a depth of 50 mm to 100 mm. More topsoil will be needed if the subsoilis rocky.

2. Location of the topsoil stockpile so that it meets specifications and does notinterfere with work on the site.

3. Allow sufficient time in scheduling for topsoil to be spread and bonded prior toseeding, sodding, or planting.

4. Care must be taken not to apply to subsoil if the two soils have contrastingtextures. Sandy topsoil over clayey subsoil is a particularly poor combination, aswater creeps along the junction between the soil layers and causes the topsoil toslough.

5. If topsoil and subsoil are not properly bonded, water will not infiltrate the soilprofile evenly and it will be difficult to establish vegetation. The best method toprevent a lack of bonding is to actually work the topsoil into the layer below for adepth of at least 150 mm.

Design Criteria

• Field exploration of the site should be made to determine if there is surface soil ofsufficient quantity and quality to justify stripping. Topsoil should be friable andloamy (loam, sandy loam, silt loam, sandy clay loam, clay loam). Areas of naturalground water recharge should be avoided.

• Stripping should be confined to the immediate construction area. A 100 mm to150 mm stripping depth is common, but depth may vary depending on theparticular soil. All surface runoff control structures shall be in place prior tostripping.

• Stockpiling of topsoil should occur in the following manner: a. Side slopes of the stockpile should not exceed 2:1.


BMP PC5: Topsoiling


b. An interceptor dike with gravel outlet and silt fence should surround alltopsoil stockpiles.

c. Erosion control seeding or covering with clear plastic or other mulchingmaterials (see BMPs E1.10, E1.20) of stockpiles should be completedwithin 7 days of the formation of the stockpile.

• Topsoil should not be placed while in a frozen or muddy condition, when thesubgrade is excessively wet, or when conditions exist that may otherwise bedetrimental to proper grading or proposed sodding or seeding.

• Previously established grades on the areas to be topsoiled should be maintainedaccording to the approved plan.

Maintenance

• Cover piles with clear plastic covering until needed.

Best Management Practices Guide for Stormwater 6-31Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementGuidelines for Structural and Biomechanical Practices


6.5 Guidelines For Structural And Biomechanical Practices

Structural and biomechanical control practices are used to either reduce erosion or retainsediment on the construction site. The BMPs in this section have been divided into twobasic groups based on these characteristics. The guidelines for each BMP are presentedin the same format used for cover practices.

Structural erosion control BMPs include measures for site stabilization (such as stabilizedconstruction entrances), slope protection (such as pipe slope drains) and drainagewayprotection (such as level spreaders). Sediment control BMPs include filter fences, berms,and sediment traps. Table 3.2 at the end of Section 3 gives the coding for these and otherBMPs in this manual.

Structural control is more effective when combined with vegetative protection andappropriate grading practices as part of an Erosion and Sediment Control (ESC) Plan (seethe supplemental guidelines on preparing an ESC plan). Control measures may be eitherpermanent or temporary depending on whether they will remain in use after developmentis complete.

Although temporary structures are emphasized in this section, they may be combined withpermanent control facilities to provide protection of downstream properties duringconstruction. Temporary ESC facilities provide siltation control, but downstream erosionprotection should also be provided. Accordingly, the allowable discharge fromdevelopment sites should not exceed 50% of the pre-development peak flow for theselected design storm (the Puget Sound manual specifies the 2 year, 24-hour designstorm).

It is also important not to disturb areas of natural ground water discharge and/or retention.To accomplish this, a permanent detention pond may have to be constructed first withmodifications allowing it to temporarily function as a sediment pond. Or, a controlstructure may be required on the outlet of the sediment pond.

The design of structural measures for erosion and sedimentation control is accomplishedby carefully predetermining appropriate factors. The design storm, maximum drainagearea, slope and other restrictions are noted for each BMP. The design criteria andlimitations are important; if they are not observed, the simplest measures will fail anderosion control will not be achieved.

In most ESC designs, especially for sites larger than 2 hectares, several small structureswill function more effectively than a single large structure. For example, a combinationof BMPs, such as filter fences, temporary dikes/swales, and several small sedimenttraps/ponds (depending on subbasin configuration) may be used as opposed to a singlelarge sediment pond.


Guidelines for Structural and Biomechanical Practices


Maintenance is also of critical importance for proper operation of structural BMPs andmust be considered in their design. Maintenance requirements and frequency vary witheach BMP and its performance criteria. At a minimum, the ESC plan should requiremonthly maintenance, or following each runoff producing storm (whichever occurs morefrequently), for silt removal and proper operation of all ESC facilities. ESC facilities mayhave to be replaced or relocated depending on their performance under field conditions.

The following factors should be considered when designing structural control measures:

• Use material available on-site whenever possible. • Keep structures simple and take advantage of permanent facilities unless the

permanent structures are for infiltration. • Install the most important control structures first. • Install BMPs correctly; visit the site during and after storms to be sure

that all structures are properly located, constructed, and functioning as designed. • Install control measures in sequences which minimize land disturbance. For

example, install interceptor dikes/swales and drainage trenches before seeding toavoid disturbing the seedbed. Avoid disturbing or removing existing vegetationwhenever possible.

• Do not block a natural drainageway. Make certain that all necessary permits havebeen obtained before starting any work in a wetland, stream, or other sensitivearea.

• Place control measures out of the way of construction operations. • Make field modifications where necessary with the approval of the local

jurisdiction. • Provide access for maintenance.

Although design and construction guidelines are presented in some detail, this section isnot a substitute for training in hydraulic and construction engineering. The materialspresented are guidelines to assist in the design of erosion control measures. Theguidelines provided should not be considered rigid requirements. Where local experiencehas shown that an alternate design will work better, it may be used as long as it meets theobjectives and is approved by the local government. Designers are encouraged tocontinuously seek out new, more reliable solutions for controlling erosion and sediment.

Best Management Practices Guide for Stormwater 6-33Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC1: Stabilized Construction Entrance and Tire Wash


6.6 Structural Erosion Control BMPs

6.6.1 BMP EC1: Stabilized Construction Entrance and Tire Wash

Definition A temporary stone-stabilized pad located at points of vehicular ingress andegress on a construction site.

Purpose

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


• Whenever traffic will be leaving a construction site and moving directly onto apublic road or other paved areas.

Advantages

• Mud on vehicle tires is significantly reduced which avoids hazards caused bydepositing mud on the public roadway.

• Sediment, which is otherwise contained on the construction site, does not enterstormwater runoff elsewhere.


Construction entrances provide an area where mud can be removed from vehicle tiresbefore they enter a public road. If the action of the vehicle traveling over the gravel padis not sufficient to remove the majority of the mud, then the tires should be washed beforethe vehicle enters a public road. If washing is used, provisions should be made tointercept the wash water and trap the sediment before it is carried off-site. Constructionentrances should be used in conjunction with the stabilization of construction roads toreduce the amount of mud picked up by vehicles.

It is important to note that this BMP will only be effective if sediment control is usedthroughout the rest of the construction site.

Design Criteria

• Material should be quarry spalls (where feasible), 100 mm to 200 mm size.


BMP EC1: Stabilized Construction Entrance and Tire Wash


• The rock pad should be at least 300 mm thick and 30 m in length for sites morethan 0.4 ha; and may be reduced to 15 m in length for sites less than 0.4 ha.

• A filter fabric fence (see BMP SR1) should be installed down-gradient from theconstruction entrance in order to contain any sediment-laden runoff from theentrance.

• Width should be the full width of the vehicle ingress and egress area (minimum6.1 m).

• Additional rock should be added periodically to maintain proper function of thepad.

• See Figure 6.4 for details.

Figure 6.4 Stabilized Construction Entrance

• Tire washing should be done before the vehicle enters a paved street. Washing should be done on an area covered with crushed rock and the wash water shouldbe 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 whencalculating the sediment trap or basin size.

Maintenance

• The entrance should be maintained in a condition which will prevent tracking orflow of mud onto public rights-of-way. This may require periodic top dressingwith 50 mm stone, as conditions demand, and repair and/or cleanout of any

Best Management Practices Guide for Stormwater 6-35Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC1: Stabilized Construction Entrance and Tire Wash


structures used to trap sediment. All materials spilled, dropped, washed, ortracked from vehicles onto roadways or into storm drains should be removedimmediately.

• All temporary erosion and sediment control measures should be removed within30 days after final site stabilization is achieved or after the temporary BMPs areno longer needed. Trapped sediment should be removed or stabilized on site. Disturbed soil areas resulting from removal should be permanently stabilized.


BMP EC2: Construction Road Stabilization


6.6.2 BMP EC2: Construction Road Stabilization


Definition The temporary stabilization with stone of access roads, subdivision roads,parking areas, and other on-site vehicle transportation routes immediately after grading.

Purpose

• To reduce erosion of temporary road beds by construction traffic during wetweather.

• To reduce the erosion and therefore regrading of permanent road beds between thetime of initial grading and final stabilization.


• Wherever rock-base roads or parking areas are constructed, whether permanent ortemporary, for use by construction traffic.

• Note: Exceptions may be in areas with gravelly soils, as approved by the localgovernment.

Advantages

• Efficiently constructed road stabilization not only reduces on-site erosion but cansignificantly speed on-site work, avoid instances of immobilized machinery anddelivery vehicles, and generally improve site efficiency and working conditionsduring adverse weather.


• Measures on temporary roads must be cheap not only to install but also todemolish if they interfere with the eventual surface treatment of the area.

• Application of aggregate to construction roads may need to be made more thanonce during a construction period.


Areas which are graded for construction vehicle transport and parking purposes areespecially susceptible to erosion. The exposed soil surface is continually disturbed,leaving no opportunity for vegetative stabilization. Such areas also tend to collect andtransport runoff waters along their surfaces. During wet weather, they often becomemuddy quagmires which generate significant quantities of sediment that may pollutenearby streams or be transported off-site on the wheels of construction vehicles. Dirtroads can become so unstable during wet weather that they are virtually unusable.

Best Management Practices Guide for Stormwater 6-37Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC2: Construction Road Stabilization


Immediate stabilization of such areas with stone may cost money at the outset, but it mayactually save money in the long run by increasing the usefulness of the road during wetweather.

Permanent roads and parking areas should be paved as soon as possible after grading. Asan alternative, the early application of stone may solve potential erosion and stabilityproblems and eliminate later regrading costs. Some of the stone will also probablyremain in place for use as part of the final base course of the road.

Design Criteria

• A 150 mm course of 50 mm to 100 mm crushed rock, gravel base, or crushedsurfacing base course should be applied immediately after grading or thecompletion of utility installation within the right-of-way. A 100 mm course ofasphalt treated base (ATB) may be used in lieu of the crushed rock, or as advisedby the local government.

• Where feasible, alternative routes should be made for construction traffic; one foruse in dry condition, the other for wet conditions which incorporate the measureslisted below.

• Temporary roads should follow the contour of the natural terrain to the maximumextent possible. Slope should not exceed 15 percent. Roadways should becarefully graded to drain transversely. Provide drainage swales on each side of theroadway in the case of a crowned section, or one side in the case of a super-elevated section. Drainage swales should be designed in accordance with theGuidelines given for BMP S9.

• Installed inlets shall be protected to prevent sediment-laden water entering thedrain sewer system (see BMP SR5 on Storm Drain Inlet Protection).

• Simple gravel berms without a trench can be used for less traveled roads. • Undisturbed buffer areas should be maintained at all stream crossings.• Areas adjacent to culvert crossings and steep slopes should be seeded and/or

covered.• Dust control should be used when necessary (see BMP EC3).

Maintenance

• Inspect stabilized areas regularly, especially after large storm events. Add crushedrock if necessary and restabilize any areas found to be eroding.



BMP EC3: Dust Control


6.6.3 BMP EC3: Dust Control


Definition Reducing surface and air movement of dust during land disturbing,demolition, and construction activities.

Purpose

To prevent surface and air movement of dust from exposed soil surfaces.


• In areas (including roadways) subject to surface and air movement of dust whereon-site and off-site damage is likely to occur if preventive measures are not taken.

Advantages

• A decrease in the amount of dust in the air will decrease the potential foraccidents and respiratory problems.


• Use of water on-site to control dust emissions, particularly in areas where the soilis already compacted, can cause a runoff problem where there wasn't one.


Construction activities inevitably result in the exposure and disturbance of soil. Fugitivedust is emitted both during the activities (i.e., excavation, demolition, vehicle traffic,human activity) and as a result of wind erosion over the exposed earth surfaces. Largequantities of dust are typically generated in "heavy" construction activities, such as roadand street construction and subdivision, commercial and industrial development, whichinvolve disturbance of significant areas of soil surface. Research at construction sites hasestablished an average dust emission rate of 2.7 tonnes/ha/month for active construction. Earthmoving activities comprise the major source of construction dust emissions, buttraffic and general disturbance of the soil also generate significant dust emissions.

In planning for dust control, remember that the less soil is exposed at any one time, theless potential there will be for dust generation. Therefore, phasing a project and utilizingtemporary stabilization practices upon the completion of grading can significantly reducedust emissions.

Best Management Practices Guide for Stormwater 6-39Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC3: Dust Control


Design Criteria

• Minimize the period of soil exposure through use of temporary ground cover andother temporary stabilization practices (see Seeding and Mulching, BMPs TC1and TC2).

• Sprinkle the site with water until surface is wet. Repeat as needed. To preventcarryout of mud onto street (see Stabilized Construction Entrance, BMP EC1).

• Spray exposed soil areas with approved dust palliative. Oil should not be used fordust suppression. Check with the local government to see which other dustpalliatives may be used in the area.

Maintenance

• Respray area as necessary to keep dust to a minimum.


BMP EC4: Pipe Slope Drains


6.6.4 BMP EC4: Pipe Slope Drains

Definition A pipe extending from the top to the bottom of a cut or fill slope anddischarging into a stabilized water course or a sediment trapping device or onto astabilization area.

Purpose

To carry concentrated runoff down steep slopes without causing gullies, channel erosion,or saturation of slide-prone soils.


• Where a temporary (or permanent) measure is needed for conveying runoff downa slope without causing erosion.

Advantages

• Slope drains provide a potentially effective method of conveying water safelydown steep slopes.


• Care should be taken to correctly site drains and not underdesign them. Also,when clearing takes place prior to installing these drains, care should be taken torevegetate the entire easement area, otherwise erosion tends to occur beneath thepipeline, resulting in gully formation.


There is often a significant lag between the time a cut or fill slope is completed and thetime a permanent drainage system can be installed. During this period, the slope isusually not stabilized and is particularly vulnerable to erosion. This situation also occurson slope construction which is temporarily delayed before final grade is reached. Temporary slope drains can provide valuable protection of exposed slopes untilpermanent drainage structures can be installed. When used in conjunction with diversiondikes, temporary slope drains can be used to convey stormwater from the entire drainagearea above a slope to the base of the slope without erosion. It is very important that thesetemporary structures be installed properly since their failure will often result in severegully erosion. The entrance section should be securely entrenched, all connections shouldbe watertight, and the conduit should be staked securely.

Best Management Practices Guide for Stormwater 6-41Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC4: Pipe Slope Drains


Design Criteria

• The capacity for temporary drains should be sufficient to handle a 10-year, 24-hour peak flow. Permanent pipe slope drains should be sized for the 25-year 24-hour peak flow.

• The maximum drainage area allowed per pipe should be 4 ha. For larger areas, arock-lined channel or more than one pipe should be installed.

• The entrance should consist of a standard flared end section for culverts 300 mmand larger with a minimum 150 mm metal toe plate to prevent runoff fromundercutting the pipe inlet. The slope of the entrance should be at least 3 percent(Figure 6.5).

Figure 6.5 Pipe Slope Drains

• The soil around and under the pipe and entrance section shall be thoroughlycompacted to prevent undercutting.

• The flared inlet section should be securely connected to the slope drain and havewatertight connecting bands.

• Slope drain sections should be securely fastened together and have gasketedwatertight fittings, and be securely anchored into the soil.

• Interceptor dikes should be used to direct runoff into a slope drain. The height ofthe dike should be at least 300 mm higher at all points than the top of the inletpipe.

• The area below the outlet should be stabilized with a riprap apron (seeBMP EC12, Outlet Protection, for the appropriate outlet material).


BMP EC4: Pipe Slope Drains


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

• Materials specifications for the type of pipe used should be set by the localgovernment.

Maintenance

• Check inlet and outlet points regularly, especially after heavy storms. The inletshould be free of undercutting, and no water should be going around the point ofentry. If there are problems, the headwall should be reinforced with compactedearth or sand bags. The outlet point should be free of erosion and installed withappropriate outlet protection (see BMP EC12).


Best Management Practices Guide for Stormwater 6-43Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC5: Subsurface Drains


6.6.5 BMP EC5: Subsurface Drains

SDC o d e : S y m b o l:

Definition A perforated conduit such as a pipe, tubing, or tile installed beneath theground to intercept and convey ground water.

Purpose

To provide a dewatering mechanism for draining excessively wet, sloping soils-usuallyconsisting of an underground perforated pipe that will intercept and convey ground water.

Conditions When Practice Applies

• Wherever excessive water must be removed from the soil. The soil should bedeep and permeable enough to allow an effective system to be installed.

Advantages

• Subsurface drains often provide the only practical method of stabilizingexcessively wet, sloping soils.


• Problems may be encountered with tree roots (see Maintenance). • Pipes cannot be located under heavy vehicle crossings.


Subsurface drainage systems are of two types; relief drains and interceptor drains. Reliefdrains are used either to lower the water table in order to improve the growth ofvegetation, or to remove surface water. They are installed along a slope and drain in thedirection of the slope. They can be installed in a gridiron pattern, a herringbone pattern,or a random pattern (Figure 6.6).

Interceptor drains are used to remove water as it seeps down a slope to prevent the soilfrom becoming saturated and subject to slippage. They are installed across a slope anddrain to the side of the slope. They usually consist of a single pipe or series of singlepipes instead of a patterned layout (Figure 6.7).


BMP EC5: Subsurface Drains


Figure 6.6 Subsurface Drain Layout

Figure 6.7 Effect of Subsurface Drain on Water Table

Best Management Practices Guide for Stormwater 6-45Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC5: Subsurface Drains


Design Criteria

• Subsurface drain should be sized for the required capacity. The minimumdiameter for a subsurface drain should be four inches.

• The minimum velocity required to prevent silting is 0.4 m/sec. The line should begraded to achieve at least this velocity.

• Filter material and fabric should be used around all drains for proper bedding andfiltration of fine materials.

• The outlet of the subsurface drain should empty into a sediment trap or pond. Iffree of sediment, it should empty into a receiving channel, swale, or stablevegetated area adequately protected from erosion and undermining.

• The strength and durability of the pipe should meet the requirements of the site inaccordance with the manufacturer's specifications.

Construction Specifications

• The trench should be constructed on a continuous grade with no reverse grades orlow spots.

• Soft or yielding soils under the drain should be stabilized with gravel or othersuitable material.

• Deformed, warped, or otherwise unsuitable pipe should not be used. • Filter material should be placed as specified with at least 75 mm of material on all

sides of the pipe. • Backfilling should be done immediately after placement of the pipe. No sections

of pipe should remain uncovered overnight or during a rainstorm. Backfillmaterial should be placed in the trench in such a manner that the drain pipe is notdisplaced or damaged.

Maintenance

• Subsurface drains should be checked periodically to ensure that they are free-flowing and not clogged with sediment.

• The outlet should be kept clean and free of debris. • Surface inlets should be kept open and free of sediment and other debris. • Trees located too close to a subsurface drain often clog the system with their

roots. If a drain becomes clogged, relocate the drain or remove the trees as a lastresort. Drain placement should be planned to minimize this problem.

• Where drains are crossed by heavy vehicles, the line should be checked to ensurethat it is not crushed.



BMP EC6: Surface Roughening


6.6.6 BMP EC6: Surface Roughening

SR SRC o d e : S y m b o l:

Definition Provision of a rough soil surface with horizontal depressions created byoperating a tiller or other suitable equipment on the contour or by leaving slopes in aroughened condition by not fine grading them.

Purpose

To aid in establishment of vegetative cover, reduce runoff velocity, increase infiltration,and provide for sediment trapping.


• All slopes steeper than 3:1, and greater than 1.5 vertical metres, require surfaceroughening; either stair-step grading, grooving, furrowing, or tracking if they areto be stabilized with vegetation.

Advantages

• Surface roughening provides some instant erosion protection on bare soil whilevegetative cover is being established.

• It is an inexpensive and simple erosion control measure.


• While this is a cheap and simple method of erosion control, it is of limitedeffectiveness in anything more than a moderate storm.


Graded areas with smooth, hard surfaces give a false impression of "finished grading" anda job well done. It is difficult to establish vegetation on such surfaces due to reducedwater infiltration and the potential for erosion. Rough slope surfaces with uneven soiland rocks left in place may appear unattractive or unfinished at first, but they encouragewater infiltration, speed the establishment of vegetation, and decrease runoff velocity.

Rough, loose soil surfaces give lime, fertilizer, and seed some natural coverage. Nichesin the surface provide microclimates which generally provide a cooler and more favorablemoisture level than hard flat surfaces; this aids seed germination.

There are different methods for achieving a roughened soil surface on a slope, and theselection of an appropriate method depends upon the type of slope. Roughening methods

Best Management Practices Guide for Stormwater 6-47Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC6: Surface Roughening


include stair-step grading, grooving, and tracking. Factors to be considered in choosing amethod are slope steepness, mowing requirements, and whether the slope is formed bycutting or filling.

1. Disturbed areas which will not require mowing may be stair-step graded, grooved,or left rough after filling.

2. Stair-step grading is particularly appropriate in soils containing large amounts ofsoft rock. Each "step" catches material which sloughs from above, and provides alevel site where vegetation can become established. Stairs should be wide enoughto work with standard earth moving equipment.

3. Areas which will be mowed (these areas should have slopes less steep than 3:1)may have small furrows left by disking, harrowing, raking, or seed-plantingmachinery operated on the contour.

4. It is important to avoid excessive compacting of the soil surface when scarifying. Tracking with bulldozer treads is preferable to not roughening at all, but is not aseffective as other forms of roughening, as the soil surface is severely compactedand runoff is increased.

Design Criteria

Graded areas with slopes greater than 3:1 but less than 2:1 should be roughened beforeseeding (Figures 6.8a and 6.8b). This can be accomplished in a variety of ways, including"track walking," or driving a crawler tractor up and down the slope, leaving a pattern ofcleat imprints parallel to slope contours.

Graded areas steeper than 2:1 should be stair-stepped with benches as shown in Figure6.9. The stair-stepping will help vegetation become established and also trap soil erodedfrom the slopes above.

Maintenance

• Areas which are graded in this manner should be seeded as quickly as possible. • Regular inspections should be made of the area. If rills appear, they should be re-

graded and re-seeded immediately.


BMP EC6: Surface Roughening


Figure 6.8a Heavy Equipment Can Be Used To Mechanically Scarify Slopes

Figure 6.8b Unvegetated Slopes Should be Temporarily Scarified to Minimize Runoff Velocities

Best Management Practices Guide for Stormwater 6-49Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC6: Surface Roughening


Figure 6.9 Stair-Stepping Cut Slopes and Grooving Slopes


BMP EC7: Gradient Terraces


6.6.7 BMP EC7: Gradient Terraces

GTC o d e : S y m b o l:

Definition An earth embankment or a ridge-and-channel constructed with suitablespacing and with an acceptable grade.

Purpose

To reduce erosion damage by intercepting surface runoff and conducting it to a stableoutlet at a nonerosive velocity. (This guideline covers the planning and design ofgradient terraces and does not apply to diversions.)


• Gradient terraces normally are limited to denuded land having a water erosionproblem. They should not be constructed on deep sands or on soils that are toostony, steep, or shallow to permit practical and economical installation andmaintenance. Gradient terraces should be used only where suitable outlets are orwill be made available.

Advantages

• Gradient terraces lower the velocity of runoff, increase the distance of overlandflow, and reduce effective hydraulic gradient. They also hold moisture andminimize sediment.


• May significantly increase cut and fill costs and cause sloughing if excessivewater infiltrates soils.

Best Management Practices Guide for Stormwater 6-51Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC7: Gradient Terraces


Design Criteria

• The maximum spacing of gradient terraces should be determined by the followingmethod: V.I. = 0.3048 (xs + y)

Where: V.I. = vertical interval in metres x = 0.8 for Washington1

s = land slope in metres per 100 metres y = a soil and cover variable with values from 1.0 to 4.02

• The minimum constructed cross-section should meet the design dimensions. • The top of the constructed ridge should not be lower at any point than the design

elevation plus the specified overfill for settlement. The opening at the outlet endof the terrace should have a cross section equal to that specified for the terracechannel.

• Channel Grade - Channel grades may be either uniform or variable with amaximum grade of 0.6 metre per 100 metre length. For short distances, terracegrades may be increased to improve alignment. The channel velocity should notexceed that which is nonerosive for the soil type with the planned treatment.

• Outlet - All gradient terraces should have adequate outlets. Such an outlet may bea grassed waterway, vegetated area, or tile outlet. In all cases the outlet mustconvey runoff from the terrace or terrace system to a point where the outflow willnot cause damage. Vegetative cover should be used in the outlet channel.

• The design elevation of the water surface of the terrace should not be lower thanthe design elevation of the water surface in the outlet at their junction, when bothare operating at design flow.

Specifications

• Vertical spacing determined by the above methods may be increased as much as150 mm or 10 percent, whichever is greater, to provide better alignment orlocation, to avoid obstacles, to adjust for equipment size, or to reach a satisfactoryoutlet (Figure 6.10).

• The drainage area above the top should not exceed the area that would be drainedby a terrace of equal length with normal spacing.

• Capacity - The terrace should have enough capacity to handle the peak runoffexpected from the design storm without overtopping (the Puget Sound manualspecifies the 2-year, 24-hour design storm).

• Cross-Section - The terrace cross-section should be proportioned to fit the landslope. The ridge height should include a reasonable settlement factor. The ridge

1 U.S. Soil Conservation Service, National Engineering Handbook 2Values of "y" are influenced by soil erodibility and cover practices. The lower values are

applicable to erosive soils where little to no residue is left on the surface. The higher value isapplicable only to erosion-resistant soils where a large amount of residue (3.4 tonnes of straw/haequivalent) is on the surface.


BMP EC7: Gradient Terraces


should have a minimum top width of 1 metre at the design height. The minimumcross-sectional area of the terrace channel should be 0.74 m2 for land slopes of5 percent or less, 0.65 m2 for slopes from 5 to 8 percent, and 0.56 m2 for slopessteeper than 8 percent. The terrace can be constructed wide enough to bemaintained using a small cat.

Figure 6.10 Gradient Terraces

Maintenance

• Maintenance should be performed as needed. Terraces should be inspectedregularly; at least once a year, and after large storm events.

Best Management Practices Guide for Stormwater 6-53Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC8: Bioengineered Protection of Very Steep Slopes


6.6.8 BMP EC8: Bioengineered Protection of Very Steep Slopes

SSPC o d e : S y m b o l:

Definition Steep slope protection using a combination of vegetative and mechanicalmeasures.

Purpose

To stabilize steep banks.


• Slopes of steep grade, cut and fill banks, and unstable soil conditions that cannotbe stabilized using ordinary vegetative techniques.

Advantages

• Vegetation reduces sheet erosion on slopes and impedes sediment at the toe of theslope.

• Where soils are unstable and liable to slip due to wet conditions, utilization of soilmoisture by vegetation can reduce the problem.

• Shrubs and trees shelter slopes against the impact of rainstorms, and the humusformed by decaying leaves further helps to impede runoff.

• Mechanical measures help to stabilize soil long enough to allow vegetation tobecome established.


• The planting of non-seeded material such as live willow brush is a specializedoperation and cannot be highly mechanized or installed by unskilled labor.

• The methods described are effective but require a complete knowledge of soil,hydrology, and other physical data to design measures that will adequately solvethe problem.

Design Criteria

The following bioengineering methods can be used after slopes have been protected bydiversion of runoff (BMP EC10) or through the terracing of slopes (BMP EC7).

• Sod walls or retaining banks are used to stabilize terraces. Sod is piled by tiltingit slightly toward the slope and should be backfilled with soil and compacted asthey are built up. Sod walls can be as steep as 1:8 but should not be higher than5 feet (Figure 6.11a).


BMP EC8: Bioengineered Protection of Very Steep Slopes


Figure 6.11(a) Sod Retaining Bank

• Timber frame stabilization is effective on gradients up to 1:1 and involves thefollowing steps in construction: 1) Lay soil retarding frames of 50 mm x 100 mmvertical members and 25 m x 100 mm horizontal members on slopes. Frames onslopes over 5 m in length need to be anchored to slope to prevent buckling. 2) Attach 14 gauge galvanized tire wires for anchoring wire mesh. 3) Fill frameswith moist topsoil and compact the soil. 4) Spread straw 150 mm deep overslope. 5) Cover straw with 14 gauge 100 mm mesh galvanized reinforced wire. 6) Secure wire mesh at least 2 m back of top slope. 7) Plant ground cover plantsthrough straw into topsoil (Figure 6.11b).

Figure 6.11b Timber Frame Stabilization

• Woven willow whips (Figure 6.11c) may be used to form live barriers forimmediate erosion control. Construction: 1) 1 m poles are spaced at 1.5 mdistances and driven into the slope to a depth of 0.6 m. 2) 0.6 m willow sticks areinserted between poles at one foot distances. 3) Live willow branches of 1.5 mlength are sunk to a depth of 25 mm and interwoven with poles and stocks. 4) Spaces between the woven 'fences' are filled with topsoil. Fences are generallyarranged parallel to the slope or in a grid pattern diagonal to the direction of theslope.

Best Management Practices Guide for Stormwater 6-55Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC8: Bioengineered Protection of Very Steep Slopes


Figure 6.11c Woven Willow Whips

• Berm Planting. 1) Excavate ditches from 1 m to 1.5 m apart along the slope andshape a berm on the downslope side. Construct ditches with 5 percentlongitudinal slope. 2) Plant rooted cuttings on 1 m centers and mulch. Suitabletrees are willow, alder, birch, pine, and selected shrubs. In extremely drysituations, rooted cuttings can be planted in biodegradable plastic bags that arewatered at the time of planting (Figure 6.11d).

Figure 6.11d Berm Planting

• Brush Layers. 1) Prepare 1 m "niches" as shown. 2) Lay unrooted 1.5 m livebranches of willow or poplar at close spacing. 3) Starting at foot of slope, backfilllower ditch with excavated material from ditch above it. Operation should becarried out during dormant season (Figure 6.11e).

Figure 6.11e Brush Layers


BMP EC8: Bioengineered Protection of Very Steep Slopes


Maintenance

• Regardless of the stabilization method used, inspections should be made on aregular basis to make sure the system is functioning correctly.

• Note: There are a number of manufacturers who provide prefabricatedbioengineered devices for the protection of steep slopes.

Best Management Practices Guide for Stormwater 6-57Appendix H: Construction Site Erosion and Sediment Control Guide for Stormwater ManagementBMP EC9: Level Spreader


6.6.9 BMP EC9: Level Spreader

LSC o d e : S y m b o l:

Definition A temporary outlet for dikes and diversions consisting of an excavateddepression constructed at zero grade across a slope.

Purpose

To convert concentrated runoff to sheet flow and release it onto areas stabilized byexisting vegetation or an engineered filter strip.

Condition Where Practice Applies

• To be constructed on undisturbed areas that are stabilized by existing vegetationand where concentrated flows are anticipated to occur at 0 percent grade.

Advantages

• Level spreaders disperse the energy of concentrated flows, reducing erosionpotential and encouraging sedimentation.


• If the level spreader has any low points, flow tends to concentrate there. Thisconcentrated flow can create channels and cause erosion. If the spreader serves asan entrance to a water quality treatment system, short-circuiting of the forebaymay happen and the system will be less effective in removing sediment andparticulate pollutants.


Interceptor dikes and swales (BMP EC10) call for a stable outlet for concentratedstormwater flows. The level spreader can be used for this purpose provided the runoff isrelatively free of sediment. If properly constructed, the level spreader will significantlyreduce the velocity of concentrated stormwater and spread it uniformly over a stableundisturbed area.

Particular care must be taken during construction to ensure that the lower lip of thestructure is level. If there are any depressions in the lip, flow will tend to concentrate atthese points and erosion will occur, resulting in failure of the outlet. This problem maybe avoided by using a grade board or a gravel lip over which the runoff must flow whenexiting the spreader. Regular maintenance is essential for this practice.


BMP EC9: Level Spreader


Design Criteria

• The grade of the channel for the last 6.1 m of the dike or interceptor entering thelevel spreader should be less than or equal to 1 percent. The grade of the levelspreader should be 0 percent to ensure uniform spreading of storm runoff(Figure 6.12).

Figure 6.12 Level Spreader

• A 150 mm high gravel berm placed across the level lip should consist of washedcrushed rock, 50 m to 100 mm or 19 m to 38 mm size.

Documents

5.2 Small Parcel Erosion and Sediment Control Guidelines · Small Parcel Erosion and Sediment Control Guidelines Greater Vancouver Sewerage & Drainage District Liquid Waste Management