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RESOLUTION NO. 98-01 A RESOLUTION ADOPTING CITY OF MOSCOW EROSION AND SEDIMENT CONTROL STANDARDS TECHNICAL GUIDANCE HANDBOOK AS GUIDANCE RELATED TO EROSION AND SEDIMENT CONTROL AMENDMENTS TO THE MOSCOW U.B.C. IN MOSCOW CITY CODE, CHAPTER 1, TITLE 7. WHEREAS, City of Moscow, Idaho, has determined that it is in the best interests of the City to adopt Moscow City Code 7-1-2, which contains Appendix Chapter 33, Section 3316.3.1 et seq., also commonly known as the Erosion and Sediment Control Ordinance; and WHEREAS, certain standards must apply to erosion and sediment control practices to promote consistency; and WHEREAS, the erosion and sediment control standards included in the Technical Guidance Handbook are commonly accepted practices within the erosion and sediment control area and have been adapted to those areas which are under the purview of the City; and WHEREAS, the standards contained within the Moscow Erosion and Sediment Control Standards Technical Guidance Handbook and this Resolution are appropriate and desirable; NOW, THEREFORE, BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF MOSCOW that the attached City of Moscow Erosion and Sediment Control Standards Technical Guidance Handbook are hereby adopted as guidance for City of Moscow, Idaho, to be applied to the Uniform Building Code, as amended. ADOPTED THIS S'h day of January, 1998. v Paul C Agidius, Mayor Resolution,Erosion & Sediment Control,.<;r RESOLUTION Page I ---PAGE BREAK--- ---PAGE BREAK--- CITY OF MOSCOW Erosion and Sediment Control Standards Technical Guidance Handbook Revisions: August, 1997 ---PAGE BREAK--- ---PAGE BREAK--- 1. Introduction 1.1 Definitions 1.2 Causes of Erosion 1.2.1 Water Erosion 1.2.2 Wind Erosion CONTENTS 1. 3 General Principles of Erosion and Sediment Control for Construction Sites 2. Site Management Practices 2.1 Construction Scheduling 2.2 Timing and Extent of Site Disturbance Activities 2.3 Handling and Storage of Hazardous Materials 2.4 General Inspection and Maintenance of ESC Practices 3. Stormwater Runoff Treatment 3.1 Definition of "Design Storms" 3.2 Estimation of Design-Storm Runoff Discharge 3.3 Estimation of Sediment Yields 3.4 Runoff Conveyance 3.4.1 Earth Berms (Dikes) and Swales 3.4.2 Water Bars 3.4.3 Terraces and Level Spreaders 3.4.4 Pipe Slope Drains 3.4.5 Subsurface Drains 3.4.6 Outlet Protection 4. Erosion Control Practices 4.1 Vegetation Maintenance and Establishment 4.1.1 Vegetative Buffer Strips 4.1.2 Mulching and Temporary Seeding 4.1.3 Establish Permanent Vegetation 4.1.4 Erosion Control Blankets 4.2 Other Construction Practices 4.2.1 Gravel Construction Entrances and Parking Areas 4.2.2 Surface Roughening 4.2.3 Tarps, Plastic Sheeting, and Similar Coverings 4.2.4 Chemical Soil Treatments 5. Sediment Control Practices 5.1 Sediment Traps and Barriers 5.1.1 Silt Fence 5.1.2 Straw Bale Barrier 5.1.3 Gravel Berm and Check Dam 5.1.4 Gravel Bag Check Dam 5.2 Sediment Ponds and Inlet Traps 5.2.1 Sediment Detention Pond 5.2.2 Sediment Retention Pond 5.2.3 Silt Filters and Inlet Traps 6. Written ESC Plan 6.1 General Format 6.2 Plan Approval 7. References 1 2 2 3 3 3 3 3 4 4 4 5 5 6 8 9 10 11 12 13 13 13 14 15 16 18 18 19 20 20 21 21 21 23 24 25 26 26 27 28 29 29 29 29 ---PAGE BREAK--- ---PAGE BREAK--- 1. INTRODUCTION 1 .1 Def i n i t i o n s For the context of this document, important terms are defined a s follows: Soil - naturally occurring, particulate, surficial deposits that overlie bedrock. Erosion - the process of displacing and transporting soil particles by the actions of gravity, moving water, or wind. Sediment • eroded soil material suspended, carried along, or deposited by moving water or by wind. Sedimentation • the deposition of eroded soil material. It is important to note that erosion control and sediment control are different treatments for different problems. Erosion control is intended to minimize that amount of soil initially detached and then transported (that is, a treatment at the source). Thus, an erosion control practice is an activity, device, structure, or land treatment designed to minimize erosion potential by protecting in-place soil from being dislodged and mobilized. · Effective erosion. control at a construction site can greatly reduce the extent and cost of sediment control, which focuses on the removal or containment of sediment that already has been mobilized. Thus, a sediment control practice is an activity, device, structure, or land treatment designed to inhibit the transport of sediment by inducing conditions to trap, settle, or othe!Wise remove sediment from the transporting media. 1. 2 Causes of Erosion 1.2.1 Water Erosion Erosion on hillslopes often is associated with precipitation or snowmelt events, and with the subsequent runoff that results. 'Erosion along stream channels also is a common geologic occurrence, but such activity is not discussed in detail within this document. The following processes contribute to the water-related erosion of a slope: • Raindrop impact and splash (impact, dispersal, and mobilization of particles). • Sheet runoff (sheetwash) occurs when the available surtace water exceeds the infiltration capacity of the soil; the velocity of flow will increase with steeper slopes and smoother surtaces. • Due to groundsurtace irregularities and the surtace tension of water, as sheetwash moves downslope it soon becomes concentrated flow, forming erosional rills (small, shallow channels with dimensions up to about one foot [30 em] deep and wide) and indicating significant erosion problems. • When a rill enlarges and becomes incised and permanent, it is classified as a gul1y. It represents severe erosion and often is difficult to remedy. Key factors that influence the amount and intensity of hillslope erosion by water. • The precipitation or snowmelt intensity and duration (the greater the intensity and duration, the greater the erosion potential). • · Soil erodability, that is, the tendency for soil particles to become detached and mobilized (generally, soils comprised mainly of silts and/or fine. sands are much more susceptible to water erosion than are clay soils and coarse sands to gravels). ' The length of a slope as measured directly downslope (or upslope) along the ground surtace (the longer a slope, the greater the potential for erosion). • Slope angle, or gradient (the steeper a slope, the greater the potential for erosion). The amount and type of cover (such as vegetation or mulch) on the ground surtace (the denser the cover. the lower the erosion potential). ---PAGE BREAK--- Land use and the condition of the ground surtace (urban development and pavement increase runoff, can cause accelerated erosion in untreated areas; ground-surtace roughening and terracing can help to reduce erosion potentia! on exposed slopes) 1. 2. 2 Wind Erosion Only the smallest soil particles can be detached by wind and then main\ained by suspension lor extended distances (particles typically less than 0.1 mm in diameter). Larger particles are moved along near the ground surface by saltation (the process of particles bouncing and skipping) and by creep (rolling and skidding). A general estimate of the total tonnage of soil eroded by wind can be divided into roughly 60% suspension. 30% saltation, and 1 0°/o creep. Key factors that influence the amount and intensity of wind erosion include: The condition of the ground surlace {loose, dry soils are more prone to wind erosion). The amount and type of ground cover (especially vegetation) available (a lack of vegetation directly contributes to the likelihood of wind erosion). The extent of open expanses lelt unpwtected and unsheltered; such a clear-span distance is know as thÁ fetch (the greater the fetch, the greater the erosion potentia!). The wind velocity and duration (greater wind intensity causes greater erosion). 1 . 3 General Principles of Erosion and Sediment Control for Construction Sites The following principles for mitigating erosion generally apply to most construction sites in and around the city of Moscow:· Minimize the area of disturbance and retain as much vegetation cover as Schedule the construction so that significant areas of bare soil are exposed only during the summer months. Use temporary vegetation, mulch, or other ground coverings on exposed areas. Divert runŦon and runoff water away from exposed areas. Maintain low runoff velocities by using slope breaks, temporary diversions, retentioQ structures, Capture sediment near the source, Inspect and maintain the erosion and sediment controls or practices that have been put in place. 2 ---PAGE BREAK--- 2. SITE MANAGEMENT PRACTICES 2 . 1 Construction Scheduling In the Moscow vicinity. the potential lor soil erosion drastically increases during the period from late autumn through late spring mid-October through mid-June). Therefore, any exposed soil areas opened up by construction activity should be treated prior to mid-October. Areas not re-vegetated will require significant artificial treatments. Due to the time and costs associated with such treatments. contractors wisely should endeavor to complete final grading work and re-vegetation as soon as possible in the construction schedule. Contractors also should be aware that beginning a project that involves earthwork during the late fall or winter wilt require extensive erosion and sediment control planning and implementation. When possible, such projects· should be avoided and re·scheduled for summer. 2. 2 Timing and Extent of Site Disturbing Activities Construction activities should be timed to minimize the extent and duration of bare-ground exposure. This may result in a phased-construction with minimal areas exposed, then treated in sequence. When possible, grade no larger an area than necessary at any one time and leave undisturbed vegetative buffer zones in stategic locations to help mitigate potential erosion and sedimentation problems. In most cases, leaving such zones of established vegetation undisturbed is much preferred to the alternative of stripping a larger area, then trying to re-establish vegetation during the course of the construction project, particularly during the heat of summer. Temporary protection of graded surfaces prior to a fall-seeding program may best be achieved by mulch or fabric covers. 2. 3 Handling and Storage of Hazardous Materials Proper management of hazardous materials on a construction site is essential to prevent such materials from leaving the site through stormwater runoff or by erosion of contaminated soils. Such materials may include the following (not an exhaustive list): Paints, • Cleaning solvents, • Concrete curing compounds and additives, as well as acids used to clean masonry surfaces, Detergents and cleaning solvents, Petroleum products, Construction adhesives, coatings. and other chemicals, Pesticides, herbicides, and fertilizers. Local waste management authorities should be consulted to determine the requirements for storing and disposing of hazardous materials. Original product labels should not be removed from their respective containers, and products should not be mixed unless specifically recommended by the manufacturer. Product labels typically contain information from the manufacturer on correct disposal of containers. It possible, store such products in a dry, covered area and provide curbs or dikes to contain any spilled materials. Heavy construction equipment should be parked repeatedly in a designated area and refueling/maintenance operations should follow procedures designed to avoid soil contamination by fuel, lubricants, or waste oil. Notify local authorities when chemicaVpetroleum spills occur, and contain and clean´up the affected area/soils as prescribed by the authorities. 2. 4 General Inspection and Maintenance o f ESC Practices Field inspections of the installed ESC practices shall occur on regular intetvals and immediately after any significant surface*water runoff event induces potential for erosion and sedimentation. Runoff diversion structures must be inspected for erosion and wear, then repaired if deficiencies are found. Silt fences and other sediment traps must be cleaned out when significant amounts of sediment have accumulated (see specific guidelines for each practice}. Depth of sedimentation in detention/retention ponds needs to be monitored closely, so that ctean-out can occur before storage capacities of the ponds are diminished below design levels. ---PAGE BREAK--- 3. STORMWATER RUNOF F TREATMENT 3. t Definition of "Design Storms" For the purpose of estimating stormwater runoff (and related sediment yield) in the City of Moscow, the well× known hydrologic Ørational method" can be used for proposed construction sites. Based on available precipitation !OF (Intensity-Duration-Frequency) curves for Moscow, and on the expect€d conditions of two specified cases, design storms are defined as follows: Case 1: Gently sloping sites slope) - 1.2 in./hr (30 for a 15-min. design storm with a 5-yr. recurrence intetval; Case 2: Steep sites slope) 1.4 in.lhr (36 mm/hr) for a 1 0 -min. design storm with a 5-yr. recurrence interval; Such rainfa!llntensities result from heavy thunderstorms that can occur during the summer in Moscow. 3. 2 Estimation o f Design-Storm Runoff Discharge The rational method for predicting stormwater runoff relies on the use of a runoff coefficient, C, typical values of which are presented in the table below. The C value represents the fraction of precipitation that results in overland runoff: for example, C=0.3 indicates that 0.30 (30%) of the precipitation results in actual runoff. EstimatiOn of runoff discharge is important for the design of runoff conveyance structures and is given by the equation: O = CiA (3.1) where: 0 =peak runoff discharge (vol./time), C =runoff coefficient, i =design-storm intensity (depth/time), and A = area of the site being studied. Given that 1.0 tt3/sec = 1.008 acreÙin./hr, for most practical purposes 0 is · expressed as ft3/sec, i as in./hr, and A as acres. Thus, equation 3.1 can be rewritten as follows: (3.2) with 0/A expressed as cfs/acre and i as inches/hr. Thus, using the design·storm precipitation intensities presented in Sec. 3.1 and the C values presented below, the design-storm runoff in cfs per acre can be calculated. Example: Flat site wilh high erosion risk; i = 1.5 in.lhr. C = 0.2 Olacre = 0.2( 1.5) = 0.3 cfslacre For a 2-acre site, the design Q is: Q = 0.3(2) = 0.6 cfs. Table 3.1. Runoff coefficients, C, for use in the rational mÚthod. Undeveloped Land Woods and Forests Sparse Trees with Good Ground Cover Light to sparse grass , (Note: For frozen ground, double the above C values) Construction and Developed Areas Bare Ground; Playgrounds Lawns, Meadows, Pastures Parks, Cemeteries Pavement and Roofs Graveled Roads and Parking Lots Light Industrial Areas Residential Areas with 3.5 to 4 units/acre 4 Flat (s5%) Steep 0.05 0.10 0.10 0.15 0.15 0.20 0.25 0.30 0.20 0.25 0.15 0.20 0.90 0.90 0.75 0.80 0.60 0.70 0.45 0.45 ---PAGE BREAK--- 3. 3 Estimation of Sediment Yields Several accepted methods are available to estimate sediment yields from design storms. This information is important for the sizing of sediment traps and the design of other similar structures One method is the Modffied Universal Soil Loss Equation (MUSLE), as presented by Williams (1982) then discussed and applied in numerous subsequent related publications. The well-known Revised Universal Soil Loss Equation (RUSLE) estimates a total annual sediment yield for a site, whereas the MUSLE predicts sediment yield from an Individual storm event Both equations use several similar input terms. Professionals experienced in the use of MUSLE should be consulted for its application to construction sites. Another method is the recently released WEPP computer model CNater Erosion Prediction Project, coordinated by the U.S. Agricultural Research Service), which can be appfied to construction sites for predicting sediment yield from given design storms (ARS, 1995). The WEPP computer code is aveuable in the public domain, but professionals experienced in its use should be consulted for its application to specific study sites. Other technical approaches also can be used for sediment yield estimates from construction sites in Moscow, provided they are reviewed and approved by the City Engineers Office. 3. 4 Runoff Conveyance An important component in any erosion control plan for a construction project is the proper handling of stormwater runoff during the various phases of construction. Carefully planned, effective runoff conveyance practices can prevent many soil erosion problems. Suggested practices are described in the sections that immediately follow. ---PAGE BREAK--- 3.4 . 1 Earth Berm s (Dikes) and Swales Description A ridge of compacted soil (berm/dike) or a small, shallow topographic valley (swale, usually vegetated) located near or on a sloping disturbed area. ·Purpose When properly installed, berms and swales serve to intercept and divert surface-water runoff to a stabilized outlet or containment area. Application Temporary earth berms (dikes) and swales divert and direct runoff to a stabilized outlet or sediment basin during the construction period and until permanent stabilization is achieved for a disturbed site. They can be used to reduce the volume and velocity of runoff across a disturbed area by intercepting runoff prior to it reaching the sae (wl)en located on the upslope side of the site) and by intercepting runoff generated on-site (when located parallel or sub-parallel to contours directly on site, which reduces the slope length !hat runoff is allowed to flow directly downhill}. They can be constructed readily from on-site materials using available grading and excavation equipment Limitation s Although berms (dikes) and swales should be constructed with a grade appropriate for adequate drainage, thÇ gradient cannot be excessive, or high velocities will cause·erosive conditions in the structures. Oesig n Criteria 1. Earth berms (dikes): Top width minimum of 0.6 m (2 tt) Height minimum of 0.5 m (1.51t), as measured from the upslope toe Side slopes maximum of 2:1 (horz.:vert.) Compaction minimum of 90% of maximum dry density per ASTM 06Q8 standard proctor test Grade maximum of 1% unless intermittent structures are used in cqnjunction with the berm Horz. spacing maximum of 90 m (300 tt) on slopes · maximum of 60 m (200 tt) on slopes 5-10% maximum of 30 m (1 00 It) on slopes 10-40% Stabilization slopes mulch and or seed (depends on time of year) immediately after construction slopes 5-40%: select stabilization materials based on expected runoff velocities; may include sod, erosion control blankets, riprap, etc. Outlet Provide energy dissipation measures· to prevent erosion at the outlet, such as riprap, erosion control blankets, level spreaders. etc. (see Outlet Protection). Runoff shall be released into a sediment trapping facility. 2. Swales: Bottom width minimum of 0.6 m (2 ft) Depth minimum of 0.3 m (1 It) Side slopes maximum of 2:1 (horz.:vert.) Grade maximum of 5% unless intermittent structures are used in conjunction with the swale Horz. spacing maximum of 90 m (300 It) on slopes maximum of 60 m (200 ft) on slopes 5·1 0% maximum of 30 m (100 It) on slopes 10-40% Stabilization slopes mulch and or seed (depends on time of year) immediately after construction slopes 5·40%: select stabilization materials based on expected runoff velocities; may include sod, erosion centro! blankets, riprap, etc. Outlet Provide energy dissipation measures to prevent erosion at the outlet, such as riprap, erosion control blankets, level spreaders, etc. (see 3.5.6 Outlet Protection). Runoff shall be released into a sediment trapping facility. Special Installation Procedures 1. Select appropriate locations for berms and swales using the design criteria given above. 2. Excavate, grade, and compact as needed to meet the design critefta. 6 ---PAGE BREAK--- . Inspection and Maintenance Construction traffic over earth berms and swales should be avoided or minimized. unless protected crossings are used. The diversion structures should be inspected regulary, as well as immediately affar any significant runoff event Repairs to structures should be completed immediately when damage is recognized. If intermittent structures are used in the berm or swale, then sediment trapped behind them should be cleaned out regularly. If berms or swales are intended to serve long·term, then permanent vegetation stabir500,000 seeds/lb.) - 40 lbsJacre Seleciion of species and other seeding instrucllons can be reccmmended by the local USDA-NRCS offiCe. Specia l lnstallailo n Procedures Seed-bed preparation at some sites may require the addition of fertilizer, lime, or other treatments to enhance vegetation survival and growth. Inspection and Maintenance Planted areas should be inspected regularly to check for any damage and to monitor plant germination and health. Seed and mulch should be re-applied as needed to produce a complete coverage of the treated area. The application of any weed-<:ontrol chemicals shall comply with label instructions and shall be wrrhin the laws and regulations set forth by the State of Idaho or by local authorities. ---PAGE BREAK--- 4.1 .4 Erosion Control Blankets Description An erosion control blanket (mat}, or ECB, is a porous net or fibrous sheet attached to the ground suriace for erosion control and slope stabilization. ECB's typically are manufactured in rol!s and must be unrolted and then stapled or staked to the ground. ECB's commonly are made of: straw, coir (cocunut fiber), jute, P u r p o se ECB's serve as a mulch layer to protect seeded areas from wind and water damage and to help hold moisture for enhanced vegetation growth. They also can serve as a temporary erosion contra! treatment even when seeding has not been done. Application This treatment is appropriate for any disturbed area. especially_ slopes, where temporary or permanent erosion protection is needed. Different ECB grades (weights) and composites ot materials allow quite a range of selections for the appropriate application. limitations Installation of ECB's is fairly labor intensive and may not be practical on some difficult sites (steep or irregular).. Incorrect installation can render the ECB's ineffective, and thus, manufacturers' recommendations must be lo!lowed. Design Criteria On a slope, it is common practice to protect the final grade by beginning blanket installation 3ft. (0.9 m) back on the flatter upslope behind the crest of the final slope. This will help prevent runoff from flowing beneath the blanket, which can cause serious erosion problems. One staple or stake should be installed for every one loot (30 em) along the outside perimeter of the blanket also to allow protection from wate-r runoff. In the event there is potential, significant runoff, the upslope edge of the ECB should be buried in a slot trench and stapled every 12 in. (30 em) along it's length prior to backfilling and compaction. ECB's should be installed according to manufacturer's specifications, with the following general guidelines: 1. On longer slope installations it is usually an advantage to start at the slope crest. Allow lor a 3 foot blanket installation over the crest and onto the flatter portion of the upper grade. Either staple or bury the blanket edge per the manufacturers recommendation. 2. Install the blanket lrom the top to the bottom by a controlled rolled method. The best way is for the installer to be on the downside of the slope in front of the blanket and walk the blanket down the slope with each backward step. Approximately every 6 m the blanket should be smoothed to take out any excess slack. Do not overţstretch the blankets as this may cause bridging. ECB's need to lay flat and allow intimate contact with the soil in order to function effectively. 3. Most manufacturers recommend a blanket width overlap of 2--4 in. along the edges). If the installation is a channel slope, always overlap upstream to 4. Continue the installation downslope and allow for a 2§tt minimum installation onto flat ground at the toe area. 5. The stapling procedure should be at a staple pattern of one staple spaced at 10-24 in. across the blanket width (horizontal) and 1 staple at 3 - 5 ft. intervals along the length (vertical) of the blankets (average of 1 - 1.5 staples per square yard). 6. On very steep slopes a staple pattern of 1 staple per 2 feet of blanket width and length has been used successfully. 7. On short slopes some manufactures allow for a horizontal installation to avoid extra handling and cutting of the blankets. The same final grade installation stapling procedures still apply. install from the base of the slope first and proceed up to over the crest to protect the final grade. 16 ---PAGE BREAK--- Special tnstaltatlon Procedures Manufacturers' recommendations should be followed for any special installations. Inspection and Maintenance ECS's should be inspected regularly to check for any damage. integrity to the blanket installation. STN'I...El Tl-E Ą H AISC1.tSC1ąH•CH n:eoiwrH E}oOC 1' AĆ CF l ST N'LE5 te«J JiCJJf SO 01 QO N H'Nt'f • 1'J..N.fCET MAñ , t:Nf!'UI' Ar t..EJS1 t5 01 tO H NC SiJ.J'\.£5 t OOTH FNR::S Af A òJaG Cf= lrify - Figure 4.3 Examples of surface roughening. 1 9 ---PAGE BREAK--- 4 . 2 . 3 Tarps, Plastic Sheeti n g , and Similar Coverings When loose soils are temporarily stockpiled. tarps or plastic sheeting can be used to protect them from wind and water erosion. Generally, such coverings are not appropriate for large. sloping disturbed areaG on native soil, because it is very difficult to keep runoff from getting beneath the coverings and causing considerable erosion damage. Even when they are used to cover stockpiles. careful planning is required to handle the runoff which will need to be collected in stabilized collection conveyances and directed oH·site. T arps and plastic typically are anchored temporarily using stakes, rocks, or soil pilesiberms positioned along their lower edges. U several pieces are needed, then ones on the lower slopes should be installed first. followed. by adjacent ones upslope in a shingled, overlap fashion. In most cases, tarps and plastic used for temporary protection at stockpiles can be saved and reoused at a later date. 4 . 2 . 4 C hemical Soil Treatments Chemical stabilization practices for soils consist of temporary measures that rely on chemical mulch, soil binders. or sOil palliatives. They are made of materials such as vinyl, asphalt, latex. and potyacrylics, which can be sprayed onto the soil surtace to help bind the soil particles and prevent erosion. Chemical soil treatments serve as an alternative to temporary seeding during dry seasons when seeding is impractical. They usually are more expensive than seeding, but can provide immediate and effective erosion protection from wind and water. These treatments should be planned carefully, because they can produce impervious surfaces, which can result in more intense stormwater runoff. Manufacturers' specifications shall be followed. 20 ---PAGE BREAK--- 5. SEDIMENT CONTROL PRACTICES 5 . 1 Sediment Traps and Barriers 5.1.1 Silt Fence OescriptionSilt fence consists of a geotextile fabric attached to supporting posts and installed on sloping disturbed areasØ the fabric may be backed by a supporting-wire mesh if needed. Purpose When properly installed, silt fence reduces the velocity of overland flow and induces setlling of water· born sediment. thus trapping sediment on the upslope side of the fence. Application Silt fences are installed along contours, typically near the perimeter of a disturbed area to trap sediment and divert runoff to a stabilized outlet structure. The fences should remain in place and be maintained until the disturbed area is permanently stabilized. A silt fence is installed after clearing and grubbing of vegetation, but before any significant soil grading or excavation occurs: Umitations Silt fence is not designed to be used where there is a concentrated flow of water in a swale or channel, or where ground conditions prevent a minimum toe-in depth of 150 mm (6 in.) for the tower edge of the fabric or prevent driving of support posts to a minimum depth of 300 mm (12 in.). Design Criteria 1 . Maximum upslope drainage area: 0.3 hectare per 100 m length of silt fence (1 ,600 tt2 per .100 ft). 2. Maximum length of fence segment: 100 m per 30 m of slope length upslope (330 It per 100 ft slope length). 3. Maximum stope steepness upslope from fence: 1:1 (horz.:vert.), or 1 00% slope. 4. Maximum slope length upslope from fence: 30 m (100 ft) for slopes < 4% 1 5 m (50 ft) for slopes 5 · 12% 8 m (26 ft) for slooes 13 - 20% 5 m ( 1 6 ft) for slopes > 20% 5. Silt fence fabric specifications: fabric may be burlap or various materials, such as pervious nylon, polypropylene, polyester, or polyethylene yam. 6. Wire mesh backing (if used): 14-gauge wire with 150-mm (6-in.) mesh grid. 7. Post specifications: minimum length of 1 .2 m (4 ft), consisting of 50x50 mm (2x2 in.) wooden stakes. and spaced a maximum of 2.4 m (8 ft) apart: a closer spacing is recommended for windy areas. Special I nstallation Procedures 1 . Select appropriate site for silt fence installation. Leave a 1-m (3.2-ft) minimum buHer zone on the lower side of the silt fence when installed near a property boundary, stormwater collection curb gutter, etc. Avoid installations in channels with concentrated water flows. 2. Excavate a toe-in trench along the planned installation path. This trench shall be approximately 150 mm (6 in. wide) and 150 to 300 mm (6 to 12 in.) deep 3. Unroll the silt fence and, if pre-fabricated with stakes attached, drive its stakes into the ground on the downhill side of the trench. It the fabric is not pre-attached to the stakes, then attach the fabric to the stakes using the and spacing specified by the design criteria given above. Large-diameter roofing nails or equivalent are preferred to staples for attaching fabric to stakes. _ 4. The lower edge of the fabric should ex1end 200 to 300 mm (8 to 12 in.) into the trench, and should be laid in an "L shape facing toward the upslope direction. 5: Backfill the trench with compacted soil, burying the shaped toe of the Iatric. Suitable trenching and toe-in of the fabric is essential to proper installation and performance of silt fence. 6. Do not attach silt fence tO·trees or other fixed natural or man-made objects. ---PAGE BREAK--- tnspectlon and Maintenance Silt fences must be inspeC.ed regulariy to assess damage {e.g . • due to animals, high winds. working equipment) and to monitor retained sediment levels. Inspections also should take place immediately after any significant runoff event. Sediment must be cleaned out when its height reaches half the silt§fence height Silt fence should not be removed until an adequate vegetation cover is established upslope. Silt fence can be re-used, provided its integrity is verified (silt fence subiected to the weather likely will lose its integrity in 1 2 to 18 months, but the user is encouraged to check with manufacturers for further guidance}. 12 ffi Figure 5.1 Recommended installation procedures for silt fence. 22 ---PAGE BREAK--- 5 . 1 . 2 Straw Sa>·ā 4arrier Description A row of properly trenched and anchored straw bales can be installed to serve as a temporary perimeter barrier or as a temporary drop structure in a ftow channeL P u rp ose sedimentation. Straw bale barriers are temporary structures used to reduce flow velocities and induce Application They can be used effectively as temporary diversion berms, and as temporary check dams across minor SNales and flow channels. Limitatio n s They are suitable only where flow rates are low(< 30 liters/sec) and where the upslope drainage area is less than 0.5 Ha. They require regular inspections and repairs, and periodic replacement if their usable Ufe is to be extended past 3 months. Also, in most cases, they must be cleaned up after the site work is completed. Design Criteria Straw bales used in a sediment barrier must be embedded into the soil to a depth of at least 100 - 150 mm (4 - 6 in.) and backfilled for the entire length of the barrier. Each bale should be securely staked to the ground using two wood stakes {50x50 mm x 1 m-long) or comparable steel rods (less desirable) driven to a depth in the ground of at least 500 mm (19 in.). Bales shall be placed on edge so that no wire or twine is in contact with the ground surface. Ends of bales should' be butted tighUy together. In most situations, straw bale barriers should not be constructed on slopes steeper than 2:1 or placed in channels with side slopes flatter than 6:1. The maximum flow path length to any given barrier is 30m; if slope exceed 30m, then intermediate barriers shall be installed. · When used as a drop structure in a flow channel, the lowest point on the top of the lowest bale.should be lower than the point on the sideslope where the uppermost bale forms a with the sideslope. For large perimeter applications, sitt fence is probably preferred to straw bales. Special Installation Procedures When used as a perimeter barrier adjacent to surface waters, an undisturbed vegetated buller 1-2 m in width is required so as to allow effective removal of the barrier and its accumulated sediments. I n spection and Maintenance Straw bale barriers should be inspected regularly, but especially after a significant storm event. Undercutting, end-around erosion, and damaged bales should be repaired immediately. Straw bale barriers often prove inefiective, and even may exacerbate erosion, due to improper installation and maintenance practices. ORrV£ St AXE THROOGH 80TH BAJ..£5 M BIJfT£0 EHOS lS££ ROT£ HO. 2J Si ME 0£PfH 1r APPOx. ur STRAW OR HAY BALE FRONT ELEVATION - EROSION BARRIER Figure 5.2 Recommended installation procedures for straw bale barriers. 23 SIJALL CHANNEL OPTION EL£11 AT/OilS - CHECK DAM ---PAGE BREAK--- 5 . 1 .3 Gravel Berm and Check Dam Description A grave! or crushed rock check dam is a small temporary barrier placed across a conveyance channel to intercept and trap sediment. Purpose These structures reduce !low velocities, induce sedimentation, and reduce channel degradation. They also can serve as small sediment traps. Application They can be used effectively in drainage swates or ditches, but should never be placed in perennial stream channels. Limitatio n s They are suitable only where water flows are intermittent and where the upslope drainage area is less than 2 Ha. They require regular inspections and repairs, and periodic replacement if their usable life is to be eX1ended past 3 months. Also, in most cases, they must be cleaned up after the site work is completed. Design Cr.iteria Gravel or crushed rock materia! shall be well--graded with less than 5% fines by weight partic!es smaller than 0.075 mm) and with the vast amount of particles between 20 and 80 mm in size. In most cases, the height of berms shall be 0.3 to 1.0 m with a side slope of 6:1.- ln plan view, the check dams will appear crescent-shaped with the limbs pointing in the upslope direction. The flow line of such a check dam, located most often near the center of the structure, should be at least 150 mm (6 in.) lower than the channel to force water to flow over the structure and not around it. When placing a series of structures down a water conveyance channel, the horizontal projection of the flow. line of one such structure shall intersect a point no higher than the base of the adjacent upstream structure. Special I nstallation Procedures In areas of highly erodible soils, the rock dams should be underlain by a geoteX1ile fabric to separate the rock fill from the underlying soils and to he!p prevent under:=:utting of the structure. Inspection and Maintenance An initial inspection should be conducted during or immediately after the first significant runoff event to assess functioning of the gravel berm system. Undercutting, end-around erosion, and damaged berms should be repaired or re-shaped immediately. t. • Jt , ! • ç :r - · · Figure 5.3 Examples of gravel check dams. i ' , \ \ 24 'I À. , w.