Stormwater Best Management Practice Silt Fences

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Stormwater Best Management Practice Silt Fences, Use J hooks as shown in Figures 5 and 6 which have ends of a gentle slope if large. turning up the slope to break up long fence runs and provide Figure 10 can be more. multiple storage areas that work like mini retention areas If the important than its slope in. fence doesn t create a ponding condition it will not work well determining sediment loss. The silt fence in Figure 7 doesn t pond water or retain sediment A silt fence should not be. Stormwater will run around the fence carrying sediment to the placed in a channel with. street which will transport the water and its sediment load to continuous flow channels. the storm sewer inlet in Figures 8 and 9 don t Figure 10 Gentle slopes may require a. have a continuous flow nor silt fence,across a narrow or steep. sided channel But when necessary a silt fence can be placed. parallel to the channel to retain sediment before it enters the. watercourse, Figure 5 Use J hook fences to break up Paved streets are major conduits of stormwater and silt and. long fence runs they drain to storm sewer inlets The best solution is to retain as. much sediment as possible before it reaches paved surfaces. Install a silt fence at the inlet side of a storm sewer or culvert. rather than at the discharge where there is greater velocity and. Figure 6 J hook silt fences less storage area Streets cut in the grade but not yet paved. provide multiple storage areas, are also prime erosion conduits If the streets are not going to. be paved right away they need a containment barrier such as. Figure 7 This silt fence doesn t work, a silt fence Finally as a construction site s dynamics change.
the silt fence layout should be adjusted when necessary to. Water flowing around the ends of a silt fence will cause maintain its effectiveness. additional erosion and defeat its purpose The bottom of each. end of the fence should be higher than the top of the middle of Designers and contractors should also consider diverting. the fence Figure 8 This insures that during an unusually heavy sediment laden runoff water to a sediment detention pond If. rain water will flow over the top rather than around either end of the site can provide a large enough area this is usually the. the fence Only fine suspended material will spill over the top most effective and economical best management practice. which is not as harmful as having erosion at the ends When for retaining sediments Silt fences are needed when there is. there is a long steep slope insufficient space for a detention pond or when roads and other. install one fence near the structures are in the way. head of the slope to reduce, the volume and velocity Adequate Amount of Fencing. of water flowing down The amount of fencing means the total linear length of the silt. the slope and another fencing runs on the construction site A reasonable rule of. fence 6 10 ft from the toe thumb for the proper amount of silt fence is 100 ft of silt fence. of the slope to create a Figure 8 Proper installation bottom of per 10 000 square foot sq ft of disturbed area Soil type slope. sediment storage area near both ends are above the top of the middle slope length rainfall and site configuration are all important. the bottom A common elements in determining the adequate silt fence protection. misconception is that you for a site and to what extent it fits the 100 ft per 10 000 sq ft. only have to worry about rule of thumb If the amount of fencing provides the volume of. water running off steep runoff storage needed then over flowing the silt fence runs will. slopes However steep be minimized This is the basic test if fences are over flowing. slopes may have a relatively after a moderate rainfall event the amount of fencing probably. small water collection area Figure 9 Poor installation water can needs to be increased to avoid undercutting washouts and. flow around the ends causing additional,The total drainage area erosion fence failures. Stormwater Best Management Practice Silt Fences, Appropriate Materials designed to slightly disrupt. There are different types of porous fabrics available e g soil upward next to the slit. woven non woven mono filament but all types tend to clog and to minimize horizontal. rapidly and don t provide lasting filtration The support posts compaction thereby. and installation method are more important than the fabric type creating an optimum. for overall sediment retention However a lightweight fabric condition for compacting. tends to tear where it is attached to the posts Posts must hold the soil vertically on. Figure 12 Static slicing machine, the fabric up and support the horizontal load of retained water both sides of the fabric. and sediment Hardwood posts 2 x 2 are potentially strong Compaction is achieved. enough to support the loads but are difficult to drive into the by rolling a tractor wheel. ground more than 6 8 To hold 2 ft of sediment and water along both sides of the slit. the posts should be driven 2 ft into the ground Steel posts in the ground 2 to 4 times to. are best because they can be driven into compacted soil to a achieve nearly the same or. depth of 2 ft The support posts should be spaced 3 4 ft apart greater compaction as the. Figure 13 Tractor wheel compacting, where water may run over the top of the fence 5 ft in most other original undisturbed soil.
areas and 6 7 ft where there isn t a considerable horizontal This vertical compaction. load Improper post depth and spacing is often the cause of reduces the air spaces. sagging fabric and falling posts Some authorities believe a between soil particles. more robust wire or chain link supported silt fence is needed which minimizes infiltration. to withstand heavy rain events However this may double the Without this compaction. cost of a silt fence installation and entails disposing of more infiltration can saturate. material in a landfill when the fence is removed Installing silt the soil and water may. fencing having five interacting features 1 proper placement find a pathway under the. based on the site s contours 2 adequate amount of fencing fence When a silt fence is. without long runs 3 heavy porous filter fabric 4 metal posts holding back several tons. with proper depth and of accumulated water and,spacing and 5 tight soil sediment it needs to be. compaction on both sides supported by posts that, of the silt fence will usually are driven 2 ft into well. obviate the need for wire compacted soil Driving in. or chain link reinforced the posts and attaching the. fencing Prefabricated silt fabric to them completes Figure 14 Silt fence installation using. fences e g fabric attached the installation the static slicing method. to wooden posts in a,Trenching Method,100 ft package doesn t. provide for posting after the Trenching machines have been used for over twenty five years. Figure 11 Chain link supported silt fence, ground is compacted or to dig a trench for burying part of the filter fabric underground. allow variable post spacing Usually the trench is about 6 wide with a 6 excavation Its. walls are often more curved,than vertical so they don t.
Silt Fence Installation,provide as much support, Two commonly used approaches for installing silt fences are for the posts and fabric. the static slicing method and the trenching method Turning the trencher is. necessary to maneuver,Static Slicing Method around obstacles follow. The static slicing machine pulls a narrow blade through the terrain contours or property. ground to create a slit 12 deep and simultaneously inserts lines and install upturns Figure 15 Trenchers make a wider. the silt fence fabric into this slit behind the blade The blade is or J hooks But trenchers excavation at turns. Stormwater Best Management Practice Silt Fences,can t turn without making damage a conventional. a wider excavation and silt fence The bottom of,this results in poorer the porous fabric is held. soil compaction which firmly against both the, allows infiltration along the ground and base of precast.
underground portion of the concrete highway barriers. Figure 16 Poor compaction has resulted, fence This infiltration leads by light colored stones An. in infiltration and water flowing under to water seeking pathways alternative installation would. this silt fence causing retained sediment Figure 18 Front of silt fence on part of. under the fence which be having the concrete,washout stock pile s perimeter. causes subsequent soil barriers rest directly on the. erosion and retained sediment washout under the fence The bottom edge of the filter fabric which would extend under the. white line on the fence in Figure 16 and red arrow both mark barriers about 10 so the barriers weight will press the fabric. the previous sediment level before the washout Post setting against the ground to prevent washout Water passing through. and fabric installation often precede compaction which make the silt fence red arrow in Figure 18 flows to a storm sewer. effective compaction more difficult to achieve EPA supported culvert inlet which is surrounded by a fabric silt fence yellow. an independent technology evaluation ASCE 2001 which arrows in Figures 17 and 18 that reduces the runoff s velocity. compared three progressively better variations of the trenching and allows settling before the water is discharged to a creek. method with the static slicing method The static slicing. method performed better than the two lower performance Bridge abutment example During the construction of a bridge. levels of the trenching method and was as good or better than over a river between two lakes an excavation on the river bank. the trenching method s highest performance level The best was needed to pour footings for the bridge abutment The silt. trenching method typically required nearly triple the time and fence along the excavation s perimeter composed of concrete. effort to achieve results comparable to the static slicing method highway barriers with orange filter fabric was designed to. prevent stormwater from,Proper Attachment washing excavated spoil. Regardless of the installation method proper attachment of into the river and to fend off. the fabric to the posts is critical to combining the strength of the river during high flows. the fabric and support posts into a unified structure It must be A portion of the orange filter. able to support 24 of sediment and water For steel posts use fabric that has blown away. three plastic ties per post 50 lb test strength located in the from the concrete barriers. top 8 of the fabric with each tie hung on a post nipple placed shows the need to overlap. diagonally to attach as many vertical and horizontal threads as and reinforce the joints. possible For wooden posts use several staples per post with a where two sections of filter Figure 19 Silt fence for bridge abutment. fabric are attached excavation,wood lath to overlay the fabric. Highway example Because of the proximity of a construction. Perimeter Silt Fences, site to a highway a concrete barrier was required by Minnesota s.
When silt fences are placed around the perimeter of a stock pile DOT to protect the highway. or a construction site the conventional silt fence design and and an underground fiber. materials discussed previously may not be sufficient optic cable next to the. highway from construction,Stock pile example A activities The concrete. stock pile of dirt and large barrier was used to support. rocks is shown in Figures 17 a silt fence along the. and 18 with a silt fence perimeter of a large amount. protecting a portion of its of dirt that was stock piled. perimeter Rocks that roll before being used for fill at. Figure 20 Silt fence protecting a,highway and underground fiber optics. down the pile would likely Figure 17 Back of silt fence on part of. a different location cable,the stock pile s perimeter. Stormwater Best Management Practice Silt Fences,Lake shore example Permanent Soil Stabilization. The lake s shoreline is being, When the land disturbing activities are sufficiently completed to.
restored with plant plugs, allow permanent soil stabilization on the site the silt fences and. and seeded with native, sediment basins are removed The fabric and damaged posts. plant species A plywood, go to the landfill Steel posts and some of the wooden posts. perimeter silt fence is used, can be reused Then the sediment is spread over the site to. to trap sediment from a, provide fertile soil and the area can be seeded and mulched to.
construction site on the, Figure 21 Silt fence protecting a lake support revegetation. shore right side of the picture,protect the lake shore from References. boat wake erosion and to prevent geese from eating the seeds. ASCE 2001 Environmental Technology Verification Report for. and young plants This fencing will be removed when 70. Installation of Silt Fence Using the Tommy Static Slicing Method. vegetative cover is achieved CERF Report 40565 Washington DC American Society of. Civil Engineers www epa gov etv pubs 08 vs tommy pdf. Inspection and Maintenance, ASTM 2003 Standard Practice for Silt Fence Installation. Silt fences should be D 6462 03 2008 West Conshohocken PA American Society. inspected routinely and of Testing Materials International. after runoff events to www astm org SEARCH search reskin html query D6462. determine whether they 03 siteType store standards searchType standards full. The silt fence in Figure 7 doesn t pond water or retain sediment Stormwater will run around the fence carrying sediment to the street which will transport the water and its sediment load to the storm sewer inlet Water flowing around the ends of a silt fence will cause additional erosion and defeat its purpose The bottom of each

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