Sediment Basin (SB) Description. Appropriate Uses. Design and Installation

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1 Descriptin A sediment basin is a temprary pnd built n a cnstructin site t capture erded r disturbed sil transprted in strm runff prir t discharge frm the site. Sediment basins are designed t capture site runff and slwly release it t allw time fr settling f sediment prir t discharge. Sediment basins are ften cnstructed in lcatins that will later be mdified t serve as pst-cnstructin strmwater basins. Apprpriate Uses Mst large cnstructin sites (typically greater than 2 acres) will require ne r mre sediment basins fr effective Phtgraph SB-1. Sediment basin at the te f a slpe. Pht curtesy f WWE. management f cnstructin site runff. On linear cnstructin prjects, sediment basins may be impractical; instead, sediment traps r ther cmbinatins f BMPs may be mre apprpriate. Sediment basins shuld nt be used as stand-alne sediment cntrls. Ersin and ther sediment cntrls shuld als be implemented upstream. When feasible, the sediment basin shuld be installed in the same lcatin where a permanent pstcnstructin detentin pnd will be lcated. Design and Installatin The design prcedure fr a sediment basin includes these steps: Basin Strage Vlume: Prvide a strage vlume f at least 3,600 cubic feet per acre f drainage area. T the extent practical, undisturbed and/r ff-site areas shuld be diverted arund sediment basins t prevent clean runff frm mixing with runff frm disturbed areas. Fr undisturbed areas (bth n-site and ff-site) that cannt be diverted arund the sediment basin, prvide a minimum f 500 ft 3 /acre f strage fr undevelped (but stable) ff-site areas in additin t the 3,600 ft 3 /acre fr disturbed areas. Fr stable, develped areas that cannt be diverted arund the sediment basin, strage vlume requirements are summarized in Table SB-1. Basin Gemetry: Design basin with a minimum length-t-width rati f 2:1 (L:W). If this cannt be achieved because f site space cnstraints, baffling may be required t extend the effective distance between the inflw pint(s) and the utlet t minimize shrt-circuiting. Sediment Basins Dam Embankment: It is recmmended that embankment slpes be 4:1 (H:V) r flatter and n steeper than 3:1 (H:V) in any lcatin. Functins Ersin Cntrl Sediment Cntrl Site/Material Management N Yes N August 2013 Urban Drainage and Fld Cntrl District SB-1

2 Sediment Basin (SB) Inflw Structure: Fr cncentrated flw entering the basin, prvide energy dissipatin at the pint f inflw. Table SB-1. Additinal Vlume Requirements fr Undisturbed and Develped Tributary Areas Draining thrugh Sediment Basins Additinal Strage Vlume (ft 3 ) Imperviusness (%) Per Acre f Tributary Area Undevelped Outlet Wrks: The utlet pipe shall extend thrugh the embankment at a minimum slpe f 0.5 percent. Outlet wrks can be designed using ne f the fllwing appraches: Riser Pipe (Simplified Detail): Detail SB-1 prvides a simplified design fr basins treating n mre than 15 acres. Orifice Plate r Riser Pipe: Fllw the design criteria fr Full Spectrum Detentin utlets in the EDB Fact Sheet prvided in Chapter 4 f this manual fr sizing f utlet perfratins with an emptying time f apprximately 72 hurs. In lieu f the trash rack, pack unifrmly sized 1½ - t 2-inch gravel in frnt f the plate r surrunding the riser pipe. This gravel will need t be cleaned ut frequently during the cnstructin perid as sediment accumulates within it. The gravel pack will need t be remved and dispsed f fllwing cnstructin t reclaim the basin fr use as a permanent detentin facility. If the basin will be used as a permanent extended detentin basin fr the site, a trash rack will need t be installed nce cntributing drainage areas have been stabilized and the gravel pack and accumulated sediment have been remved. Flating Skimmer: If a flating skimmer is used, install it using manufacturer s recmmendatins. Illustratin SB-1 prvides an illustratin f a Fairclth Skimmer Flating Outlet, ne f the mre cmmnly used flating skimmer utlets. A skimmer shuld be designed t release the design vlume in n less than 48 hurs. The use f a flating skimmer utlet can increase the sediment capture efficiency f a basin significantly. A flating utlet cntinually decants cleanest water ff the surface f the pnd and releases cleaner water than wuld discharge frm a perfrated riser pipe r plate. SB-2 Urban Drainage and Fld Cntrl District August 2013

3 Illustratin SB-1. Outlet structure fr a temprary sediment basin - Fairclth Skimmer Flating Outlet. Illustratin curtesy f J. W. Fairclth & Sns, Inc., FairclthSkimmer.cm. Outlet Prtectin and Spillway: Cnsider all flw paths fr runff leaving the basin, including prtectin at the typical pint f discharge as well as vertpping. Outlet Prtectin: Outlet prtectin shuld be prvided where the velcity f flw will exceed the maximum permissible velcity f the material f the waterway int which discharge ccurs. This may require the use f a riprap aprn at the utlet lcatin and/r ther measures t keep the waterway frm erding. Emergency Spillway: Prvide a stabilized emergency verflw spillway fr rainstrms that exceed the capacity f the sediment basin vlume and its utlet. Prtect basin embankments frm ersin and vertpping. If the sediment basin will be cnverted t a permanent detentin basin, design and cnstruct the emergency spillway(s) as required fr the permanent facility. If the sediment basin will nt becme a permanent detentin basin, it may be pssible t substitute a heavy plyvinyl membrane r prperly bedded rck cver t line the spillway and dwnstream embankment, depending n the height, slpe, and width f the embankments. August 2013 Urban Drainage and Fld Cntrl District SB-3

4 Sediment Basin (SB) Maintenance and Remval Maintenance activities include the fllwing: Dredge sediment frm the basin, as needed t maintain BMP effectiveness, typically when the design strage vlume is n mre than ne-third filled with sediment. Inspect the sediment basin embankments fr stability and seepage. Inspect the inlet and utlet f the basin, repair damage, and remve debris. Remve, clean and replace the gravel arund the utlet n a regular basis t remve the accumulated sediment within it and keep the utlet functining. Be aware that remval f a sediment basin may require dewatering and assciated permit requirements. D nt remve a sediment basin until the upstream area has been stabilized with vegetatin. Final dispsitin f the sediment basin depends n whether the basin will be cnverted t a permanent pst-cnstructin strmwater basin r whether the basin area will be returned t grade. Fr basins being cnverted t permanent detentin basins, remve accumulated sediment and recnfigure the basin and utlet t meet the requirements f the final design fr the detentin facility. If the sediment basin is nt t be used as a permanent detentin facility, fill the excavated area with sil and stabilize with vegetatin. SB-4 Urban Drainage and Fld Cntrl District August 2013

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6 Sediment Basin (SB) SB-6 Urban Drainage and Fld Cntrl District August 2013

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