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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br /> 1 Also,sediment traps and ponds are attracrive ro children and can be very dangerous. If fencing o( <br /> the pond is planned,the type of fence and its location shall be shawn on the Erosion and <br /> Sediment Control Plan. <br /> 2. Struchucs having a maximum slorage capaciry at the top of the dam of 10 acre-feet(435,600 ft') <br /> or more are subjcet to the Washington Dam Safcty Rcgulauons(Chapter 173-175 WAC). <br /> 3. See DCSS Drawing#s 217 through 219. <br /> 4. ff permanent runoff control facilitics are paR of the project,they mey be used for sediment <br /> retention. However,the surtace azea roquirements of the sediment pond must be met. This may <br /> require enlarging the permanent pond to comply wit6 the surface area requirements or providing <br /> additional capacity in a temporary sediment control facility. !(a permanent control structure is <br /> used,it mey be advisable to partially restrict the lower orifice with gravel to increase residcnce <br /> timc while stip allowing dewatering of the pond. <br /> 5. Use of infiltration facilides for sedimentation ponds duting construction tends to clog the soils <br /> and reduce their capacity to infiltrate. [f infilhation facilities aze to be used,the sides and bouom <br /> of the facility shall only be rough excavated ta a minimum of 2 feet abave final grade. Final <br /> grading oCthe infiltration facifiry shall occur only when all contributing drainage areas are fully <br /> stabiGud. The infiltration pretreatment facility shall be fully constructed and used with the <br /> sedimentation pond te help prevent ciogging. <br /> Determining Pond Ceometry: <br /> I. Obtain the discharge from the hydrologic calculations of the peak flow for the 2-yeaz recurrence <br /> interval flow(Q=). The 10-year recurrence interval peek flow shall bc used if thc project size, <br /> lexpected timing and durauon of construction,or downstream conditians warrant a 6igher levcl of <br /> i protection. For sediment traps having a cootributing area with a time of concentration less than <br /> or equal to 60 minutes,the rational method may be used to detetvtitte the design flow. Otherwise, <br /> a continuous mnoff model(WWHM ar an approved altemative,see Volume I,Chaptcr 5)shall <br /> be used to detemvne the design flow, 'Ctte developed(unmitigated) land use condition shall be <br /> used in thc hydrologic analysis. <br /> 2. The basic geometry of the pond shall meet the following design criteria: <br /> a. Surface uea at the top of the riser equal to 2,083 square feet per cubic feet per second <br /> (cfs)of intlow <br /> b. Minimum 3.5•foot dcpth from top of riser to bottom of pond <br /> c. Maximum 3:1 interior side slopes and maximum 2 1 exterior slopes. Thc interior slopes <br /> can be increased to a maximum of 2:1 if fencing is provided at or above the maximum <br /> watcr surfacc <br /> d. One foot of freeboard betwccn the top of thc riser and the crest of the cmcrgcncy <br /> spillway <br /> c. Flat bottom <br /> f. Minimum t-foot d�ep spillwa�� <br /> g. Length-to-width ratio betwcen 3:I and 6:I. <br /> Sizing oP Discharge�techanisms: <br /> I. The outlet for the pond consists of a combination of principal and emergency spillways. These <br /> outlets must pass the pcak runoff expected fram Ihe conhibuting drainege arca for a 100•year <br /> - recu[reuce interval stocm. If,due 10 site conditions and pond geometq�,a separate emergency <br /> Volume II- Construction Stortnwater Pollufion Prevenfion <br /> 3-62 <br />