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1 <br />pretreatment facility should be fully constructed and used with the sedimentation <br />basin to help prevent clogging. <br />. Determining Pond Geometry <br />Obtain the discharge from the hydrologic calculations of the peak flow for the 2- <br />year runoff event (Q2). The 10-year peak flow shall be used if the project size, <br />expected timing and duration of construction, or downstream conditions warrant a <br />higher level of protection. If no hydrologic analysis is required, the Rational Method <br />may be used. <br />Determine the required surface area at the top of the riser pipe with the equation: <br />SA = 2 x Q2/0.00096 <br />or <br />2080 square feet per cfs of inflow <br />See BMP C240: Sediment Trap (p.383) for more information on the derivation of <br />the surface area calculation. <br />The basic geometry of the pond can now be determined using the following design <br />criteria: <br />. Required surface area SA (from Step 2 above) at top of riser. <br />. Minimum 3.5-foot depth from top of riser to bottom of pond. <br />. Maximum 3H:1 V interior side slopes and maximum 2H:1 V exterior slopes. The <br />interior slopes can be increased to a maximum of 2H:1 V if fencing is provided at or <br />above the maximum water surface. <br />• One foot of freeboard between the top of the riser and the crest of the emergency <br />spillway. <br />. Flat bottom. <br />. Minimum 1-foot deep spillway. <br />. Length -to -width ratio between 3:1 and 6:1. <br />• Sizing of Discharge Mechanisms. <br />The outlet for the basin consists of a combination of principal and emergency spill- <br />ways. These outlets must pass the peak runoff expected from the contributing drain- <br />age area for a 100-year storm. If, due to site conditions and basin geometry, a <br />separate emergency spill -way is not feasible, the principal spillway must pass the <br />entire peak runoff expected from the 100-year storm. However, an attempt to <br />provide a separate emergency spillway should always be made. The runoff cal- <br />culations should be based on the site conditions during construction. The flow <br />1 <br />2014 Stormwater Management Manual for Western Washington <br />Volume 11 - Chapter 4 - Page 389 <br />