405 RIVERSIDE RD 2023-01-30 - Laserfiche WebLink

I Determine the required surface area at the top of the riser pipe with the equation: ISA = 2 x Q2/0.00096 or 2080 square feet per cfs of inflow I See BMP C240 for more information on the derivation of the surface area calculation. The basic geometry of the pond can now be determined using the Ifollowing design criteria: • Required surface area SA (from Step 2 above) at top of riser. I • Minimum 3.5-foot depth from top of riser to bottom of pond. • Maximum 3:1 interior side slopes and maximum 2:1 exterior slopes. The interior slopes can be increased to a maximum of 2:1 if fencing is Iprovided at or above the maximum water surface. • One foot of freeboard between the top of the riser and the crest of the I emergency spillway. • Flat bottom. • Minimum 1-foot deep spillway. • Length-to-width ratio between 3:1 and 6:1. • Sizing of Discharge Mechanisms. IThe outlet for the basin consists of a combination of principal and emergency spillways. These outlets must pass the peak runoff expected from the contributing drainage area for a 100-year storm. If, due to site Iconditions and basin geometry, a separate emergency spill-way is not feasible, the principal spillway must pass the entire peak runoff expected from the 100-year storm. However, an attempt to provide a separate I emergency spillway should always be made. The runoff calculations should be based on the site conditions during construction. The flow through the dewatering orifice cannot be utilized when calculating the I 100-year storm elevation because of its potential to become clogged; therefore, available spillway storage must begin at the principal spillway riser crest. IThe principal spillway designed by the procedures contained in this standard will result in some reduction in the peak rate of runoff. However,the riser outlet design will not adequately control the basin I discharge to the predevelopment discharge limitations as stated in Minimum Requirement#7: Flow Control. However, if the basin for a permanent stormwater detention pond is used for a temporary I sedimentation basin,the control structure for the permanent pond can be used to maintain predevelopment discharge limitations. The size of the basin, the expected life of the construction project, the anticipated I downstream effects and the anticipated weather conditions during construction, should be considered to determine the need of additional discharge control. See Figure 4.28 for riser inflow curves. 4-106 Volume ll—Construction Stormwater Pollution Prevention February 2005