Laserfiche WebLink
the project size, expected timing and duration of construction, or <br /> downstream conditions warrant a higher level of protection. If no <br /> hydrologic analysis is required, the Rational Method may be used. <br /> Determine the required surface area at the top of the riser pipe with the <br /> equation: <br /> SA = 2 x Q2/0.00096 or <br /> 2080 square feet per cfs of inflow <br /> See BMP C240 for more information on the derivation of the surface <br /> area calculation. <br /> The basic geometry of the pond can now be deternined using the <br /> following design 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:1V interior side slopes and maximum 2H:1V exterior <br /> slopes. The interior slopes can be increased to a maximum of 2H;1 V if <br /> fencing is provided at or above the maximum water surface. <br /> • One foot of freeboard between the top of the riser and the crest of the <br /> emergency 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 <br /> emergency spillways. These outlets must pass the peak runoff <br /> expected from the contributing drainage area for a 100-year storm. If, <br /> due to site conditions and basin geometry, a separate emergency spill- <br /> way is not feasible, the principal spillway must pass the entire peak <br /> runoff expected from the 100-year stonn. However, an attempt to <br /> provide a separate emergency spillway should always be made. The <br /> runoff calculations should be based on the site conditions during <br /> construction. The flow through the dewatering orifice cannot be <br /> utilized when calculating the 100-year storm elevation because of its <br /> potential to become clogged; therefore, available spillway storage <br /> must begin at the principal spillway riser crest. <br /> The principal spillway designed by the procedures contained in this <br /> standard will result in some reduction in the peak rate of runoff. <br /> However, the riser outlet design will not adequately control the basin <br /> discharge to the predevelopment discharge limitations as stated in <br /> Minimum Requirement#7: Flow Control. However, if the basin for a <br /> permanent stormwater detention pond is used for a temporary <br /> Vohnne II—Construction Slogmwaler-Pollution Pi-evention-August 2012 <br /> 4-105 <br />