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Detention Time <br />All of the ponds are designed as flow-through structures once the design sedi- <br />ment storage volume has been reached. Prior to that point, runoff will accumu- <br />late below the dewatering device and evaporate or percolate through the sides <br />and bottom of the reservoir. The principal spillway is a CMP riser connected <br />to a CMP conduit extending through the embankment. The riser contains a per- <br />foration at the maximum sediment volume level which will allow detention of <br />runoff for at least 24 hours. The detention times were generated as part of <br />the DEPOSITS model developed at the University of Kentucky. The detention <br />times were all found to exceed 24 hours. The input to this model, including <br />inflow and outflow calculations, was checked and found to be adequate. <br />The applicant is in compliance. <br />Dewatering <br /> <br />z <br />Dewatering of all ponds for runoff from at least the 10 year-24 hour storm will <br />be accomplished by the principal spillway. The spillway consists of a CMP ris- <br />er connected to a CMP conduit extending through the embankment. The top of the <br />riser is equipped with a trash rack and anti-vortex plate. A perforation in <br />the riser at the: maximum sediment level will provide, a detention time exceeding <br />24 hours for runoff from a 10 year-24 hour. storm. In the event that this storm <br />is exceeded and additional storage is unavailable, the principal spillway has <br />been designed to pass the peak flow from a 10 year-24 hour precipitation event <br />with a maximum allowable head of one foot above• the riser crest (page IV-18 of <br />the permit .application). The riser and conduit were sized using Figures 6-25 <br />through 6-27 presented in Chapter 6 of the SCS Engineering Field Manual. A1- <br />though;the Refuse Disposal Area ponds were designed according to these specifi- <br />cations, the D-Portal pond, with a design flow of 32 cfs and a 30-inch riser, <br />requires a head of 1.2 feet. In order to fulfill the specifications the ap- <br />plicant should re-size the spillway to allow a maximum_F~ad of only one foot <br />above the riser crest durin9__a_10 vear-24 hour runoff event thereby ensuring <br />that outflow will not occur through the emergency spillway (See "Stipulations" <br />in this section of the TEA). <br />20 <br />