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the flow from a 25 year-24 hour run-off event. The D-Portal Area pond has an <br />embankment with a height greater than 10 feet. Therefore, the emergency spill- <br />way for this pond has been designed to safely pass the flow from a 100 year-24 <br />hour run-off event. <br />The applicant is in compliance. <br />Emergency Spillway Grades and Allowable Velocities <br />,Emergency spillways for all the sedimentation ponds are weir-type spillways <br />with trapezoidal configurations. Base widths of the spillway crests are a min- <br />imum of ten feet, and channel sideslopes are set at 2h:ly. With the exception <br />of pond R2-3, the spillway channels are constructed with that configuration <br />until they discharge into the natural drainage system. The spillway at pond <br />R2-3, however, is a trapezoidal section only at the berm where it is located at <br />the southwest corner of the reservoir. From that point, the remainder of the <br />spillway channel becomes part of an existing natural drainage channel. None of <br />the emergency spillways are constructed in the embankment, although the D-Port- <br />al pond spillway control section shares a side slope with the embankment fill. <br />In this case, the entire spillway channel is lined with 24-inch riprap. The <br />other spillways are typically lined with 12-inch riprap at the upper segment of <br />the channel and 6-inch riprap at the downstream segment. All spillways are <br />designed with a minimum freeboard of one foot between the design depth and the <br />top of the dam. Plan views, a typical cross-section and design information are <br />shown on Maps 34, 78, 79, and 80. Calculations for the spillway capacities <br />are provided in Illustrations 4, 5, 6, and 7. <br />Design inflows were derived from the SCS TP 149 method. A check of the calcu- <br />lations showed the figures to be correct. <br />Velocities in the emergency spillway channels vary from 2.5 fps to 4.9 fps in <br />the control sections and from 5.3 fps to 12.5 fps in the outlet channels. The <br />spillways are riprapped, therefore, the higher velocities should not pose an <br />erosion hazard. No specifications have been included, however, for erosion <br />controls at the outlets of the emergency and principal spillways. The appli- <br />cant should provide additional riprap at the emergency spillway intersection <br />22 <br />