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Copper Spur - Spillway Adequacy Memo <br />DAMID: 350xxx <br />DATE: December 21, 2017 <br />Page 3 of 3 <br />NOAA rainfall, NRC soils report and calculation sheets are included as attachments to <br />this memorandum. <br />OF Determination and Spillway Hydraulics <br />HEC -HMS modeling indicated the OF would be 95 cfs. This value was in good <br />agreement with the 25 -year streamflow for the basin of 94 cfs. <br />Spillway hydraulics were modeled using Flow Master software and the spillway <br />geometry as it is shown in the Plans. Given the longitudinal slope of the spillway was <br />indiscernible from the plans, a slope of two percent was assumed. A channel <br />roughness of 0.069 was selected to represent the 9 -inch D5o riprap lining. Hydraulic <br />modeling results demonstrated the spillway for Copper Spur would not adequately <br />convey the IDF, resulting in a flow depth of 1.46 feet within the 1 -foot deep spillway, <br />or 0.46 feet overtopping the dam crest. <br />HEC -HMS and Flow Master results and included within the attachments. <br />Spillway Recommendations <br />Modeling results demonstrate that the spillway lacks sufficient capacity to convey the <br />OF without overtopping the dam. Furthermore, the Notice of Intent form specifies <br />that the total depth from the spillway crest to the dam crest cannot be less than 3 - <br />feet. <br />Preliminary hydraulic modeling indicates that widening the spillway to a bottom width <br />of 30 -feet would convey the OF with zero residual freeboard. To widen the spillway <br />further would encroach onto the location of the Agridrain, which is positioned at the <br />maximum section of the dam. It is therefore recommended to raise the crest of the <br />dam by two feet to ensure a compliant spillway. <br />Last, a hazard study may need to be prepared to confirm the dam is NPH. <br />