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<br />',.] <br /> <br />] <br /> <br />i'.( <br />;:':; <br /> <br />;>1 <br />~'.,;J <br />:~'J <br /> <br />,~,.., <br /> <br />~ .'"; <br /> <br />iA <br /> <br />",' <br />.~, .~ <br />i:J <br /> <br />fi~ <br />1;\1 <br /> <br />'1 <br /> <br />Based on the criteria in "Design of Small Dams", <br />the probable maximum precipitation thunderstorm, <br />adjusted for the Hyannis Reservoir zone and drainage <br />area, is 9.25 inches in one hour. To determine <br />spillway width design, the low hazard (100 year), <br />probable maximum (500 year) and intermediate moder- <br />ate hazard (250 year) thunderstorms were routed <br />through the reservoir. Ih all routings, a dam <br />height of 80 feet was assumed, with a spillway ele- <br />vation set at 74.6 feet such that the high water <br />level would impound 2123 acre-feet of water. As a <br />conservative measure, it was decided to design the <br />reservoir to contain the moderate hazard thunderstorm. <br /> <br />Based on the flood routing information developed <br />for each of the storms (Appendix E) it was determined <br />that five feet of freeboard would wholly contain the <br />100 year maximum inflow of 1930 cfs, therefore any <br />width of spillway would be adequate. It was then <br />determined that probable maximum flood would <br />require a spillway width of 150 feet and 5.90 feet <br />of freeboard to contain a maximum inflow of 8866 <br />cfs. <br /> <br />To determine the spillway width desired in this <br />project, a moderate hazard thunderstorm was then <br />routed. It was found that an 100 foot spillway would <br />contain a maximum inflow of 8866 cfs, with 4.6 <br />feet of freeboard. Figure 6 in the inflow/outflow <br />hydrograph for this moderate hazard, 100 foot spill- <br />way width event. Figure 7 shows the inflow/outflow <br />hydrograph for the probable maximum precipitation <br />event at this 100 foot spillway width design. <br /> <br />1. <br /> <br />Environmental Assessment Report <br /> <br />The National Environmental Policy Act of 1969, P. L. <br />92-190, requires completion of an assessment of the <br />environmental impact of a proposed project if federal <br />funds are being used. Because the proposed reservoir <br />is located on National Forest land, an environmental <br />assessment report (and determination for need of an <br />Environmental Impact Statement) may be completed in con- <br />junction with the application for a special use permit. <br /> <br />-27- <br />