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from the standpoint of dam safety, the hydrololgic design of a dam <br />and spillway is based upon avoiding overtopping. Overtopping is espe- <br />cially dangerous for an earth dam because the down rush of waters over the <br />crest will erode the dam face and, if continued long enough, can breach <br />the dam embankment and release ail the stored water rapidly into the <br />downstream floodplain. The safe hydrologic design of a dam dictates ade- <br />quate spillway discharge capability, in combination with an embankment <br />crest height or flood freeboard such that a very large and exceedingly <br />rare flood can be accommodated without overtopping. <br />The Corps of Engineers designs its dams to safely pass the Probable <br />Maximum Flood that is estimated could 6e generated from the upstream <br />watershed. This is the generally accepted criterion for major dams <br />throughout the world, and is the standard of dam safety where overtopping <br />would pose any threat to human life. Although dams that do not fully <br />meet this Corps standard wii] not be evaluated as "unsafe", the Corps <br />considers any dam located in a high hazard potential area to be <br />"seriously inadequate and unsafe" if it cannot pass one-half the Probable <br />Maximum Flood without overtopping. <br /> <br /> <br />Further, the State Engineer's Office (SEO) requires that a high hazard <br />dam pass the full Probable Maximum Flood (PMF) based upon the 6-hour rain- <br />fall from "Design of Small Dams" or one-half the PMF determined from the <br />Corps of Engineers criteria (whichever is greater). Based upon this cri- <br />teria and the following results of the inflow hydrograph development and <br />reservoir flood routing analysis, Monument Dam and Reservoir is considered <br />"seriously inadequate and unsafe" as it cannot pass the PMGS or PMTS floods <br />without overtopping. <br /> Flood Routing Comp arison <br /> Rain Inflow Outflow Overtopping Max. Depth <br />Source inches Peak Peak Duration Over Dam Crest <br /> <br />Hydro-Met X49 PMTS 6.60 9,890 cfs 9,330 cfs 2.2 hr. 3.6 ft. <br />Sma17 Oams PMTS 7.01 10,490 cfs 9,880 cfs 2.0 hr. 3.8 ft. <br />Hydro-Met p49 PMGS 4.60 2,340 cfs 2,300 cfs 2.5 hr. 0.9 ft. <br />Small..Dams PMGS 3.00 1,690 cfs 1,fi60 cfs 2.0 hr. 0.5 ft. <br />One half of both the "Hydro-Meteorologi cal Report No. 49" Probable Maximum <br />Thunderstorm and "Design of Small Dams" Probable Maximum Thunderstorm result in <br />overtopping of the dam as well. . <br />5DX PMP Flood Routing Comparison <br /> <br /> Rain Inflow Outflow Overtapping Max. Depth <br />Source inches Peak Peak Duration Over Dam Crest <br /> <br />Hydro-Met X49 PMTS 3.30 2,600 cfs 1,710 cfs 0.8 hr. 0.6 ft. <br />Small Dams PMTS 3.50 2,850 cfs 1,490 cfs 2.7 hr. 3.4 ft. <br />Hydro-Met $49 PMGS 2.30 570 cfs 350 cfs Does not overtop. <br />Small Dams PMGS 1.50 480 cfs 270 cfs Does not overtop. <br />-6- <br />