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.:":<O:".~' o ~ .. - .-z. . -.. 0 . ~ o . , '. .. .' '/'tJl' Cherry Creek Reservoir Site-Specific Probable Maximum Precipitation Study CWCS Fact Sheet o..)U)ltI\lX) :.~i 1.l"UllW'''<'f NATURAL 1U'.sc"UIUT.\ Y Background: the Flood Control Act of 1941 and 1944 authorized The Cherry Creek Lake project. Construction was completed in 1950. An existing spillway hydraulically connects the Cherry Creek basin with the adjacent Toll Gate and Sand creek basins. The Project controls 386 square miles of the 410 square mile Ch~rry Creek Basin.:.r-- o \)","' 7" \lty The Corps,. Study: The Army Corps of Engineers (USACE) . Omaha District started a study that concluded in with A Probable Maximum Precipitation event and the resulting Probable Maximum Flood event could potentially overtop the dam and put 100.000 people at risk and could cause $10-15 billion in damages. The PMP study also concluded that the cl!Lrent project could safely control 75% of the PMF under existing conditions with adequate freeboard. A 1993 Dam Safety Assurance Evaluation report produced some alternatives that included that have been evaluated and further studied to include: roller compacted concrete hardening of the dam. a 15 foot dam raise. 300~400 foot new spillway on Cherry Creek Dam with a 9.foot dam raise. dry dam at Castlewood Dam Site with no dam raise. dry dam at West Cherry Creek with a 7.5 foot dam raise, and a dray dam at Scott Road with a 12 foot dam raise. fr:> ,l"i-i..j <6">' cu;;:G.('\'''~~ \, _. . . The State's Site-Specific Study: The ".ult of ubllo eAlln in 1 QQ9 \'" -t:: 1.-,':::::::;:::."::." ~".8. A~~ r- l''''?: JOlT ;.r("", ~Q:i ~~ the Corps, Ule.st.at9 ~n~ local mterests r ~ was that -t .n was ut on h Resolution (99-023) required the state to conduct an independent peer review of the National Weather Service's (NWS) Probable Maximum Precipitation Study (PMP) used by the USACE. Responsibility for the st~y was delegated to the Colorado Water Conservation Board (CWCB). -A- R Tomllnsofluf Applied Weather Associates was contracted in 2000 to a it11rly Ipam ;:and r:omr!ptQ thQ independent review and conduct a study specific to the C~eny C;l.e~~B~k>r .178,000. The state funded half of the study and the,:tOf'ill'nun~ies' of Greenw~ ~illage. Aurora, Englew~. -:: I and Douglas County paid for the remaining half of the fundlnSi.- - :-:-~, Repr~sentati~es from those communities w~re aCJive participants ~lrt- .t: ,- -~.- ~ I meetlngsdunng the study....,'v ("'L ~ {<"",-,b' " Yrc3eLd (\\I.:~ 0'" ~"'J({(ttJc +0055.5..1-- v...-.d-"'. -l4 """.tt~6-1 ~"'^. ._:::-..:'S'2_- AWA approach V5. the NWS approach: The NWS Study started with the generalized PMP values from Hydro-Meteorological Reports (HMR), This process uses regional storms and envelops and smoothes the storms across different topographic and areas and fits them into the basin. It then uses procedures to apply the generalized PMP values down in spatial scale to the specific watershed. The AWA approach started with observed rainfall data from the Front Range identified the most extreme storms with occurred over locations, which are topographically and climatologically similar to the study basin. AWA then applied the same procedures used to develop the HMR and PMP values but specifically applied them to the study basin. To the right you can see results from the AWA analysis. the colored areas were the storm path and rainfall amounts for one day of the June 1965 flood. The red line is the outline of the Cherry Creek Basin. The storm was moved into the Cherry Creek Basin and centered to allow for the most amount of rainfall to come up with PMP estimates for the study. \. , END - h .- . .. . ..