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II'''''''' '" ~> , . . . , jO , ~ . '. . . '1'11' C01DRAIX1 ..--- ~1 :~ I:l'PIO.RT'If.I",Tllf NATURAL Rf..'oil:'UIU:E.."i Cherry Creek Reservoir Site-Specific Probable Maximum Precipitation Study CWCS Fact Sheet 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 Cherry Creek Basin. .. 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 S10-15 billion in damages. The PM? study also concluded that the current 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 Castfewood 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. The State's Site-Specific Study: The result of public meetings in 1999 with the Corps, the state and local interests regarding the dam improvements was that the Corps funding was put on hold and Colorado State Senate 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 study was delegated to the Colorado Water Conservation Board (CWCS). A Request lor Qualifications was released in 1999 by the CWCS and Ed Tomlinson of Applied Weather Associates was contracted in 2000 to develop a study team and complete the independent review and conduct a study specific to the Cherry Creek Basin for 5175,000. The state funded half of the study and the communities of Greenwood Village, Aurora, Englewood, and Douglas County paid for the remaining half of the funding. Representatives from those communities were active participants at meetings during the study_ ---- '~-"'-"- --- ---..- .--.-.--- '.,.- .J,. '.',- , ~ -.tj ."t..'~El . :'-1;t;, AWA approach vs. 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. AWA Study findings: