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~.., .t~ ~~ Flow Technologies Write-ups for the Cherry Creek Site- Specific PMP Study CHERRY CREEK BASIN RAINFALL/RUNOFF MODELING FOR TRANSPOSED STORMS Flood hydro graphs were developed for Cherry Creek Basin at Cherry Creek Reservoir by utilizing the HEC-I Flood Hydrograph package, Model runs were based on USACE hydro graph parameters as reported in a USACE Cherry Creek Dam safety evaluation study report.. 1 Hydrograph parameters were as follows: . Unit hydro graph: Synder . Lag-time computation: Snyder . Infiltration: Initiallconstant (0.5inlO,5inlhr) . Effective Basin Contributing Area: 0.84 · Rainfall distribution: 1995 Site-Specific PMP Hourly Rainfall Because the Cherry Creek Basin HEC-l PMF model as used by USACE has not yet been obtained by A W A (but requested through CWCB and Flow Technologies), that model was duplicated as-best-as possible based on readily available data and information as reported above, The model was verified by simulating the USACE 1995 Cherry Creek Dam PMF as reported in reference no. 1 (footnote no. 1). Initial comparison of peak discharge and volume were very close. However, the model was calibrated by adjusting lag-time until the peak discharges between the model and USACE PMF compared favorably. The resulting model was then considered a "duplicate effective" model of US ACE's 1995 Cherry Creek Basin PMF model. The duplicate effective model was applied to various area-weighted average rainfall depths over the basin for the purpose of evaluating historic storm transpositioning and maximization on resultant flood hydro graphs. The model can be used to evaluate rainfalllrunoff for other purposes as well, However, the study team would prefer to use USACE's model for determining flood results to eliminate any uncertainty regarding if the two models yield the same results, 1 Cherry Creek Dam Safety Evaluation Study, Spillway Design Flood adn Probable Maximum Flood History, u.s. Army Engineer District, Omaha, January 1997,