1400000062 Droz Creek Inundation Report

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Droz Creek Reservoir Breach Inundation Report | Breach Modeling 2 BREACH MODELING The first task of this analysis was to develop a flood hydrograph resulting from a “sunny-day” breach of the dam assuming the reservoir was at normal capacity prior to the breach. As such, no inflow into Droz Creek Dam at the time of the breach was assumed. According to guidance published under the CDWR’s Guidelines for Hazard Classification (CDWR, Nov 2010), the Droz Creek Dam is considered a small dam. Pursuant to the CDWR’s Guidelines for Dam Breach Analysis (CDWR, Feb 2010), for Small Dams with a medium storage intensity, the dam breach parameters should be based on either the empirical breach relationships developed by David C. Froehlich (Froehlich, 1987) or the MacDonald & Langridge Monopolis with Washington State failure time (MLM). Because a sunny day breach was modeled, an overtopping breach is not considered likely, so only the piping failure mode was considered. The Froehlich and MLM breach calculations require certain parameters of the dam be specified. These parameters, and the values selected for this analysis, are as follows: Parameter Value Source Maximum Water Depth (Hw) 30.5 ft Dam height per spillway (8623.9’) and outlet (8593.4’) elevations Reservoir Volume (Vw) 155.3 ac-ft Storage at spillway height (8623.9) per stage-storage table Reservoir Surface Area (As) 13.3 ac Maximum surface area per Stage/Storage curve Height of Breach (Hb) 34.6 ft Dam Crest (8628) and outlet (8593.4’) Crest Width of Dam (C) 15 ft From as-constructed drawings Slope of Upstream Dam Face (Zu) 3 From as-constructed drawings Slope of Downstream Dam Face (Zd) 2.5 From as-constructed drawings The resulting breach parameters for the empirical relationships can be found in Appendix A. The predicted peak breach discharge based on the Froelich Method is 14,533 cfs. The MLM Method resulted in a predicted peak breach discharge of 6,912 cfs. The breach hydrograph was developed by performing a dam breach analysis using HEC-HMS v3.5. The calculated Froelich breach parameters were utilized for the dam breach model as they were the most conservative. The analysis simulated the reservoir as a storage area with a defined stage- storage relationship provided by the as-constructed drawings, included in Appendix B. The peak breach discharge predicted by the HEC-HMS model was approximately 7,266 cfs. Because there are four smaller dams downstream of the Droz Creek Dam, the outflow from the Droz Creek Dam breach was routed through each, and overtopping failure was assumed for these four ponds in a cascading model. The final peak outflow from the system is calculated as 7,252 cfs. The modeled peak discharge is lower than anticipated by the Froehlich methodology, which is considered more conservative. The dam breach discharge hydrograph can be found in Appendix C.