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 Page 17 Upper and Lower Emerald Valley Dam Failure Report <br /> January 23, 2015 <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />Initial model runs used the bottom widths and side slopes surveyed during the <br />September 25 site visit. Using those parameters, the model produced <br />relatively large breach flows. It was felt that the relatively wide breach <br />bottom widths measured in the field were the result of additional erosion of <br />those embankments as the remainder of the storm flows passed through them <br />continuously for 13 days after breaching. Our final breach bottom widths were <br />therefore adjusted based on that knowledge and validated using DSB <br />spreadsheets for determining dam breach parameters using the methods of <br />Froelich (DSB, 2010). <br /> <br />Breach development time is the total time for the breach to form and the <br />reservoir to drain through that breach. The time starts when the reservoir <br />meets the downstream slope of the eroded embankment, and continues until <br />the reservoir is fully drained through the resulting opening. Based on <br />discussions with the on-site dam representative, the failure development time <br />for the dams was judged to be about 25 minutes and utilized in the final <br />modeling. Table 6 presents a summary of the overtopping failure breach <br />parameters used in the modeling. <br /> <br />Triggering the initiation of failure can be accomplished by three methods in <br />HEC-HMS: elevation, duration at elevation, and a specific time. Through the <br />investigation process we were able to obtain eyewitness timelines of the <br />failures, as have been previously described. Those trigger times of failures <br />were used in the modeling and are also shown in Table 6. <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />Figure 20: LEV Breach Cross Section, facing downstream