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recommended empirical methods are summarized in Table 1 below along with the <br />surveyed breach width, reported time of failure and modeled input parameters and <br />peak breach discharge. <br />Table 1 - Havana Street Pond Dam Breach Parameters <br />7.4 Analysis of Incremental Impacts <br />The incremental impacts of the failure of the spillway were determined by modeling <br />the reservoir assuming that the spillway had not failed for a baseline condition, and <br />then modeling the event with the dam failure. Since the failure mode was easily <br />determined, and the primary purpose of the modeling was to determine impacts of the <br />dam failure on downstream flooding, determining the exact inflow to the reservoir <br />through calibration was deemed unnecessary. The volume impounded by the reservoir <br />was easily determined by the high water marks surveyed during the field investigation <br />and the known stage - storage relationship for the reservoir. <br />The results of the HMS modeling show that the incremental impacts of the dam failure <br />were significant in terms of peak discharge, but relatively low in terms of impacts and <br />damage. The peak discharge calculated with both the HMS and RAS models is <br />approximately 5,600 - 6,400 cfs. This is approximately 3.0 - 3.5 times the discharge <br />that was entering the reservoir (peak inflow was modeled to be approximately 1,600 <br />cfs). However, the duration of this peak flow occurred over a period of approximately <br />30 minutes was short in comparison to the overall event which lasted for several days. <br />The peak flow discharging from the dam breach was likely significantly attenuated in <br />the wide, flat floodplain and by the railroad embankment downstream of the dam as <br />MLM -WA <br />Bav (ft) 73.3 <br />Zb (_H:1V) 0.5 <br />Tf (hr) 0.36 <br />cfs) 10,223 <br />Froehlich <br />Bav, (ft) 81.4 <br />Zb (_H:1V) 0.7 <br />Tf(hr) 1.20 <br />cfs) 7,553 <br />Surveyed' <br />t) <br />ZB(° Hf 1.5 <br />Reported Tf(hr) 0.75 <br />Modeled <br />HEC -HMS) <br />Bav (ft) 65 <br />Tf (hr) 1.0 <br />Zb (_H:1V) 2.5 <br />hr) 1 5,600 <br />Progression I Sine Wave <br />Qa calculated with SMPDBK Equation <br />Ultimate Geometry Surveyed on September 23, 2013 <br />Q, includes approximately 1,600 cfs of inflow <br />7.4 Analysis of Incremental Impacts <br />The incremental impacts of the failure of the spillway were determined by modeling <br />the reservoir assuming that the spillway had not failed for a baseline condition, and <br />then modeling the event with the dam failure. Since the failure mode was easily <br />determined, and the primary purpose of the modeling was to determine impacts of the <br />dam failure on downstream flooding, determining the exact inflow to the reservoir <br />through calibration was deemed unnecessary. The volume impounded by the reservoir <br />was easily determined by the high water marks surveyed during the field investigation <br />and the known stage - storage relationship for the reservoir. <br />The results of the HMS modeling show that the incremental impacts of the dam failure <br />were significant in terms of peak discharge, but relatively low in terms of impacts and <br />damage. The peak discharge calculated with both the HMS and RAS models is <br />approximately 5,600 - 6,400 cfs. This is approximately 3.0 - 3.5 times the discharge <br />that was entering the reservoir (peak inflow was modeled to be approximately 1,600 <br />cfs). However, the duration of this peak flow occurred over a period of approximately <br />30 minutes was short in comparison to the overall event which lasted for several days. <br />The peak flow discharging from the dam breach was likely significantly attenuated in <br />the wide, flat floodplain and by the railroad embankment downstream of the dam as