4.3. ERMUSR 06-16-2015 - Laserfiche WebLink

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failure on the Elk River is. As discussed below, the assumption of a zero-freeboard flood on the Elk River and a 10-year concurrent flood on the Mississippi River resulted in the dambreak flood wave height being almost negligible on the Mississippi, and only about one foot at the most on the Elk River. Therefore, this combination of flood discharges was determined to provide an appropriate upper limit for identifying potentially significant dambreak hazards. Breach Dimensions and Timing. A breach having a bottom width of 40 feet, a bottom elevation of 858 feet, 1:1 side slopes, and a formation time of 0.5 hour was modeled. The assumed failure parameters correspond to an average breach width three times the height of the dam. The assumed breach is located in the left embankment to the left of the main overflow spillway and powerhouse. The failure was assumed to extend down to the deepest portion of the Orono Lake bed just upstream of the dam. Following failure, the upstream water level would draw down very quickly, and continued enlargement of the breach would be unlikely and inconsequential if it did occur. The breach plot from the HEC-RAS model is attached as Appendix C. Consequences of Failure Tables 2 and 3 summarize the consequences of failure during the zero freeboard flood at the Orono Dam and 10-year flood on the Mississippi River (Table 2) and the normal flow case (Table 3). In the case of the flood failure the incremental consequences of failure, relative to the pre-existing flood, become negligible as the flood wave moves towards the Mississippi River. The only significantly impacted properties for the flood failure case are along the Elk River within the first quarter mile downstream of the dam. In the case of a failure at normal flow, the dambreak flood wave is higher relative to the pre- existing profile but is for the most part contained within the river banks. For this case the computed wave heights and travel times are listed to assist in warning recreationists or persons working on or near the river. Inundation maps of the flood failure and normal failure flow are attached as Appendix D. These maps also show the cross section locations referenced in Tables 2 and 3. File:k:\water resource eng\elk river mn\2015 dfa&map\report\report 0320.docx 4 184