4.3. ERMUSR 06-16-2015 - Laserfiche WebLink

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Summary The discharges, flooding depths, inundation area, and flood wave travel times associated with a failure of the Orono Dam were estimated using the unsteady-flow routines of the Corps of Engineers HEC-RAS software. A normal-flow failure and a failure during a flood equivalent to the zero freeboard flood (16,600 cubic feet per second (cfs))with a 10-year flood on the Mississippi River were considered. The zero freeboard flood failure produces a worst-case inundation scenario for residential, commercial, and public use areas in the city of Elk River and is documented in the Emergency Action Plan inundation maps. The normal-flow failure would cause little overbank flooding, but poses hazards to recreationists in and on the banks of the Elk River and Mississippi River. Description of Dam The Orono Dam consists of earthen embankments and an approximately 225-foot long, 15-foot high concrete dam structure on the Elk River in Sherburne County, Minnesota. Originally constructed in 1916 for hydroelectric power, the dam no longer produces power but provides recreation benefits. The upstream pool level is maintained by overflow weirs with crest elevations of 871.3 feet. Floods are passed over the dam through the 117-foot-wide "main overflow weir" on the left side of the dam, the 24-foot-wide "south overflow weir" on the right, and three Tainter gates between the overflow spillways. Fuse plugs are located on the left and right sides of the dam and construction drawings indicate they were originally designed to deploy at floods exceeding the 500 year flood. On the right side, riprap has been added in the area of the fuse plug and may inhibit its operation. A plan view of the dam, taken from the 1980 CH2M Hill Elk River Dam Rehabilitation plan set, is included in Appendix A. The Elk River joins the Mississippi River approximately one mile downstream of the dam. Flood Frequency An updated flood frequency curve for the Orono Dam was developed using two methods: a Log Pearson Type 3 (LP3) analysis and drainage area transfer from USGS Gage 05275000 (Elk River near Big Lake); and the Minnesota regional equations applied through the USGS StreamStats online software. The LP3 analysis of 80 annual peak flows at USGS Gage 05275000 was completed using the HEC-SSP software. The HEC-SSP summary table is attached as Appendix B. The drainage area at the gage site is 559 square miles, 92 percent of the area at Orono Dam. Drainage area exponents for each recurrence interval were calculated from the regional equations given in USGS Scientific Investigations Report 2009-5250, Techniques for Estimating the Magnitude and Frequency of Peak Flows on Small Streams in Minnesota Based on Data through Water Year 2005. The SSP results and transfer to the dam site are summarized in Table 1 below. Also shown in Table 1 are the StreamStats results for the dam site. 181