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Table 1 Flood Frequency Calculations for the Orono Dam Return Qgage from Drainage area exponent Qdam by drainage HEC-SSP (cfs) area transfer(cfs) Qdam from period (Minnesota equations - Drainage area Drainage area StreamStats (years) SIR 2009-5250) = 605 mil 2 1,684 0.785 1,790 1,820 5 2,835 0.759 3,010 2,910 10 3,661 0.753 3,890 3,910 25 4,700 0.740 4,980 5,100 50 5,585 0.726 5,920 5,870 100 6,432 0.718 6,810 6,850 500 8,454 0.700 8,940 9,060 The two sets of estimates are very close to one another. Using the higher figures from the StreamStats procedure, the 100-year flood of 6,850 cfs would be passed at a reservoir elevation of approximately 875 feet with all gates open. The 500-year flood of 9,060 cfs could be passed at a reservoir elevation of approximately 876 feet. The minimum dike crest elevation is 879 feet; all elevations are in NAVD88. The spillway rating curve was scaled from the CH2M Hill rehabilitation plans (Appendix A). The same drawings show flood frequency estimates which are higher than those shown in Table 1, presumably because they lack the 33 additional years of gage record now available. Hydraulics Hydraulic Model Development The Corps of Engineers HEC-RAS model, Version 4.1.0, was used to simulate the normal-flow and overtopping flood failures of the Orono Dam. Data sources used in the HEC-RAS model development included the following: • HEC-2 model SHERMUL1.DAT of the Mississippi River, provided by the Minnesota DNR. • FEMA National Flood Hazard Layer cross sections, used to adjust the cross section distances specified in the HEC-2 Model. • HEC-RAS model of the Elk River from a point in Orono Lake just downstream of US Highway 10 to the upstream extent of the model, provided by the Minnesota DNR. • Lidar topography downloaded from the Minnesota DNR's website. File:k:\water resource eng\elk river mn\2015 dfa&map\report\report 0320.docx 2 182