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• 1971 Orono Lake map from the State of Minnesota Department of Conservation. • Sherburne County FIS dated November 16, 2011. • 1980 CH2M Hill Elk River Dam Rehabilitation plan set. Three bridges (USH 10 over Orono Lake, Parrish Avenue NE over the Mississippi River, and Highway 101 over the Mississippi River)were included in the Minnesota DNR's Elk River HEC- RAS model and the Mississippi River HEC-2 model, and are incorporated in the new HEC-RAS model. A fourth bridge, the Main Street NW bridge, crosses the Elk River immediately below the dam. Its low beam elevation is approximately the same as the overflow spillway crest elevation. Preliminary flood simulations showed that the highest flow modeled (the overtopping flood plus the breach flow)would pass below the low beam of the Main Street bridge. Therefore, this bridge was omitted from the model to improve model stability and performance. In terms of estimating the downstream impacts of failure, this is a conservative approach because the Main Street bridge, if it did impede flow at the downstream face of the dam, would tend to store and submerge the breach outflow. The three modeled bridges also proved to be unaffected by the dam overtopping flow on the Elk River and 10-year flood level of the Mississippi River. The model extends from a point on the Elk River 2.6 miles upstream of the dam to a point on the Mississippi River 6.3 miles downstream of the dam. The upstream boundary was chosen because it represents the upstream limit of impoundment by Orono Dam. The dam is 1.0 mile upstream of the confluence with the Mississippi River. The Mississippi River portion of the model extends 2.8 miles upstream and 5.3 miles downstream of the confluence with the Elk River. The 2.8-mile Mississippi River reach upstream of the confluence was included because a sudden flood wave entering the junction from the Elk River would affect stages and flows on the Mississippi both upstream and downstream of the confluence. The downstream boundary condition on the Mississippi River was a normal depth condition, with the friction slope being taken from water surface profiles shown in the Sherburne County Flood Insurance Study. Hydraulic Model Assumptions Pre-Failure River Flows. For the normal-flow failure, the assumed Elk River discharge was 300 cfs and was selected based on a review of flow data for USGS gage 05275000 upstream of the reservoir. The Mississippi River was assumed to have a base flow of 6,000 cfs, based on US Army Corps of Engineers data for USGS gage 05288500 downstream of the confluence of the two rivers. The zero freeboard flood flow of 16,600 cfs for the Orono Dam was taken from the 1980 CH2M Hill Elk River Dam Rehabilitation plan set. This zero freeboard flow assumes that the fuse plug on the left side of the dam is not utilized. The Mississippi River was assumed to have a 10-year flow at the time of the failure. The 10-year flow of the Mississippi River provided in the 2011 FIS for"Above the confluence of the Elk River" is 36,400 cfs. There is a reasonable likelihood that the Mississippi River would be experiencing a flood larger than the 10-year event during the zero-freeboard flood at the Orono Dam. However, the larger the pre-failure flood is on the Mississippi, the less consequential the additional flow from a dam File:k:\water resource eng\elk river mn\2015 dfa&map\report\report 0320.docx 3 183