5N, Approve a Contract with Kimley Horn, Inc., for Old Highway 10 Bridge Design

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Mr. Mall Woodruff, P.E. October IS, 20l0 Page 3 of 4 structure will continue to deteriorate, with the core of the deterioration occurring at the deck spall under the southbound lanes_ Based upon normal deterioration levels, we anticipate this structure will be considered Structurally Deficient in the next few years. The existing bridge also does not provide the necessary width to ailow two-way vehicular traffic and the desired shared-use (bicycle and pedestrian) path along Oid Snelling Avenue. Either a separate structure wouid be required for the path, or an unsafe condition constricting the vehicles and pedestrians onto the existing width that would not meet Mn/DOT State Aid standards. Structure Recommendation Based upon the condition af the existing bridge, the narrow deck width, and the desire to add a trail along this alignment, we recommend the City consider repiacing this bridge as part of the Old Snelling Avenue improvements. A replacement structure would accommodate the loading capacity of a two-lane roadway and a multi-use traii and would be constructed to Mn/DOT State Aid Standards. The structure lype will likely be a culvert. This type of str�cture would allow the roadway and trail to span over the drainage channel and a culvert is the most cost effective structure type. Culverts are also a very low maintenance structure when constructed correctly. In review of the existing hydraulics, we assume that we will need to �rovide a similar waterway opening as the existing bridge so that base flood elevations are not significantly affected. Based upon the Flood Insurance Study, the Josephine Lake Discharge Channel has a 100-year (i% annual chance) flood elevation of 880.1 immediately upstream of the Old Sneiling Bridge, and 879.9 immediately downstream. The low chord on the existing bridge is 886.12, approximately 6- feet above the flood elevation. Based upon the survey of the existing channel, the waterway opening is approximately 20-feet long and 2 feet high. We anticipate two lOx4 box culverts will provide tt�e required waterway opening, and allow for a 1-foot freeboard to the low-ct�ord. The box culverts are estimated to have a total length of 90-feet, to provide the roadway and trail section across the channel with the anticipated recovery zones. During preliminary and final design of the structure, a detailed hydraulic analysis will need to be conducted. The culvert sizing anticipated in this report would be finalized during this phase of the project. We have conservatively assumed that the waterway area provided by the culvert would need to closely resemble what is provided by the existing bridge. This will provide a conservative cost estimate for the str�cture. Since the culvert construction would have impacts on the surrounding wetlands, floodplain, and floodway, several permits would be required. These permits include: