DCPFLO-2015-00010 - Laserfiche WebLink

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PROPOSED STRUCTURE AND APPROACHES The proposed structure is a 48-foot long single-span concrete flat slab bridge on 6-foot tall vertical pile-encased (A5) abutm ent. The wings will be angled back 45 degrees to provide protection to the approaches during highw ater events. The bridge will have a 30-foot clear roadway width with a Type "M" railing and will have a normal crown of 2%. The bridge is being built 6-feet wider than the typical based on Town preference to accommodate farm equipment. The structure will not be skewed. The bridge will be shifted 4 feet to the north to avoid filling-in the upstream channel near the NW corner of the bridge. At the regional flood discharge of 1,200 cfs, a low flow condition exists that results in a highwater elevation of 895.45. The waterway opening is approximately 307 square feet, resulting in a velocity of 3.9 feet per second through the opening. The proposed structure's navigational and design regional flood clearances are 7.5 feet and 0.4 feet, respectively at the north abutment. The clearances are 0.1 feet more at the south abutment. The proposed horizontal alignment is straight at the bridge. The proposed vertical profile consisting of a straight grade that will raise the profile of Elder Lane approximately 0.2 feet at the bridge. SCOUR To prevent scour at the bridge, heavy riprap will be used at the abutm ents and at all four wing locations. Risk to the structure should not be a concern since the 3.9 feet per second velocity is less than the 4.9 feet per second velocity of the existing structure and is within the acceptable limits for the type of streambed material found at this site. In addition, standard heavy riprap is effective up to a velocity of 10 feet per second. The Scour Critical Code was determined to be 8 for the proposed structure. SUMMARY Based on hydraulic results, environmental impacts, cost comparisons, site geometry, and various risk considerations, the 48-foot long single-span concrete flat slab bridge on 6-foot tall vertical pile-encased (A5) abutments is the most economical way to accomplish the goals of this bridge replacement. A shorter bridge would increase the upstream flood stage elevation and velocity to an unacceptable level. A longer bridge would increase the project cost without significantly improving hydraulic performance. 5 u:\41-0668.00-dane county,town bristol,elder lane\reports\bsr\b-13-672_siz.doc 41-0668.00