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No realignment is proposed at this site. The straight horizontal alignment will remain the same. <br /> The vertical profiles will be raised approximately 0.2 feet at the bridge. <br /> Based on flood history, ice and floating debris are not concerns at this site. <br /> Sizing the structure consisted of determining the smallest opening that would match the <br /> upstream flood elevation while not encroaching excessively into the river. Risk to the stru cture <br /> and upstream property was the basis for this determination. <br /> ALTERNATIVES CONSIDERED <br /> The primary structure types considered at this site were single-span concrete flat slab bridges <br /> on vertical pile-encased (A5) abutm ents. Some of the sizes studied and the respective costs <br /> are shown in the attached Table 2. <br /> Prestressed concrete box beam bridges were removed from consideration because they <br /> historically have had greater initial construction and future maintenance costs associated with <br /> them, for the span lengths required , when compared to the other structure ty pes studied. In <br /> addition, vertical clearance for false work is not an issue at this site. <br /> Box culvert structures were removed from consideration due to the relatively high flow rates and <br /> river geometry associated with this site. <br /> Single-span arch structures (CONT ECH or CONSPAN)with natural stream bottoms and <br /> aluminum box culverts were removed from consideration because they were not cost <br /> competitive with other culvert options. <br /> Prestressed concrete girder and steel girder alternatives w ere removed from consideration <br /> because they lack the minimum required 2 feet of clearance from the proposed structure's low <br /> chord to the regional flood water surface. WisDOT BOS' current policy is to use a concrete flat <br /> slab bridge, prestressed concrete box beam bridge, or culvert where this 2 feet of freeboard <br /> cannot be met. If the proposed gradeline were raised to accommodate the minimum flooding <br /> clearance, both the structure length and associated roadway cost would increase substantially. <br /> Timber structures were removed from consideration because they historically have had greater <br /> initial construction and future maintenance costs associated with them, for the span lengths <br /> required,when compared to the other structure ty pes studied. <br /> The proposed structure matches the upstream flood stage elevation, reduces the velocity at the <br /> bridge, geometrically fits the site, is economical, and should have minimal environmental <br /> impacts. <br /> 4 u:\41-0668.00-dane county,town bristol,elder lane\reports\bsr\b-13-672_siz.doc <br /> 41-0668.00 <br />