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<br />North St. Vrain & St. Vrain Creeks <br />Floodplain Update <br /> <br />Town of Lyons <br /> <br />channel constructions at bridges and culverts, thus creating a damming effect. This damming <br />effect may cause overbank flow, which in turn may erode bridge abutments and approaches. <br />Where debris had collected at channel constrictions, increasing flood flows may cause a mass of <br />debris to break loose, causing a wall of water and debris to surge downstream until another <br />obstruction is encountered. In some instances, debris may collect to the point where structural <br />capacity of a bridge is exceeded, causing the bridge to fail. <br /> <br />Bridge and culvert debris blockage remains consistent in this re-analysis with the 1987 FIR. <br />Bridges with center piers and multiple culvert installations have a 25% debris blockage of the <br />opening area. No blockage was assumed for bridges without piers or for single culvert <br />installations. To model the blockage, the low chord of the culvert or bridge was lowered to <br />reduce the opening area. The culvert information obtained by the U.S. army Corps of Engineers <br />for the 1987 FIR was used where appropriate. <br /> <br />As in the previous two studies, several culverts/bridges were assumed to be washed out during <br />the IOO-year flood. This assumption was based on a visual inspection of the structural integrity <br />of the crossing by Boulder County, the Colorado Water Conservation Board, and the consultant <br />for the 1987 FIR. The crossings assumed to wash out are noted in Table 1, summary of <br />culvert/bridge data. <br /> <br />TABLE 1 <br />SUMMARY OF CUL VERT/BRIDGE INFORMATION <br /> <br /> Low Top of Bottom Pier Opening Side <br /> Invert Chord Road Width Width Area Slope <br />Location (MSL) (MSL) (MSL) (ft) (ft) (sq. ft.) (H:V) Blockage Notes <br />U.s. Highway 36 5253.5 5264.9 5267.4 65 4.0 822 2 25% <br />(1662+55) Assumed <br /> washed out <br />2nd Avenue 5308.5 5320.8 5323.0 113 1.2 1636 25% <br />(1729+10) <br />Footbridge Assumed <br />(1743+80) washed out <br />Highway 7 5329.0 5340.6 5342.0 60 552 <br />(1750+05) <br />U.s. Highway 36 5364.5 5372.3 5375.9 58 344 <br />(1786+30) <br />u.s. Highway 36 5387.5 5399.2 5404.7 59 690 <br />(1811 +23) <br />Access Bridge Assumed <br />(1828+50) washed out <br /> <br />Floodplain Information Report 8 <br />K:\356001004\FLOODPLAIN INFORMATION REPORT.doc <br /> <br />June 1998 <br />