Robert E. Hammond Lora K. Striker 1997 <p>This report provides the results of a detailed Level II analysis of scour potential at structure ANDOVT00110038 on State Route 11 crossing the Middle Branch Williams River, Andover, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including a quantitative analysis of stream stability and scour (U.S. Department of Transportation, 1993). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D.</p> <br/> <p>The site is in the Green Mountain section of the New England physiographic province in south central Vermont. The 5.65-mi² drainage area is in a predominantly rural and forested basin. Upstream and downstream of the study site banks and overbanks are forested.</p> <br/> <p>In the study area, the Middle Branch Williams River has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 44 ft and an average bank height of 4 ft. The channel bed material ranges from gravel to boulders with a median grain size (D₅₀) of 54.0 mm (0.177 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 5, 1996, indicated that the reach was stable.</p> <br/> <p>The State Route 11 crossing of the Middle Branch Williams River is a 33-ft-long, two-lane bridge consisting of one 31-foot concrete T-beam span (Vermont Agency of Transportation, written communication, March 29, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 55 degrees to the opening while the measured opening-skew-to-roadway is 45 degrees. </p> <br/> <p>There were no scour problems observed during the Level I assessment. Type-4 stone fill (less than 60 inches diameter) and type-3 stone fill (less than 48 inches diameter) was present on the left bank upstream and right bank upstream respectively. Type-2 stone fill (less than 36 inches diameter) was present in the upstream left wing wall area. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E.</p> <br/> <p>Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows.</p> <br/> <p>Contraction scour for all modelled flows ranged from 1.8 to 3.4 ft. The worst-case contraction scour occurred at the 500-year flow. Abutment scour ranged from 12.0 to 14.0 ft. The worst-case abutment scour occurred at the 500-year flow at the right abutment. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. </p> <br/> <p>It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.</p> application/pdf 10.3133/ofr97368 en U.S. Geological Survey Level II scour analysis for Bridge 38 (ANDOVT00110038) on State Route 11, crossing the Middle Branch Williams River, Andover, Vermont reports