Scott A. Olson 1997 This report provides the results of a detailed Level II analysis of scour potential at structure ALBUTH00150006 on Town Highway 15 crossing Mud Creek, Alburg, 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. The site is in the Champlain section of the St. Lawrence Valley physiographic province in northwestern Vermont. The 2.90-mi² drainage area is in a predominantly rural and forested basin. However, nearly a third of the drainage, including the location of the study site, is wetland. In the study area, Mud Creek has an sinuous channel through wetland with a slope of approximately 0.0002 ft/ft, an average channel top width of 42 ft and an average bank height of 2 ft. The channel bed material ranges from clay to sand with an estimated median grain size (D₅₀) of 0.047 mm (0.00015 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 26, 1995, indicated that the reach was stable. The Town Highway 15 crossing of Mud Creek is a 30-ft-long, one-lane bridge consisting of one 28-foot steel-beam span (Vermont Agency of Transportation, written communication, March 7, 1995). The opening length of the structure parallel to the bridge face is 26.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed zero degrees to the opening and the opening-skew-to-roadway is also zero degrees. Channel scour, approximately 2 ft deeper than the mean thalweg depth, was observed in the middle of the channel extending from 5 to 35 ft upstream of the bridge. The only scour countermeasure observed at this site was some small stone, possibly type-1 stone fill (less than 12 inches diameter), partially covering the channel bed under the bridge. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. 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. Contraction scour for all modelled flows ranged from 6.2 to 7.2 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 2.0 to 2.4 ft and 2.1 to 2.6 ft on the left and right abutments respectively. The worst-case abutment scour occurred at the 500-year discharge. 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 crosssection 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. 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. application/pdf 10.3133/ofr97397 en U.S. Geological Survey Level II scour analysis for Bridge 6 (ALBUTH00150006) on Town Highway 15, crossing Mud Creek, Alburg, Vermont reports