Level II scour analysis for Bridge 92 (WSTOVT01000092) on State Highway 100, crossing the West River, Weston, Vermont

Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure WSTOVT01000092 on Vermont Highway 100 crossing the West River, Weston, 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 Green Mountain section of the New England physiographic province in south-central Vermont. The 32.7-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover upstream of the bridge is primarily forest with pasture on the upstream left overbank. Upstream and downstream, the immediate banks have brush and dense forest cover. Downstream of the bridge is forested. In the study area, the West River has an incised, sinuous channel with a slope of approximately 0.006 ft/ft, an average channel top width of 111 ft and an average channel depth of 3 ft. The predominant channel bed material is very coarse gravel and cobbles with a median grain size (D50) of 67.7 mm (0.222 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 19, 1996 indicated that the reach was laterally unstable based on the fine bank material, sinuosity of the stream, point bars and cutbanks. The state highway 100 crossing of the West River is a 113-ft-long, two-lane bridge consisting of one 110-foot steel-beam span (Vermont Agency of Transportation, written communication, March 31, 1995). The bridge is supported by vertical, concrete abutments without wingwalls. The channel is skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is 25 degrees. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) along the entire base length of the left and right abutments. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and 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 0.4 to 2.1 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.4 to 30.7 ft. The worst-case abutment scour occurred at the 500-year discharge along the left 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. 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.