Level II scour analysis for Bridge 46 (CHELTH00680046) on Town Highway 68, crossing the First Branch of the White River, Chelsea, Vermont

Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure CHELTH00680046 on town highway 68 crossing the First Branch of the White River, Chelsea, 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 of central Vermont in the town of Chelsea. The 58.2-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the banks have dense woody vegetation coverage. In the study area, the First Branch of the White River has a sinuous channel with a slope of approximately 0.0054 ft/ft, an average channel top width of 92 ft and an average channel depth of 4 ft. The predominant channel bed material is gravel and cobble (D 50 is 52.7 mm or 0.173 ft). The geomorphic assessment at the time of the Level I and Level II site visit on November 16, 1994, indicated that the reach was stable. The town highway 68 crossing of the First Branch of the White River is a 61-ft-long, onelane covered bridge with a 52-foot clear-span (Vermont Agency of Transportation, written commun., August 26, 1994). The bridge is supported by vertical, stone abutments with a concrete wingwall on the downstream right. The left abutment is laid-up stone supported by concrete at the upstream and downstream ends of the laid-up stone abutment. The channel is skewed approximately 40 degrees to the opening while the opening-skew-to-roadway is 15 degrees. A scour hole 1.5 ft deeper than the mean thalweg depth was observed under the bridge during the Level I assessment. The scour protection measures in place at the site were type- 2 stone fill (less than 36 inches diameter) at the road approach embankments except the downstream left embankment which had no protection. The upstream right road embankment, impacted by the channel bend, has an extensive covering of stone fill for erosion protection. Type-3 stone fill (less than 48 inches diameter) was noted along the right abutment. 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 0.9 to 2.6 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 14.3 to 24.0 ft. 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 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. The left abutment sits atop a bedrock outcrop. The results of the calculated scour depths will be limited by the bedrock. 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.