Level II scour analysis for Bridge 32 (BETHTH00380032) on Town Highway 038, crossing Camp Brook, Bethel, Vermont

Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure BETHTH00380032 on town highway 38 crossing Camp Brook, Bethel, 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). A Level I study is included in Appendix E of this report. A Level I study provides a qualitative geomorphic characterization of the study site. Information on the bridge available from VTAOT files were compiled prior to conducting Level I and Level II analyses and can be found in Appendix D. The site is in the Green Mountain physiographic province of central Vermont in the town of Bethel. The 7.57-mi 2 drainage area is predominantly rural and forested. In the vicinity of the study site, the banks have dense woody vegetation coverage. In the study area, Camp Brook is an incised, mildly sinuous channel with a slope of approximately 0.018 ft/ft, an average channel top width of 50 ft and an average channel depth of 4 ft. The predominant channel bed material is gravel and cobble (D 50 is 66.4 mm or 0.218 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 29, 1994, indicated that the reach was stable. The town highway 38 crossing of Camp Brook is a 32-ft-long, one-lane bridge consisting of one 29-foot span steel beam with timber deck (Vermont Agency of Transportation, written commun., August 23, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 5 degrees to the opening while the opening-skew-to-roadway is 0 degrees. The scour protection measures at the site include type-1 stone fill (less than 12 inches) at both of the US wingwalls, type-2 stone fill (less than 36 inches) at the US and DS right and DS left road approaches. The US right bank is protected by an artificial levee with a mix of stone fill. 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, 1993). Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution. The scour analysis results are presented in tables 1 and 2 and a graph of the scour depths is presented in figure 8.