Level II scour analysis for Bridge 43 (BRIDTH00040043) on Town Highway 004, crossing Dailey Hollow Branch, Bridgewater, Vermont

Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure BRIDTH00040043 on town highway 4 crossing Dailey Hollow Branch, Bridgewater, 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 was 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. The 9.80-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the left and right banks are forested. Upstream of bridge 43, Town Highway 4 runs parallel to the left bank and DS of the bridge, parallel to the right bank. In the study area, Dailey Hollow Branch has an incised channel with a slope of approximately 0.02 ft/ft, an average channel top width of 63 ft and an average channel depth of 5 ft. The predominant channel bed materials are gravel and cobble (D 50 is 77.0 mm or 0.253 ft). The geomorphic assessment at the time of the Level I and Level II site visit on November 1, 1994, indicated that the reach was stable. The town highway 4 crossing of Dailey Hollow Branch is a 38-ft-long, one-lane bridge consisting of one 34-foot steel beam span with a timber deck (Vermont Agency of Transportation, written commun., August 25, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 34 degrees to the opening while the opening-skew-to-roadway is 30 degrees. The footings of both abutments are exposed and the left abutment has experienced some undermining. Abutments are not protected by stone fill. However, upstream and downstream wingwalls are protected by type-2 stone fill (less than 36 inches). 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.