Level II scour analysis for Bridge 23 (WODSTH00180023) on Town Highway 18, crossing North Bridgewater Brook, Woodstock, Vermont

Open-File Report 96-197

Prepared in cooperation with Vermont Agency of Transportation and Federal Highway Administration



This report provides the results of a detailed Level II analysis of scour potential at structure WODSTH00180023 on town highway 18 crossing North Bridgewater Brook, Woodstock, 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 New England Upland physiographic division of east-central Vermont. The 4.26-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the left and right banks are covered by moderate tree cover. In the study area, North Bridgewater Brook has a sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 38 ft and an average channel depth of 5 ft. The predominant channel bed materials are gravel and cobbles (D50 is 63.3 mm or 0.208 ft). The geomorphic assessment at the time of the Level I site visit on December 9, 1994 indicated that the reach was laterally unstable. Evidence of the instability included anabranching and extensive stone fill on channel bends. The town highway 18 crossing of North Bridgewater Brook is a 25-ft-long, one-lane bridge consisting of one 22-ft steel-beam span (Vermont Agency of Transportation, written commun., August 3, 1994). The bridge is supported by vertical, concrete abutments with no wingwalls. Type-2 stone fill (less than 36 inches) was noted at the ends of the right abutment and type-1 stone fill (less than 12 inches) was noted at the ends of the left abutment. A stone wall of type-2 and -3 stone fill (less than 36 inches and 48 inches, respectively), carefully placed, protects the upstream right channel bank extending from the bridge to more than 50 feet upstream. Although significant protection has been placed, both abutments are experiencing undermining. The channel is skewed approximately 15 degrees to the opening while the opening-skew-to-roadway is 5 degrees. 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.

Study Area

Additional publication details

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USGS Numbered Series
Level II scour analysis for Bridge 23 (WODSTH00180023) on Town Highway 18, crossing North Bridgewater Brook, Woodstock, Vermont
Series title:
Open-File Report
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U.S. Geological Survey
iv, 31 p.
United States
Other Geospatial:
North Bridgewater Brook