Level II scour analysis for Bridge 22 (WODSTH00180022) on Town Highway 18, crossing north Bridgewater Brook, Woodstock, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WODSTH00180022 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 Green Mountain physiographic province of central Vermont. The 4.34-mi²
drainage area is in a predominantly rural and forested basin. In the vicinity of the study site,
the left bank is forested and the right bank is covered by shrub and brush and is adjacent to
woods. The right bank of North Bridgewater Brook is parallel to town highway 18
upstream of the bridge.
In the study area, North Bridgewater Brook has a sinuous channel with a slope of
approximately 0.038 ft/ft, an average channel top width of 48 ft and an average channel
depth of 7 ft. The predominant channel bed materials are gravel and cobbles (D₅₀ is 63.2
mm or 0.207 ft). Due to cut banks and other channel features the geomorphic assessment at
the time of the Level I and Level II site visit on August 16, 1994, indicated that the reach
was laterally unstable. In addition, the stream approach to the bridge has a high gradient.
However, near the upstream face of the bridge the gradient shallows. Approximately 100
feet downstream of the bridge the gradient steepens again.
The town highway 18 crossing of North Bridgewater Brook is a 40-ft-long, two-lane bridge
consisting of one 35-foot concrete span (Vermont Agency of Transportation, written
commun., August 2, 1994). The bridge is supported by vertical, concrete abutments with
wingwalls. The left and right abutments are not protected by 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.