Level II scour analysis for Bridge 23 (WODSTH00180023) 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
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.