Level II scour analysis for Bridge 27 (BRIDTH00490027) on Town Highway 049, crossing Broad Brook, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00490027 on town highway 49 crossing Broad Brook, 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 in the town of
Bridgewater. The 13.9-mi² drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the left and right banks are pasture with moderate tree cover on
the immediate banks. Upstream of bridge 27, a gravel road runs parallel to the left bank.
In the study area, the Broad Brook has an incised channel with a slope of approximately
0.007 ft/ft, an average channel top width of 54 ft and an average channel depth of 4 ft. The
predominant channel bed materials are gravel and cobble with a D₅₀ (median diameter)
of
77.9 mm or 0.256 ft. The geomorphic assessment at the time of the Level I and Level II site
visit on November 9, 1994, indicated that the reach was stable.
The town highway 49 crossing of the Broad Brook is a 32-ft-long, one-lane bridge
consisting of one 31-ft steel-beam span (Vermont Agency of Transportation, written
commun., August 24, 1994). The bridge is supported by vertical, concrete abutments with
wingwalls. The left abutment is noted as settled due to previous undermining. Type-2 (less
than 3 ft diameter) stone fill protects the upstream left and right wingwalls, the downstream
right wingwall, the right abutment, the upstream right road embankment, and the
downstream left and right road embankments. Type-3 (less than 4 ft diameter) stone fill
protects the downstream left wingwall, but it’s condition was reported as slumping. The
channel is skewed approximately 10 degrees to the opening; the opening-skew-to-roadway
is also 10 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.