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

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-mi²
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 (D₅₀ 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.