Level II scour analysis for Bridge 32 (BRIDTH00050032) on Town Highway 005, crossing North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00050032 on town highway 5 crossing the North Branch Ottauquechee River,
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 4.37-mi²
drainage area is in a predominantly rural and forested basin. In the vicinity of the study site,
the US left bank is lawn; DS left bank is pasture; US right bank is shrub and brushland and
the DS right bank is forested.
In the study area, the North Branch Ottauquechee River has an incised channel with a slope
of approximately 0.0148 ft/ft, an average channel top width of 25 ft and an average channel
depth of 9 ft. The predominant channel bed materials are gravel and cobbles (D₅₀ is 79.6
mm or 0.261 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on November 4, 1994, indicated that the reach was stable.
The town highway 5 crossing of the North Branch Ottauquechee Riveris a 27-ft-long, onelane bridge consisting of one 25-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 DS left road approach is protected by type-1 stone
fill (less than 12 inches). The DS right road approach is not protected and is eroded slightly
by road wash. The US left bank is protected by type-2 stone fill (less than 36 inches). A
stream confluence is located approximately 130 US of the bridge. The channel is skewed
approximately 30 degrees to the opening. 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.