This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00050034 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 division of central Vermont in the town of
Bridgewater. The 5.45-mi2 drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, left and right banks are forested. Upstream of bridge 34, Town
Highway 5 runs parallel to the right bank and DS of the bridge, parallel to the left bank.
In the study area, the North Branch Ottauquechee River has an incised channel with a slope
of approximately 0.015 ft/ft, an average channel top width of 32 ft and an average channel
depth of 7 ft. The predominant channel bed material is sand and gravel (D50 is 105 mm or
0.345 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
November 3, 1994, indicated that the reach was stable.
The town highway 31 crossing of Lilliesville Brook is a 41-ft-long, one-lane bridge consisting of one 39-foot steel-beam span with a timber deck (Vermont Agency of Transportation, written commun., August 24, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 35 degrees to the opening while the opening-skew-to-roadway is 0 degrees. Scour protection measures in place at the site were type-1 stone fill (less than 12 inches diameter) at the downstream left wingwall, left abutment, and upstream and downstream sides of the left road embankment. 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.
Additional publication details
USGS Numbered Series
Level II scour analysis for Bridge 34 (BRIDTH00050034) on Town Highway 005, crossing North Branch Ottauquechee River, Bridgewater, Vermont