This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00070038 on town highway 7 crossing Gilead Brook, Bethel, 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
Bethel. The 8.83-mi2 drainage area is predominantly rural and forested. In the vicinity of
the study site, the banks have dense woody vegetation coverage.
In the study area, Gilead Brook is an incised, straight channel with a slope of approximately
0.028 ft/ft, an average channel top width of 46 ft and an average channel depth of 3 ft. The
predominant channel bed material is gravel (D50 is 43.9 mm or 0.144 ft). The geomorphic
assessment at the time of the Level I and Level II site visit on October 18, 1994, indicated
that the reach was stable.
The town highway 5 crossing of the Black River is a 70-ft-long, two-lane bridge consisting of one 65-foot clear span (Vermont Agency of Transportation, written commun., August 2, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. There is also a retaining wall along the upstream side of the road embankments. The channel is skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is 15 degrees. A scour hole 3.0 ft deeper than the mean thalweg depth was observed along the right abutment. The scour hole was 27 feet long, 15 feet wide, and was 2.5 feet below the abutment footing at the time of the Level I assessment. This right abutment had numerous cracks and had settled. 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 38 (BETHTH00070038) on Town Highway 007, crossing Gilead Brook, Bethel, Vermont