Level II scour analysis for Bridge 32 (BETHTH00380032) on Town Highway 038, crossing Camp Brook, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00380032 on town highway 38 crossing Camp 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 were 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 7.57-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, Camp Brook is an incised, mildly sinuous channel with a slope of
approximately 0.018 ft/ft, an average channel top width of 50 ft and an average channel
depth of 4 ft. The predominant channel bed material is gravel and cobble (D₅₀ is 66.4 mm or
0.218 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
September 29, 1994, indicated that the reach was stable.
The town highway 38 crossing of Camp Brook is a 32-ft-long, one-lane bridge consisting of
one 29-foot span steel beam with timber deck (Vermont Agency of Transportation, written
commun., August 23, 1994). The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 5 degrees to the opening while the
opening-skew-to-roadway is 0 degrees.
The scour protection measures at the site include type-1 stone fill (less than 12 inches) at
both of the US wingwalls, type-2 stone fill (less than 36 inches) at the US and DS right and
DS left road approaches. The US right bank is protected by an artificial levee with a mix of
stone fill. 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.