Level II scour analysis for Bridge 40 (BETHTH00230040) on Town Highway 23, crossing Gilead Brook, Bethel, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BETHTH00230040 on town highway 23 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, gleaned from
VTAOT files, was compiled prior to conducting the 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 10.2-mi² drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the banks have moderately dense woody vegetation coverage.
In the study area, the Gilead Brook has an incised, sinuous channel downstream of the site
and a meandering channel upstream, with narrow flood plains and a slope of approximately
0.015 ft/ft, an average channel top width of 47.0 ft and an average channel depth of 2.75 ft.
The predominant channel bed materials are gravel and cobble (D₅₀ is 94.8 mm or 0.311 ft).
The geomorphic assessment at the time of the Level I and Level II site visit on October 14,
1994, indicated that the reach was laterally unstable.
The town highway 23 crossing of Gilead Brook is a 37-ft-long, one-lane bridge consisting
of one 34-foot span steel-stringer type superstructure (Vermont Agency of Transportation,
written commun., August 24, 1994). The bridge is supported by vertical, concrete
abutments with concrete wingwalls. The channel is skewed 25 degrees to the opening and
the opening-skew-to-roadway is zero degrees.
A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the downstream
right wingwall during the Level I assessment. The scour protection measures at the site
were type-2 stone fill (less than 36 inches diameter) on the upstream and downstream right
roadway embankments, at the extreme upstream and downstream ends of the upstream and
downstream right wingwalls, and along the entire base length of the downstream left
wingwall. 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.