Level II scour analysis for Bridge 10 (NORWTH00120010) Town Highway 012 Bloody Brook, Norwich, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NORWTH00120010 on town highway 12 crossing Bloody Brook, Norwich, 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 the Level I and Level II
analyses and can be found in Appendix D.

The site is in the New England Upland physiographic province in east-central Vermont. The
8.98-mi²
drainage area is in a predominantly rural and forested basin. In the vicinity of the
study site, the left bank upstream and the left and right banks downstream are forested. The
immediate right bank upstream is covered by shrub and brush with pasture on the overbank.
Town Highway 12 runs along the valley of Bloody Brook; however, at structure
NORWTH00120010 the road crosses Bloody Brook at a 90-degree angle.

In the study area, Bloody Brook has a sinuous channel with a slope of approximately 0.014
ft/ft, an average channel top width of 41 ft and an average channel depth of 3 ft. The
predominant channel bed materials are gravel and cobble (D₅₀ is 51.0 mm or 0.167 ft). The
geomorphic assessment at the time of the Level I site visit on October 31, 1994, indicated
that the reach was unstable.

The town highway 12 crossing of Bloody Brook is a 34-ft-long, two-lane bridge consisting
of one 30-foot clear span (Vermont Agency of Transportation, written commun., July 29,
1994). The bridge is supported by vertical, concrete abutments with wingwalls. The right
abutment is protected by sparse type-2 stone fill (less than 24 inches diameter). The channel
is skewed 0 degrees to the opening and the opening-skew-to-roadway is 0 degrees.
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.