Level II scour analysis for Bridge 2 (CRAFTH00590002) on Town Highway 59, crossing Black River, Craftsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CRAFTH00590002 on town highway 59 crossing the Black River, Craftsbury, 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 New England Upland physiographic province of north-central Vermont in
the town of Craftsbury. The 35.5-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the banks have no woody vegetation coverage except
for the upstream right bank, which has dense brush cover.
In the study area, the Black River is not incised, has a sinuous channel with a slope of
approximately 0.0004 ft/ft, an average channel top width of 49 ft, and an average channel
depth of 5 ft. The predominant channel bed material is sand (D₅₀ is 0.371 mm or 0.00122
ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 8,
1995, indicated that the reach was laterally unstable.
The town highway 59 crossing of the Black Riveris a 32-ft-long, one-lane bridge consisting
of one 29-foot clear-span timber and steel beam type structure with a timber deck (Vermont
Agency of Transportation, written commun., August 3, 1994). The bridge is supported by
log-crib abutments with no wingwalls. The channel is not skewed and the opening-skew-toroadway is 5 degrees.
A scour hole 8 ft deeper than the mean thalweg depth was observed 50 ft downstream of the
bridge during the Level I assessment. There is little to no protection at the site. 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.