Level II scour analysis for Bridge 26 (CRAFTH00250026) on Town Highway 25, crossing Black River, Craftsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CRAFTH00250026 on town highway 25 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 37.8-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the flood plains are pasture and the banks have no
woody vegetation coverage.
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 32 ft, and an average channel
depth of 4 ft. The predominant channel bed material is sand (D₅₀ is 0.802 mm or 0.00263
ft). The geomorphic assessment at the time of the Level I and Level II site visit on October
6, 1994, indicated that the reach was laterally unstable.
The town highway 25 crossing of the Black Riveris a 42-ft-long, one-lane bridge consisting
of one 40-foot clear-span steel stringer type structure with a timber deck (Vermont Agency
of Transportation, written commun., August 4, 1994). The bridge is supported by concrete capped, granite-block abutments with wingwalls (except for the downstream side of the left
abutment). The channel is skewed to the opening by approximately 20 degrees and there is
no opening-skew-to-roadway.
A scour hole 5 ft deeper than the mean thalweg depth was observed centered at 50 ft
upstream of the bridge during the Level I assessment. There is sparse type-2 stone fill along
the right abutment and right wingwalls. 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.