Level II scour analysis for Bridge 22 (CRAFTH00180022) on Town Highway 18, crossing Black River, Craftsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CRAFTH00180022 on town highway 18 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 41.6-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the banks and floodplains are pasture and have no
woody vegetation coverage.
In the study area, the Black River is not incised, has a meandering channel with a slope of
approximately 0.0004 ft/ft, an average channel top width of 66 ft, and an average channel
depth of 3 ft. The predominant channel bed material is sand (D₅₀ is 0.148 mm or 0.000487
ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 13,
1995, indicated that the reach was laterally unstable.
The town highway 18 crossing of the Black Riveris a 60-ft-long, one-lane bridge consisting
of one 55-foot clear-span riveted through-girder type structure with a timber deck (Vermont
Agency of Transportation, written commun., August 3, 1994). The bridge is supported by
concrete abutments with no wingwalls. The bridge skew is approximately 5 degrees and
there is no opening-skew-to-roadway.
A scour hole 3 ft deeper than the mean thalweg depth was observed 35 ft downstream of the
bridge during the Level I assessment. There is also approximately 2 ft of channel scour
upstream of the bridge. There is type-one (less than 12 in diameter) protection on both
abutments that may act, in part, as spill-through abutments; the material however is partially
made up of the remnants of older log-cribbing abutments, when the bridge length was
significantly less than the current structure. 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.