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

Open-File Report 96-155

Prepared in cooperation with Vermont Agency of Transportation and Federal Highway Administration



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.

Study Area

Additional publication details

Publication type:
Publication Subtype:
USGS Numbered Series
Level II scour analysis for Bridge 26 (CRAFTH00250026) on Town Highway 25, crossing Black River, Craftsbury, Vermont
Series title:
Open-File Report
Series number:
Year Published:
U.S. Geological Survey
Publisher location:
Pembroke, NH
iv, 31 p.
United States
Other Geospatial:
Black River