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.