This report provides the results of a detailed Level II analysis of scour potential at structure RANDTH00BR0054 on Brook Street crossing Thayer Brook, Randolph, 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 Green Mountain physiographic division of central Vermont in the town of Randolph. The 5.39-mi² drainage area is in a predominantly rural basin. In the vicinity of the study site, the immediate banks are forested.

In the study area, Thayer Brook has an incised, sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 60 ft and an average channel depth of 3 ft. The predominant channel bed materials are gravel and cobble (D₅₀ is 42.4 mm or 0.139 ft). The geomorphic assessment at the time of the Level I and Level II site visits on August 3, 1994 and December 5, 1994, indicated that the reach was vertically and laterally unstable. This assessment was due to the extreme channel misalignment with the bridge opening and the presence of a drop structure downstream of the bridge protecting against channel degradation.

The Brook Street crossing of Thayer Brook is a 34-ft-long, two-lane bridge consisting of one 31-foot concrete span (Vermont Agency of Transportation, written communication, August 2, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. Streamflow attacks the upstream right wingwall and has undermined the upstream end of the right abutment. Type-2 stone fill (less than 36 inches diameter) exists only on the upstream and downstream sides of the left road embankment. No other protection was noted. The bank full channel skew at the bridge face is approximately 20 degrees; the opening-skew-to-roadway is also 20 degrees. 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). Total scour at a highway crossing is comprised of three components: 1) long-term streambed degradation; 2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge) and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is the sum of the three components. Equations are available to compute depths for contraction and local scour and a summary of the results of these computations follows.

Contraction scour for all modelled flows ranged from 1.3 to 2.7 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.3 to 15.1 ft. and the worst-case abutment scour also occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution.

It is generally accepted that the Froehlich equation (abutment scour) gives “excessively conservative estimates of scour depths” (Richardson and others, 1993, p. 48). Many factors, including historical performance during flood events, the geomorphic assessment, scour protection measures, and the results of the hydraulic analyses, must be considered to properly assess the validity of abutment scour results. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein, based on the consideration of additional contributing factors and experienced engineering judgement.