This report provides the results of a detailed Level II analysis of scour potential at structure WALLVT01030049 on State Highway 103 crossing Freeman Brook, Wallingford, 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). Results of a Level I scour investigation also are included in Appendix E of this report. A Level I investigation provides a qualitative geomorphic characterization of the study site. Information on the bridge, gleaned from Vermont Agency of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is found in Appendix D.

The site is in the Green Mountain section of the New England physiographic province in south-central Vermont. The 11.7-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture with trees and brush on the immediate banks except for the upstream left overbank which is tree covered. A levee composed of stone fill was constructed along the upstream left bank in order to keep flow from reaching the flood plain left (south) of the brook.

In the study area, Freeman Brook has an incised, straight channel with a slope of approximately 0.02 ft/ft, an average channel top width of 56 ft and an average channel depth of 6 ft. The predominant channel bed materials are gravel and cobbles with a median grain size (D₅₀) of 62.9 mm (0.206 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 10, 1995, indicated that the reach was stable.

The State Highway 103 crossing of the Freeman Brook is a 54-ft-long, two-lane bridge consisting of one 50-foot concrete T-beam span (Vermont Agency of Transportation, written communication, March 15, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 25 degrees to the opening while the opening-skew-to-roadway is zero degrees.

A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the downstream end of the left abutment and downstream left wingwall during the Level I assessment. The scour protection measures at the site included type-2 stone fill (less than 36 inches diameter) along the entire base length of the upstream left and downstream right wingwall and type-1 stone fill (less than 12 inches diameter) along the upstream end of the upstream right wingwall. Type-4 stone fill (less than 60 inches diameter) was found along the upstream left and right banks. 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, 1995). 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 0.0 to 1.4 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.6 to 21.4 ft. The worst-case abutment scour was predicted 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, 1995, p. 47). Usually, computed scour depths are evaluated in combination with other information including (but not limited to) historical performance during flood events, the geomorphic stability assessment, existing scour protection measures, and the results of the hydraulic analyses. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein.