This report provides the results of a detailed Level II analysis of scour potential at structure RIPTTH00180017 on Town Highway 18 crossing the South Branch Middlebury River, Ripton, 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 west-central Vermont. The 15.5-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest except on the upstream left bank where it is shrubs and brush.

In the study area, the South Branch Middlebury River has an incised, sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 86 ft and an average bank height of 10 ft. The channel bed material ranges from gravel to boulders with a median grain size (D₅₀) of 111 mm (0.364 ft). In addition, there is a bedrock outcrop across the channel downstream of the bridge. The geomorphic assessment at the time of the Level I and Level II site visit on June 10, 1996, indicated that the reach was stable.

The Town Highway 18 crossing of the South Branch Middlebury River is a 61-ft-long, one-lane bridge consisting of one 58-foot steel-beam span (Vermont Agency of Transportation, written communication, November 30, 1995). The opening length of the structure parallel to the bridge face is 56.8 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 40 degrees to the opening while the computed opening-skew-to-roadway is 30.

A scour hole 1.25 ft deeper than the mean thalweg depth was observed along the right abutment and the downstream right wingwall during the Level I assessment. The scour protection measures at the site include type-2 stone fill (less than 36 inches diameter) along the left abutment and it’s wingwalls and at the upstream end of the right abutment. Also, type-3 stone fill (less than 48 inches diameter) is along the upstream right wingwall. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E.

Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995) for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping discharge is determined and analyzed as another potential worst-case scour scenario. 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 0.1 to 1.1 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.6 to 9.0 ft. The worst-case abutment scour 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, 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.