This report provides the results of a detailed Level II analysis of scour potential at structure RYEGTH00020002 on Town Highway 2 crossing the Wells River, Ryegate, 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 New England Upland section of the New England physiographic province in east-central Vermont. The 75.7-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover consists of cut grass, trees, and brush on the flood plains while the immediate banks have dense woody vegetation.

In the study area, the Wells River has an incised, sinuous channel with a slope of approximately 0.006 ft/ft, an average channel top width of 110 ft and an average bank height of 12 ft. The channel bed material ranges from sand to boulder with a median grain size (D₅₀) of 82.3 mm (0.270 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 24, 1995, indicated that the reach was laterally unstable with moderate fluvial erosion and meandering downstream of the bridge.

The Town Highway 2 crossing of the Wells River is a 79-ft-long, two-lane bridge consisting of one 75-foot steel-beam span (Vermont Agency of Transportation, written communication, March 27, 1995). The opening length of the structure parallel to the bridge face is 75.1 ft. The bridge is supported by vertical, concrete abutments, the left has a spill-through embankment, with wingwalls. The channel is not skewed to the opening and the opening-skew-to-roadway is zero degrees.

A scour hole 3 ft deeper than the mean thalweg depth was observed in the channel from upstream and through the bridge during the Level I assessment. The scour protection counter-measures at the site included type-4 stone fill (less than 60 inches diameter) along the base of the left abutment forming a spill-through embankment. There was also type-2 stone fill (less than 36 inches diameter) along the entire base length of the upstream right wingwall, the upstream right bank and downstream left bank. There was a stone wall along the upstream left bank extending 130 ft from the bridge. In addition there was type-1 stone fill (less than 12 inches diameter) along the downstream right bank. 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. 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 was zero. Abutment scour ranged from 7.1 to 11.4 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.