This report provides the results of a detailed Level II analysis of scour potential at structure BENNCYPARK0002 on the Park Street crossing of Furnace Brook, Bennington, 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 southwestern Vermont. The 12.8-mi² drainage area is a predominantly rural and forested basin. The bridge site is located within an urban setting in the Town of Bennington with buildings, homes, lawns, and pavement on the overbanks.

In the study area, Furnace Brook has a mildly sinuous channel located on a delta and has a slope of approximately 0.01 ft/ft, an average channel top width of 35 ft and an average bank height of 4 ft. The predominant channel bed materials are gravel and cobble with a median grain size (D₅₀) of 58.4 mm (0.192 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 6, 1996, indicated that the reach was unstable. However, in the immediate vicinity of the bridge the reach has been stabilized with bank protection. Upstream of the protection, there is bank cutting and channel scour.

The Park Street crossing of Furnace Brook is a 29-ft-long, two-lane bridge consisting of one 26-foot concrete span (Vermont Agency of Transportation, written communication, December 14, 1995). The width of the bridge opening parallel to the downstream bridge face is 25.3 feet. The bridge is supported by vertical, concrete abutments with no wingwalls. The upstream channel is skewed approximately 45 degrees to the opening while the opening-skew-to-roadway is 10 degrees.

Scour countermeasures at the site include type-2 stone fill (less than 36 inches diameter) on the right banks upstream and downstream of the bridge and type-3 stone fill (less than 48 inches diameter) on the upstream left 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). 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.

There was no contraction scour computed for any of the modelled flows. Computed left abutment scour ranged from 2.5 to 5.6 ft. with the worst-case scour occurring at the 500- year discharge. Computed right abutment scour ranged from 5.6 to 8.4 ft. with the worst- case scour also occurring at the 100-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.