This report provides the results of a detailed Level II analysis of scour potential at structure WWINTH00370034 on Town Highway 37 crossing Mill Brook, West Windsor, 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 (FHWA, 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 16.6-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture except for the upstream left bank where there is mostly shrubs and brush.

In the study area, Mill Brook has a sinuous channel with a slope of approximately 0.003 ft/ ft, an average channel top width of 52 ft and an average bank height of 5 ft. The channel bed material ranges from sand to cobbles with a median grain size (D₅₀) of 43.4 mm (0.142 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 5, 1996, indicated that the reach was laterally unstable. Point bars were observed upstream and downstream of this site. Furthermore, slip failure of the bank material was noted downstream at a cut-bank on the left side of the channel across from a point bar.

The Town Highway 37 crossing of Mill Brook is a 37-ft-long, one-lane covered bridge consisting of one 32-foot wood thru-truss span (Vermont Agency of Transportation, written communication, March 23, 1995). The opening length of the structure parallel to the bridge face is 29.6 ft. The bridge is supported by vertical, laid-up stone abutment walls with concrete facing and laid-up stone wingwalls. The channel is skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is zero degrees. 

A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the right abutment during the Level I assessment. Scour protection measures at the site included type-3 (less than 48 inches diameter) and type-4 (less than 60 inches diameter) stone fill. Type-3 stone fill was observed along the upstream right bank and along the right abutments. Type-4 stone fill was observed at the upstream end of 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 Davis, 1995) for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping discharge was 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.

There was no contraction scour predicted for any of the modeled flows. Abutment scour at the left abutment ranged from 5.7 to 7.3 ft, while that at the right abutment ranged from 11.6 to 17.7 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 Davis, 1995, p. 46). 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.