This report provides the results of a detailed Level II analysis of scour potential at structure WALDTH00060023 on Town Highway 6 crossing Stannard Brook, Walden, 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 eastern Vermont. The 5.61-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the upstream surface cover is shrub and brushland with some trees. The downstream surface cover is forest.

In the study area, Stannard Brook has an incised, straight channel with a slope of approximately 0.02 ft/ft, an average channel top width of 54 ft and an average bank height of 9 ft. The channel bed material ranges from gravel to boulder with a median grain size (D₅₀) of 64.0 mm (0.210 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 8, 1995, indicated that the reach was stable.

The Town Highway 6 crossing of Stannard Brook is a 59-ft-long (bottom width), two-lane pipe arch culvert consisting of one 22-foot corrugated plate pipe arch span (Vermont Agency of Transportation, written communication, March 28, 1995). The opening length of the structure parallel to the bridge face is 21.9 ft.The pipe arch is supported by vertical, concrete kneewalls. 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 upstream end of the right kneewall during the Level I assessment. There was also a scour hole 0.5 ft deeper than the mean thalweg depth observed along the downstream end of the left kneewall. The scour counter measures at the site included type-3 stone fill (less than 48 inches diameter) at the upstream and downstream end of the left and right kneewall. There was also type-2 stone fill (less than 36 inches diameter) along the upstream 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. 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 kneewalls). 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 2.3 ft. The worst-case contraction scour occurred at the incipient roadway-overtopping discharge, which was greater than the 100-year discharge. Left kneewall scour ranged from 11.7 to 16.8 ft. The worst-case left kneewall scour occurred at the 500-year discharge. Right kneewall scour ranged from 13.7 to 16.7 ft. The worst-case right kneewall scour occurred at the incipient roadway-overtopping 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. During the Level I survey ledge was discovered at the upstream end of the right abutment. The ledge in the channel may limit scour depths.

It is generally accepted that the Froehlich equation (abutment/ kneewall 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.