This report provides the results of a detailed Level II analysis of scour potential at structure CRAFTH00040004 on town highway 4 crossing Whitney Brook, Craftsbury, 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 secton of the New England physiographic province of north-central Vermont in the town of Craftsbury. The 13.3-mi² drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the banks have dense woody vegetation coverage.

In the study area, Whitney Brook has an incised, sinuous channel with a slope of approximately 0.014 ft/ft, an average channel top width of 40 ft and an average channel depth of 3 ft. The observed predominant channel bed material is cobble and boulder while results from the pebble count provided a D₅₀ of 78.5 mm or 0.258 ft. The geomorphic assessment at the time of the Level I and Level II site visit on June 7, 1995 indicated that the reach was stable.

The town highway 4 crossing of Whitney Brook is a 41-ft-long, one-lane bridge consisting of one 39-foot span steel-beam and concrete superstructure (Vermont Agency of Transportation, written commun., August 3, 1994). The bridge is supported by slightly sloping, mortared stone block abutments with wingwalls. The channel is skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is 20 degrees.

A scour hole 2 ft deeper than the mean thalweg depth was observed along the upstream right wingwall and right abutment during the Level I assessment. There were no scour protection measures evident at the site. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E.

Scour depths and rock riprap 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 degradation; 2) contraction scour (due to accelerated flow caused by 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 computed scour results follow.

Contraction scour for all modelled flows ranged from 0.7 to 1.7 feet. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 10.7 to 15.3 feet. The worst-case abutment scour also 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.