|Abstract:||This report provides the results of a detailed Level II analysis of scour potential at structure RANDTH00650035 on town highway 65 crossing the Second Branch White River, Randolph, 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 of central Vermont. The 47.2-mi2 drainage area is in a predominantly rural basin. In the vicinity of the study site, the surface cover is pasture except for the downstream left bank which is forested. There is some woody vegetation on the immediate channel banks upstream of the bridge.
In the study area, the Second Branch White River has a sinuous channel with alluvial boundaries and a slope of approximately 0.002 ft/ft, an average channel top width of 52 ft and an average channel depth of 7 ft. The predominant channel bed materials are sand and gravel with a median grain size (D50) of 1.37 mm (0.0045 ft). The geomorphic assessment at the time of the Level I site visits on August 11, 1994 and December 1, 1994, indicated that the reach was laterally unstable.
The town highway 65 crossing of the Second Branch White River is a 33-ft-long, one-lane bridge consisting of one 28-foot steel-beam span (Vermont Agency of Transportation, written communication, July 29, 1994). The bridge is supported by vertical, stone abutments with wingwalls. The channel is skewed approximately 25 degrees to the opening while the opening-skew-to-roadway is 15 degrees. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E.
Scour depths and 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.
Contraction scour for all modelled flows ranged from 0.0 to 2.4 ft. The worst-case contraction scour occurred at the incipient-overtopping discharge which was
5,870 cfs less than the 100-year discharge. Abutment scour at the left abutment ranged from 5.7 to 13.9 ft. with the worst-case occurring at the 500-year discharge. Abutment scour at the right abutment ranged from 9.2 to 11.3 ft. with the worst-case occurring at the incipient-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.
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.