Level II scour analysis for Bridge 9 (BARRUSO3020009) on U.S. Route 302, crossing Jail Branch, Barre, Vermont
This report provides the results of a detailed Level II analysis of scour potential at structure BARRUS03020009 on U.S. Route 302 crossing Jail Branch, Barre, 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 central Vermont. The 42.8-mi 2 drainage area is in a predominantly rural and forested basin. A flood control reservoir with a usable capacity of 525 million cubic feet is located just upstream of the bridge. In the vicinity of the study site, the surface cover left of the channel consists of trees and brush. Right of the channel, the immediate bank is covered by trees and brush while the overbank is grass covered with several buildings. In the study area, Jail Branch has an incised, sinuous channel with a slope of approximately 0.008 ft/ft, an average channel top width of 86 ft and an average channel depth of 5 ft. The channel bed material ranged from gravel to boulder with a median grain size (D 50 ) of 73.5 mm (0.241 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 17, 1996, indicated that the reach was laterally unstable due to its sinuosity, cut banks, point bars, and extensive bank protection. The U.S. Route 302 crossing of Jail Branch is a 74-ft-long, two-lane bridge consisting of one 72-foot steel-beam span (Vermont Agency of Transportation, written communication, October 13, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 30 degrees to the opening while there is no opening- skew-to-roadway. There is evidence of channel scour along the right bank from 190 feet upstream of the bridge and extending through the bridge along the right abutment. Under the bridge, the scour depth is approximately 0.5 feet below the mean thalweg depth. Scour protection measures at the site include type-3 stone fill (less than 48 inches diameter) along the right bank extending from the bridge to 192 feet upstream. Type-2 stone fill (less than 36 inches diameter) is along the right abutment and the right downstream bank to 205 feet downtream of the bridge. 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 0.2 to 0.5 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.3 to 7.5 ft. The worst-case abutment scour occurred at the 500-year discharge. Computed scour for the 100-year event does not go below the abutment footings. 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.