Level II scour analysis for Bridge 35 (BRNATH00680035) on Town Highway 68, crossing Locust Creek, Barnard, Vermont

Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure BRNATH00680035 on town highway 68 crossing Locust Creek, Barnard, 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). A Level I study is included in Appendix E of this report. A Level I study provides a qualitative geomorphic characterization of the study site. Information on the bridge available from VTAOT files was compiled prior to conducting Level I and Level II analyses and can be found in Appendix D. The site is in the Green Mountain physiographic province of central Vermont in the town of Barnard. The 24.1-mi 2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the banks have woody vegetation coverage. In the study area, Locust Creek has an incised sinuous channel with a slope of approximately 0.0133 ft/ft, an average channel top width of 58 ft and an average channel depth of 5 ft. The predominant channel bed material is cobble (D 50 is 135 mm or 0.443 ft). The geomorphic assessment at the time of the Level I and Level II site visits on September 21 and 27, respectively, with a check on December 15, 1994, indicated that the reach was stable. The town highway 68 crossing of Locust Creek is a 30-ft-long, one-lane bridge consisting of one 28-foot concrete slab type superstructure (Vermont Agency of Transportation, written commun., August 23, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is not skewed to the opening and the opening-skew-to-roadway is zero degrees. A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the right abutment and downstream right wingwall during the Level I assessment. The only scour protection measure in place at the site was type-2 stone fill (less than 36 inches diameter) at the left abutment and wingwalls except the downstream right wingwall. Additional details describing conditions at the site are included in the Level II Summary and Appendices D. Scour depths and rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1993). 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 3.4 ft. The worst-case contraction scour occurred at the incipient overtopping discharge, which was between the 100- and 500-year discharges. Abutment scour ranged from 11.5 to 25.7 ft. with the worst-case scenario occurring 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, 1993, p. 48). Many factors, including historical performance during flood events, the geomorphic assessment, scour protection measures, and the results of the hydraulic analyses, must be considered to properly assess the validity of abutment scour results. Therefore, scour depths adopted by VTAOT may differ from the computed values documented herein, based on the consideration of additional contributing factors and experienced engineering judgement. and E.