Level II scour analysis for Bridge 5 (IRASTH00010005) on Town Highway 1, crossing Lords Creek, Irasburg, Vermont

Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure IRASTH00010005 on town highway 1 crossing Lords Creek, Irasburg, 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 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 New England Upland section of the New England physiographic province of north-central Vermont in the town of Irasburg. The 15.1-mi 2 drainage area is in a predominantly rural and forested basin with some pasture land mainly along the valley bottom. In the vicinity of the study site, the bank vegetation coverage is pasture grasses. In the study area, Lords Creek has a meandering channel with a slope of approximately 0.0026 ft/ft, an average channel top width of 32 ft and an average channel depth of 3 ft. The channel bed material ranged from gravel (D50 is 46.6 mm or 0.153 ft) to silt/clay material (D 50 of 1.006 mm or 0.0033 ft). The geomorphic assessment at the time of the Level I and Level II site visit on October 5 and 6, 1994, indicated that the reach was laterally unstable. The town highway 1 crossing of Lords Creek is a 65-ft-long, two-lane bridge consisting of one 61-foot, steel-beam span (Vermont Agency of Transportation, written communication, August 2, 1994). The bridge is supported by vertical, concrete abutments on wooden piles driven to bedrock with no wingwalls. Each abutment wall has a spill-through slope protected with type-2 stone fill (less than 36 inches diameter). 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 2.4 to 4.6 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.2 to 9.8 ft. 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.