Level II scour analysis for Bridge 71 (WODSTH00050071) on Town Highway 5, crossing Kedron Brook, Woodstock, Vermont

Open-File Report 97-807

Prepared in cooperation with Vermont Agency of Transportation and the Federal Highway Administration



This report provides the results of a detailed Level II analysis of scour potential at structure WODSTH00050071 on Town Highway 5 crossing Kedron Brook, Woodstock, 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 east-central Vermont. The 16.1-mi2 drainage area is in a predominantly rural and forested basin. However, the bridge site is within the Village of Woodstock. In the vicinity of the study site, the surface cover is best described as suburban downstream of the bridge and forest and brush upstream of the bridge. In the study area, Kedron Brook has an incised, sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 33 ft and an average bank height of 11 ft. The predominant channel bed material is cobble with a median grain size (D50) of 112 mm (0.368 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 14, 1994, indicated that the reach was vertically degraded. Evidence of the degradation was observed at the outlet of the bridge where the stream bed is 4 ft below the downstream invert of the structure (see figure 6). The Town Highway 5 crossing of Kedron Brook is a 30-ft-long, two-lane bridge/box culvert consisting of one 25-foot concrete span (Vermont Agency of Transportation, written communication, August 3, 1994). The opening length of the structure parallel to the bridge face is 23.5 ft.The bridge is supported by vertical, concrete abutments with wingwalls. The channel bed under the bridge is covered entirely by a concrete slab. The channel is skewed approximately 45 degrees to the opening and the opening-skew-to-roadway is also 45 degrees. Scour countermeasures at the site include concrete retaining walls on both the left and right downstream banks extending approximately 130 ft downstream; a drywall constructed of stone on the upstream right bank extending to the next bridge upstream; type-2 stone fill (less than 36 inches diameter) along the upstream left bank, at the upstream end of the upstream right wingwall, and along the base of the retaining wall on the downstream left bank; and type-3 stone-fill (less than 48 inches diameter) along the base of the retaining wall on the downstream right bank. In addition, the channel under the bridge is concrete. Further details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995) for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping discharge is determined and analyzed as another potential worst-case scour scenario. 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 2.5 ft. The worst-case contraction scour occurred at the incipient roadway-overtopping discharge, which was less than the 100-year discharge. The contraction scour depths do not take the concrete channel bed under the bridge into account. Abutment scour ranged from 8.7 to 18.2 ft. The worstcase abutment scour occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize 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.

Study Area

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USGS Numbered Series
Level II scour analysis for Bridge 71 (WODSTH00050071) on Town Highway 5, crossing Kedron Brook, Woodstock, Vermont
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Open-File Report
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U.S. Geological Survey
Publisher location:
Pembroke, NH
51 p.
United States
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