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Level II scour analysis for Bridge 39 (TOPSTH00510039) on Town Highway 51, crossing Tabor Branch Waits River, Topsham, Vermont

Open-File Report 97-797

Prepared in cooperation with Vermont Agency of Transportation and Federal Highway Administration
By:
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Abstract

This report provides the results of a detailed Level II analysis of scour potential at structure TOPSTH00510039 on Town Highway 51 crossing the Tabor Branch Waits River, Topsham, 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 17.4-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is predominantly pasture. However, beyond one bridge length on the right bank upstream the surface cover abruptly changes to forest. In the study area, the Tabor Branch Waits River has a sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 53 ft and an average bank height of 6 ft. The predominant channel bed material is cobbles with a median grain size (D50) of 86.4 mm (0.283 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 30, 1995, indicated that the reach was stable. The Town Highway 51 crossing of the Tabor Branch Waits River is a 34-ft-long, one-lane bridge consisting of one 32-foot concrete slab span (Vermont Agency of Transportation, written communication, March 28, 1995). The opening length of the structure parallel to the bridge face is 31.0 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 5 degrees to the opening while the opening-skew-to-roadway is 10 degrees. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) along the left and right bank upstream, along the base of the upstream left wingwall, upstream right wingwall, left abutment, right abutment, downstream left wingwall, downstream right wingwall, and along the left and right bank downstream. Additional 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 0.4 ft. The worst-case contraction scour occurred at the maximum free surface flow discharge, which was less than the 100-year discharge. Abutment scour ranged from 4.8 to 8.0 ft. The worst-case abutment scour occurred at 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 crosssection 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.

Study Area

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Level II scour analysis for Bridge 39 (TOPSTH00510039) on Town Highway 51, crossing Tabor Branch Waits River, Topsham, Vermont
Series title:
Open-File Report
Series number:
97-797
Year Published:
1997
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Pembroke, NH
Description:
iv, 51 p.
Country:
United States
State:
Vermont
City:
Topsham
Other Geospatial:
Tabor Branch Waits River
Scale:
24000