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Level II scour analysis for Bridge 24 (WODSTH00190024) on Town Highway 19, crossing North Bridgewater Brook, Woodstock, Vermont

Open-File Report 96-238

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 WODSTH00190024 on Town Highway 19 crossing North Bridgewater 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). 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 east-central Vermont in the town of Woodstock. The 3.6-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the left and right banks are covered by moderate tree cover along the immediate banks with some pasture/ grassland beyond. In the study area, the North Bridgewater Brook has a sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 44 ft and an average channel depth of 4 ft. The channel bed materials ranges from sand to boulders with a D50 (median diameter)of 70.1 mm or 0.229 ft. The geomorphic assessment at the time of the Level I and Level II site visits on August 17, 1994 and December 13, 1994, indicated that the reach was stable. Localized bank cutting existed at the immediate downstream left bank. The Town Highway 19 crossing of the North Bridgewater Brook is a 26-ft-long, one-lane bridge consisting of one 23-ft steel-beam span (Vermont Agency of Transportation, written commun., August 3, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. Type-2 (less than 3 ft diameter) stone fill protects the upstream left wingwall which is impacted by flow. The channel bed under the bridge is constructed of wood. This construction is preventing channel degradation along the impacted left abutment.The channel is skewed approximately 40 degrees to the opening; the opening-skew-to-roadway is 10 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, 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 0.8 ft. Abutment scour ranged from 6.6 to 14.9 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.

Geospatial Extents

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Level II scour analysis for Bridge 24 (WODSTH00190024) on Town Highway 19, crossing North Bridgewater Brook, Woodstock, Vermont
Series title:
Open-File Report
Series number:
96-238
Edition:
-
Year Published:
1996
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Pembroke, NH
Description:
iv, 53 p.
Number of Pages:
57
Country:
United States
State:
Vermont
City:
Woodstock
Other Geospatial:
North Bridgewater Brook