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Level II scour analysis for Bridge 10 (HANCTH00010010) on Town Highway 1, crossing the White River, Hancock, Vermont

Open-File Report 96-570

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

This report provides the results of a detailed Level II analysis of scour potential at structure HANCTH00010010 on town highway 1 crossing the White River, Hancock, 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 Green Mountain section of the New England physiographic province of central Vermont. The 59.8-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is primarily grass with trees and brush on the immediate channel banks.


In the study area, the White River has a sinuous channel with a slope of approximately 0.005 ft/ft, an average channel top width of 104 ft and an average channel depth of 6 ft. The predominant channel bed materials are gravel and cobble with a median grain size (D50) of 98.9 mm (0.325 ft). The geomorphic assessment at the time of the Level I and Level II site visit on November 15, 1994, indicated that the reach was stable.


The town highway 1 crossing of the White River is a 91-ft-long, two-lane bridge consisting of one 89-foot steel-beam span (Vermont Agency of Transportation, written communication, August 26, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 15 degrees to the opening while the opening-skew-to-roadway is 0 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 was 0.0 feet. Abutment scour ranged from 13.1 to 17.1 ft. The worst-case 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”. 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.

Study Area

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Level II scour analysis for Bridge 10 (HANCTH00010010) on Town Highway 1, crossing the White River, Hancock, Vermont
Series title:
Open-File Report
Series number:
96-570
Year Published:
1996
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Pembroke, NH
Description:
iv, 51 p.
Country:
United States
State:
Vermont
City:
Hancock
Other Geospatial:
White River
Scale:
24000