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Level II scour analysis for Bridge 28 (CAMBTH00460028) on Town Highway 46, crossing the Seymour River, Cambridge, Vermont

Open-File Report 97-648

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 CAMBTH00460028 on Town Highway 46 crossing the Seymour River, Cambridge, 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 in northwestern Vermont. The 9.94-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture while the immediate banks have dense woody vegetation.


In the study area, the Seymour River has an incised, straight channel with a slope of approximately 0.02 ft/ft, an average channel top width of 81 ft and an average bank height of 5 ft. The channel bed material ranges from gravel to boulder with a median grain size (D50) of 62.0 mm (0.204 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 11, 1995, indicated that the reach was stable.


The Town Highway 46 crossing of the Seymour River is a 38-ft-long, one-lane bridge consisting of one 33-foot steel-beam span (Vermont Agency of Transportation, written communication, March 8, 1995). The opening length of the structure parallel to the bridge face is 30.6 ft.The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 5 degrees to the opening while the measured opening-skew-to-roadway is 10 degrees.


A scour hole 0.2 ft deeper than the mean thalweg depth was observed along the upstream right wingwall and right abutment during the Level I assessment. The only scour protection measure at the site was type-1 stone fill (less than 12 inches diameter) along the upstream left road embankment. 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 0.8 ft. The worst-case contraction scour occurred at the incipient roadway-overtopping discharge. Left abutment scour ranged from 4.2 to 4.9 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right abutment scour ranged from 8.8 to 9.7 ft. The worst-case right abutment scour occurred at the incipient roadway-overtopping 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 28 (CAMBTH00460028) on Town Highway 46, crossing the Seymour River, Cambridge, Vermont
Series title:
Open-File Report
Series number:
97-648
Year Published:
1997
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Pembroke, NH
Description:
iv, 50 p.
Country:
United States
State:
Vermont
City:
Cambridge
Other Geospatial:
Seymour River
Scale:
24000