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Level II scour analysis for Bridge 34 (SHERUS00040034) on U.S. Highway 4, crossing the Ottauquechee River, Sherburne, Vermont

Open-File Report 97-9

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 SHERUS00040034 on US Route 4 crossing the Ottauquechee River, Sherburne, 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 central Vermont. The 25.8-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture upstream of the bridge while the immediate banks have dense woody vegetation. Downstream of the bridge, the banks are forested. In the study area, the Ottauquechee River has an incised, straight channel with a slope of approximately 0.028 ft/ft, an average channel top width of 66 ft and an average channel depth of 5 ft. The channel bed material ranges from gravel to boulder with a median grain size (D50) of 118.1 mm (0.387 ft). The geomorphic assessment at the time of the Level I and Level II site visit on September 25, 1995, indicated that the reach was stable. The US Route 4 crossing of the Ottauquechee River is a 187-ft-long, two-lane bridge consisting of three steel-beam spans (Vermont Agency of Transportation, written communication, March 14, 1995). The bridge is supported by vertical, concrete abutments above spill-through stone fill (< 36 inches diameter). The channel is skewed approximately 60 degrees to the opening while the opening-skew-to-roadway is 60 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 ft. Abutment scour ranged from 4.7 to 7.4 ft. The worst-case abutment scour occurred at the left abutment for the 500-year discharge. Pier scour ranged from 7.5 to 11.4 ft. The worst-case pier scour occurred at the incipientovertopping 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.

Geospatial Extents

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Level II scour analysis for Bridge 34 (SHERUS00040034) on U.S. Highway 4, crossing the Ottauquechee River, Sherburne, Vermont
Series title:
Open-File Report
Series number:
97-9
Year Published:
1997
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Pembroke, NH
Description:
iv, 51 p.
Number of Pages:
56
Country:
United States
State:
Vermont
City:
Sherburne
Other Geospatial:
Ottauquechee River
Scale:
24000