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Level II scour analysis for Bridge 17 (SHEFTH00380017) on Town Highway 38, crossing Miller Run, Sheffield, Vermont

Open-File Report 97-782

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 SHEFTH00380017 on Town Highway 38 crossing Miller Run, Sheffield, 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 White Mountain section of the New England physiographic province in northeastern Vermont. The 24.2-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is pasture along the right bank while the immediate banks are covered by trees, shrubs, and brush. The surface cover along the left bank is grass and Route 122 with shrubs and brush along the immediate banks. In the study area, Miller Run has a sinuous channel with a slope of approximately 0.01 ft/ft, an average channel top width of 52 ft and an average bank height of 3 ft. The channel bed material ranges from sand to bedrock with a median grain size (D50) of 80.5 mm (0.264 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 1, 1995, indicated that the reach was stable. The Town Highway 38 crossing of Miller Run is a 52-ft-long, one-lane bridge consisting of one 48-foot steel I-beam span with a wooden deck (Vermont Agency of Transportation, written communication, March 28, 1995). The opening length of the structure parallel to the bridge face is 42.4 ft. The bridge is supported by vertical, concrete abutments with wingwalls on the upstream end. The channel is skewed approximately 30 degrees to the opening while the computed opening-skew-to-roadway is 5 degrees. A scour hole 3.0 ft deeper than the mean thalweg depth was observed under the bridge during the Level I assessment. The only scour protection measure at the site was type-4 stone fill (less than 60 inches diameter) at the upstream end of the upstream left wingwall. 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 modelled flows ranged from 0.0 to 2.4 ft. Abutment scour ranged from 6.1 to 7.9 ft at the left abutment and 11.4 to 17.4 ft at the right abutment. The worstcase contraction and 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. 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 17 (SHEFTH00380017) on Town Highway 38, crossing Miller Run, Sheffield, Vermont
Series title:
Open-File Report
Series number:
97-782
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:
Sheffield
Other Geospatial:
Miller Run
Scale:
24000