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Level II scour analysis for Bridge 39 (PEACTH00620039) on Town Highway 62, crossing South Peacham Brook, Peacham, Vermont

Open-File Report 97-775

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 PEACTH00620039 on Town Highway 62 crossing South Peacham Brook, Peacham, 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 New England Upland section of the New England physiographic province in northeastern Vermont. The 9.1-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest on the left bank upstream and downstream of the bridge. The surface cover on the right bank upstream and downstream is shrubs and brush. In the study area, South Peacham Brook has an incised, straight channel with a slope of approximately 0.02 ft/ft, an average channel top width of 43 ft and an average bank height of 8 ft. The channel bed material ranges from sand to boulder with a median grain size (D50) of 51.4 mm (0.168 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 23, 1995, indicated that the reach was stable. The Town Highway 62 crossing of South Peacham Brook is a 23-ft-long, one-lane bridge consisting of one 22-foot steel-beam span (Vermont Agency of Transportation, written communication, March 27, 1995). The opening length of the structure parallel to the bridge face is 20.1 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 15 degrees to the opening while the computed openingskew-to-roadway is 10 degrees. The footing on the right abutment and the footing on the upstream left wingwall were exposed during the Level I assessment. The scour countermeasures at the site included type- 2 stone fill (less than 36 inches diameter) along the upstream and downstream right wingwalls and at the upstream end of the upstream left wingwall and at the downstream end of the downstream left wingwall. Type-3 stone fill (less than 48 inches diameter) was along the upstream left and right banks and the downstream right bank. On the downstream left bank, the scour countermeasure was a stone wall. 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 1.0 to 1.6 ft. The worst-case contraction scour occurred at the 100-year discharge. Abutment scour ranged from 5.9 to 7.4 ft. The worst-case abutment scour occurred at the incipient roadway-overtopping discharge, which is less than the 100-year discharge. Additional information on scour depths and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize distribution. However, there is a bedrock outcrop across the channel just upstream of the bridge. 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 39 (PEACTH00620039) on Town Highway 62, crossing South Peacham Brook, Peacham, Vermont
Series title:
Open-File Report
Series number:
97-775
Year Published:
1997
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Pembroke, NH
Description:
iv, 51 p.
Country:
United States
State:
Vermont
City:
Peacham
Other Geospatial:
Peacham Brook
Scale:
24000