Level II scour analysis for Bridge 65 (NEWBTH00500065) on Town Highway 50, crossing Peach Brook, Newbury, Vermont

Open-File Report 97-804

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



This report provides the results of a detailed Level II analysis of scour potential at structure NEWBTH00500065 on Town Highway 50 crossing Peach Brook, Newbury, 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 east-central Vermont. The 15.3-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest upstream of the bridge and shrub and brushland downstream of the bridge. In the study area, Peach Brook has an incised, sinuous channel with a slope of approximately 0.005 ft/ft, an average channel top width of 40 ft and an average bank height of 8 ft. The channel bed material ranges from cobble to boulder with a median grain size (D50) of 83.1 mm (0.273 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 29, 1995, indicated that the reach was stable. The Town Highway 50 crossing of the Peach Brook is a 29-ft-long, two-lane bridge consisting of one 25-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 24.9 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 50 degrees to the opening while the computed openingskew-to-roadway is 20 degrees. A channel scour hole 0.75 ft deeper than the mean thalweg depth was observed under the bridge during the Level I assessment. Also observed was channel scour 0.75 ft deeper than the mean thalweg at the upstream face of the bridge and channel scour 0.25 ft deeper than the mean thalweg along the right bank downstream. The scour protection measures at the site included type-1 stone fill (less than 12 inches diameter) along the upstream and downstream right wingwalls and type-2 stone fill (less than 36 inches diameter) along the upstream right bank and along the downstream left wingwall and bank. In addition, there are four 3 ft square concrete blocks at the corner where the upstream right wingwall joins the right abutment. The upstream left wingwall and upstream half of the left abutment were constructed on top of a bedrock outcrop. 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 1.3 ft. The worst-case contraction scour occurred at the incipient roadway-overtopping discharge, which was less than the 100-year discharge. The right abutment scour ranged from 6.1 to 7.2 ft. The worstcase right abutment scour occurred at the incipient roadway-overtopping discharge. The left abutment scour ranged from 7.1 to 10.3 ft. The worst-case left 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 he

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Level II scour analysis for Bridge 65 (NEWBTH00500065) on Town Highway 50, crossing Peach Brook, Newbury, Vermont
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Open-File Report
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U.S. Geological Survey
Publisher location:
Pembroke, NH
51 p.
United States
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