Level II scour analysis for Bridge 16 (GROTTH00170016) on Town Highway 17, crossing the Wells River, Groton, Vermont

Open-File Report 97-805

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 GROTTH00170016 on Town Highway 17 crossing the Wells River, Groton, 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 eastern Vermont. The 43.4-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is predominantly shrub and brushland, while the left bank downstream is forested. In the study area, the Wells River has an incised, straight channel with a slope of approximately 0.003 ft/ft, an average channel top width of 57 ft and an average bank height of 4 ft. The channel bed material ranges from sand to boulder with a median grain size (D50) of 77.8 mm (0.255 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 17 crossing of the Wells River is a 43-ft-long, one-lane bridge consisting of one 41-foot steel-beam span with a concrete deck (Vermont Agency of Transportation, written communication, March 24, 1995). The opening length of the structure parallel to the bridge face is 39.4 ft. The bridge is supported by vertical, concrete abutments. The channel is skewed approximately 0 degrees and the opening-skew-toroadway is also zero degrees. A scour hole 1.7 ft deeper than the mean thalweg depth was observed from 30 ft upstream to 70 ft downstream in mid-channel during the Level I assessment. Scour protection measures at the site included: type-3 stone fill (less than 48 inches diameter) along the left and right bank upstream, and along the left and right bank downstream. The protection along the banks begins in the road embankment areas where the wingwalls would be located. 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 was 0 ft. Abutment scour ranged from 7.6 to 8.4 ft at the left abutment and from 9.9 to 14.8 ft at the right abutment. The worst-case 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 crosssection 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

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Publication Subtype:
USGS Numbered Series
Level II scour analysis for Bridge 16 (GROTTH00170016) on Town Highway 17, crossing the Wells River, Groton, Vermont
Series title:
Open-File Report
Series number:
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U.S. Geological Survey
Publisher location:
Pembroke, NH
51 p.
United States
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