Level II scour analysis for Bridge 8 (ATHETH00090008) on Town Highway 9, crossing Bull Creek, Athens, Vermont

Open-File Report 97-818

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



This report provides the results of a detailed Level II analysis of scour potential at structure ATHETH00090008 on Town Highway 9 crossing Bull Creek in Athens, 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 southeastern Vermont. The 9.04-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the left overbank surface cover is shrub and brushland and the right overbank surface cover is pasture. In the study area, Bull Creek has an sinuous channel with a slope of approximately 0.01 ft/ ft, an average channel top width of 41 ft and an average bank height of 4 ft. The predominant channel bed materials are cobbles and gravel with a median grain size (D50) of 72.1 mm (0.236 ft). The geomorphic assessment at the time of the Level I and Level II site visit on August 14, 1996, indicated that the reach was laterally unstable. There are several point bars and cut banks along the reach in the vicinity of this site. The Town Highway 9 crossing of Bull Creek is a 32-ft-long, one-lane bridge consisting of one 28-foot steel-beam span (Vermont Agency of Transportation, written communication, April 5, 1995). The bridge is supported by vertical, “laid-up” stone abutments with concrete caps and no wingwalls. The channel is skewed approximately 15 degrees to the opening. The VTAOT bridge records indicate the opening-skew-to-roadway is 9 degrees while that computed from surveyed points is 5 degrees. A scour hole 1.75 feet deeper than the mean thalweg depth was observed under the bridge during the Level I assessment. The scour hole has lowered the streambed along the entire length of the left abutment and the upstream end of the right abutment. The scour depth at each abutment wall is 0.75 feet deeper than the mean thalweg depth elsewhere in the reach. The only scour protection measure at the site was type-2 stone fill (less than 36 inches diameter) on the upstream banks and downstream left bank. 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). 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 1.4 feet. The worst-case contraction scour occurred at the incipient-overtopping discharge of 1730 cubic feet per second, which was less than the 100-year discharge. Abutment scour ranged from 7.6 to 11.4 feet. 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 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.

Study Area

Additional publication details

Publication type:
Publication Subtype:
USGS Numbered Series
Level II scour analysis for Bridge 8 (ATHETH00090008) on Town Highway 9, crossing Bull Creek, Athens, Vermont
Series title:
Open-File Report
Series number:
Year Published:
U.S. Geological Survey
Publisher location:
Pembroke, NH
iv, 50 p.
United States
Other Geospatial:
Bull Creek