Level II scour analysis for Bridge 45 (NFIETH00250045) on Town Highway 25, crossing Union Brook, Northfield, Vermont

Open-File Report 97-407

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 NFIETH00250045 on Town Highway 25 crossing Union Brook, Northfield, 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 Green Mountain section of the New England physiographic province in central Vermont. The 4.04-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, surface cover consists of shrubs and brush on all of the banks except the upstream right bank which is forested. In the study area, Union Brook has an incised, meandering channel with a slope of approximately 0.018 ft/ft, an average channel top width of 41 ft and an average bank height of 2 ft. The channel bed material ranges from sand to cobble with a median grain size (D50) of 65.8 mm (0.216 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 24, 1996, indicated that the reach was unstable. The stream meanders and there is a cut bank on the upstream right bank and trees are falling into the channel. The Town Highway 25 crossing of Union Brook is a 28-ft-long, two-lane bridge consisting of one 26-foot concrete slab span (Vermont Agency of Transportation, written communication, October 13, 1995). The opening length of the structure parallel to the bridge face is 23.8 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 50 degrees to the opening while the opening-skew-to-roadway is 0 degrees. During the Level I assessment, a scour hole 3.0 ft deeper than the mean thalweg depth was observed at the upstream face of the bridge that extended from the center of the channel to the front of the upstream left wingwall. An additional scour hole 1.5 ft deeper than the mean thalweg depth was observed along the downstream right bank near the bridge. The scour counter measures at the site were a laid-up wall of concrete slabs along the upstream right bank beginning at the end of the upstream right wingwall and type-1 stone fill (less than 12 inches diameter) along the downstream right wingwall and bank, and type-2 stone fill (less than 36 inches diameter) along the downstream left wingwall and 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 0.4 to 0.9 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.5 to 9.1 ft. 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 45 (NFIETH00250045) on Town Highway 25, crossing Union Brook, Northfield, 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:
Union Brook