Level II scour analysis for Bridge 14 (FFIETH00010014) on Town Highway 1, crossing the Fairfield River, Fairfield, Vermont

Open-File Report 96-742

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 FFIETH00010014 on Town Highway 1 crossing the Fairfield River, Fairfield, 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 northwestern Vermont. The 7.84-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover on the upstream left and downstream right is row crops. The surface cover on the upstream right and downstream left is pasture. In the study area, the Fairfield River has a sinuous channel with a slope of approximately 0.006 ft/ft, an average channel top width of 33 ft and an average channel depth of 3 ft. The channel bed material ranges from silt to gravel with a median grain size (D50) of 15.4 mm (0.0505 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 11, 1995, indicated that the reach was laterally unstable. The Town Highway 1 crossing of the Fairfield River is a 26-ft-long, two-lane bridge consisting of one 23-foot concrete span (Vermont Agency of Transportation, written communication, March 9, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 27 degrees to the opening while there is no opening-skew-to-roadway. The bridge is located on a sharp channel bend. Th e left abutment is impacted due to this bend. A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the left abutment during the Level I asse ssment. Scour protection measures at the site include type- 3 stone fill (less than 48 inches diameter ) along the immediate upstream banks and along the base of the upstream left wi ngwall. Type-2 stone fill (le ss than 36 inches diameter) was present along the downstream left wingwall. A dditional details describing conditions at the site are included in the Level II Summary and Appendices D and E. Scour depths and 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 5.4 to 8.0 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 3.0 to 8.3 ft. The worst-case abutment scour also occurred at the 500-year discharge. Additional information on scour depths and depths to armoring are include d 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 14 (FFIETH00010014) on Town Highway 1, crossing the Fairfield River, Fairfield, 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:
Fairfield River