Level II scour analysis for Bridge 36 (RICHVT01050036) on State Route 105, crossing Stanhope Brook, Richford, Vermont

Open-File Report 96-584

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 RICHVT01050036 on State Route 105 crossing Stanhope Brook, Richford, 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 north-central Vermont. The 7.03-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is short grass except for the upstream left overbank area which is forested. In the study area, Stanhope Brook has a steep, sinuous channel with a slope of approximately 0.03 ft/ft, an average channel top width of 47 ft and an average channel depth of 5 ft. The predominant channel bed material is cobble with a median grain size (D50) of 132 mm (0.432 ft). The geomorphic assessment at the time of the Level I and Level II site visit on June 28, 1995, indicated that the reach was laterally unstable. The State Route 105 crossing of Stanhope Brook is a 42-ft-long, two-lane bridge consisting of one 38-foot concrete T-beam span (Vermont Agency of Transportation, written communication, March 8, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 15 degrees to the opening while the opening-skew-to-roadway is 20 degrees. A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the downstream end of the right abutment wall during the Level I assessment. The scour protection measures at this site were type-2 stone fill (less than 36 inches diameter) along the entire lengths of the upstream wingwalls, at the corner of the downstream left abutment and downstream left wingwall and the downstream end of the downstream right wingwall. Additional 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 0.0 to 0.3 ft. The worst-case contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.6 to 9.4 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

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Publication Subtype:
USGS Numbered Series
Level II scour analysis for Bridge 36 (RICHVT01050036) on State Route 105, crossing Stanhope Brook, Richford, Vermont
Series title:
Open-File Report
Series number:
Year Published:
U.S. Geological Survey
iv, 47 p.
United States
Other Geospatial:
Stanhope Brook