Level II scour analysis for Bridge 36 (DUXBTH00040036) on Town Highway 4, crossing Crossett Brook, Duxbury, Vermont
Open-File Report 97-661
Prepared in cooperation with Vermont Agency of Transportation and Federal Highway Administration
- Emily C. Wild and James R. Degnan
This report provides the results of a detailed Level II analysis of scour potential at structure DUXBTH00040036 on Town Highway 4 crossing the Crossett Brook, Duxbury, 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 4.9-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 overbank is pasture. The upstream and downstream right overbanks are forested. The downstream left overbank is brushland, while the immediate banks have dense woody vegetation.
In the study area, the Crossett Brook has an incised, sinuous channel with a slope of approximately 0.006 ft/ft, an average channel top width of 55 ft and an average bank height of 9 ft. The channel bed material ranges from gravel to bedrock with a median grain size (D50) of 51.6 mm (0.169 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 1, 1996, indicated that the reach was stable.
The Town Highway 4 crossing of the Crossett Brook is a 29-ft-long, two-lane bridge consisting of a 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 26 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 35 degrees to the opening while the computed opening-skew-to-roadway is 5 degrees.
A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the upstream left wingwall and the right abutment during the Level I assessment. Scour countermeasures at the site includes type-2 stone fill (less than 36 inches diameter) at the upstream end of the upstream left and right wingwalls and the upstream left and right banks and road embankments. 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 1.7 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 6.4 to 8.3 ft. Right abutment scour ranged from 6.0 to 7.0 ft. The worst-case left and right 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.
Additional Publication Details
- Publication type:
- Publication Subtype:
- USGS Numbered Series
- Level II scour analysis for Bridge 36 (DUXBTH00040036) on Town Highway 4, crossing Crossett Brook, Duxbury, Vermont
- Series title:
- Open-File Report
- Series number:
- Year Published:
- U.S. Geological Survey
- Publisher location:
- Pembroke, NH
- iv, 48 p.
- United States
- Other Geospatial:
- Crossett Brook