This report provides the results of a detailed Level II analysis of scour potential at structure STOWTH00160039 on Town Highway 16 crossing Moss Glen Brook, Stowe, 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 4.75-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover is forest upstream and on the right bank downstream. The downstream left bank is pasture while the immediate bank has dense woody vegetation.
In the study area, Moss Glen Brook has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 52 ft and an average bank height of 7 ft. The channel bed material ranges from sand to cobble with a median grain size (D50) of 56.5 mm (0.185 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 10, 1996, indicated that the reach was stable.
The Town Highway 16 crossing of Moss Glen Brook is a 22-ft-long galvanized plate arch culvert with an opening span width of 21 ft (Vermont Agency of Transportation, written communication, October 13, 1995). The opening length of the structure parallel to the culvert face is 20.6 ft. The culvert is supported by vertical, concrete abutments with no wingwalls. The channel is skewed approximately zero degrees to the opening. The opening skew-to-roadway value from the VTAOT database is 5 degrees while zero degrees was computed from surveyed points.
The only scour counter measure at the site was type-3 stone fill (less than 48 inches diameter) at the upstream and downstream ends of the left and right abutments and extending along the banks upstream and downstream. 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) for the 100- and 500-year discharges. 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 1.2 ft. The worst-case contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 12.6 to 16.2 ft. Right abutment scour ranged from 12.1 to 14.3 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.
Additional Publication Details
USGS Numbered Series
Level II scour analysis for Bridge 39 (STOWTH00160039) on Town Highway 16, crossing Moss Glen Brook, Stowe, Vermont