Level II scour analysis for Bridge 36 (RANDTH00480036) on Town Highway 48, crossing Snows Brook, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RANDTH00480036 on town highway 48 crossing Snows Brook, Randolph, 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 of
central Vermont. The 3.72-mi2
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is forest.
In the study area, Snows Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 27 ft and an average channel
depth of 3 ft. The predominant channel bed material is cobble with a median grain size
(D50) of 72.7 mm (0.238 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 9, 1994, indicated that the reach was laterally unstable.
The town highway 48 crossing of Snows Brook is a 32-ft-long, two-lane bridge consisting
of one 28-foot concrete span (Vermont Agency of Transportation, written communication,
July 29, 1994). The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 40 degrees to the opening while the opening-skew-toroadway is 45 degrees. 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 0.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.1 to
11.6 ft. The worst-case abutment scour occurred at the incipient-overtopping discharge,
which was 50 cfs lower than the 500-year discharge. Additional information on scour
depths and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize 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.