Level II scour analysis for Bridge 38 (RANDTH00640038) on Town Highway 64, crossing the Second Branch of the White River, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RANDTH00640038 on town highway 64 crossing the Second Branch of the White River,
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 46.5-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the The upstream left bank is forested, the upstream right
bank is covered primarily by brush, the surface cover is pasture on the downstream left and
row crops on the downstream right.
In the study area, the Second Branch of the White River has an incised, sinuous channel
with a slope of approximately 0.0015 ft/ft, an average channel top width of 71 ft and an
average channel depth of 8 ft. The predominant channel bed material is gravel with a
median grain size (D₅₀) of 32.0 mm (0.105 ft). The geomorphic assessment at the time of
the Level I site visits on August 10, 1994 and December 5, 1994 indicated that the reach
was laterally unstable.
The town highway 64 crossing of the Second Branch of the White Riveris a 43-ft-long,
one-lane covered bridge consisting of one 35-foot steel-beam span (Vermont Agency of
Transportation, written communication, August 1, 1994). The bridge is supported by
vertical, stone abutments with upstream wingwalls. The channel bends sharply at it’s
approach to the bridge, however, at the bridge face, the channel is skewed approximately 0
degrees to the opening. The opening-skew-to-roadway is also 0 degrees.
A scour hole 2 ft deeper than the mean thalweg depth was observed upstream of the bridge
along the outside of the channel bend. Other scour problems at this site included
undermining of the right abutment at it’s upstream and downstream ends. 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 1.7 to 2.6 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.2 to
24.2 ft. The worst-case abutment scour also 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.