This report provides the results of a detailed Level II analysis of scour potential at structure
CHESVT00110046 on State Route 11 crossing the Middle Branch Williams River, Chester,
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 and New England Upland sections of the New England
physiographic province in southeastern Vermont. The 28.0-mi2
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the surface cover is
forested on the upstream left and downstream right overbanks. The upstream right and
downstream left overbanks are pasture while the immediate banks have dense woody
In the study area, the the Middle Branch Williams River has an incised, sinuous channel
with a slope of approximately 0.013 ft/ft, an average channel top width of 81 ft and an
average bank height of 11 ft. The channel bed material ranges from gravel to bedrock with a
median grain size (D50) of 70.7 mm (0.232 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on September 12, 1996, indicated that the reach was
The State Route 11 crossing of the Middle Branch Williams River is a 118-ft-long, two-lane
steel stringer type bridge consisting of a 114-foot steel plate deck (Vermont Agency of
Transportation, written communication, March 29, 1995). The opening length of the
structure parallel to the bridge face is 109 ft.The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 45 degrees to the opening
while the opening-skew-to-roadway is 50 degrees.
A scour hole 2 ft deeper than the mean thalweg depth was observed 128 feet downstream
during the Level I assessment. Type-1 (less than 1 foot) stone fill protects the downstream
right wingwall. Type-2 (less than 3 ft diameter) stone fill protects the upstream right
wingwall, the left and right abutments, the upstream left and right 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
There was no computed contraction scour for any modelled flows. Abutment scour ranged
from 7.0 to 10.3 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