This report provides the results of a detailed Level II analysis of scour potential at structure
CHESTH00030010 on Town Highway 3 (VT 35) crossing the South 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 New England Upland section of the New England physiographic province
in southeastern Vermont. The 9.44-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, the South Branch Williams River has an incised, sinuous channel with a
slope of approximately 0.03 ft/ft, an average channel top width of 67 ft and an average
bank height of 5 ft. The channel bed material ranges from gravel to boulder with a median
grain size (D50) of 69.0 mm (0.226 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on August 26-27, 1996, indicated that the reach was stable.
The Town Highway 3 (VT 35) crossing of the South Branch Williams River is a 69-foot-long, two-lane bridge consisting of one 67-foot steel-stringer span with a concrete deck
(Vermont Agency of Transportation, written communication, August 23, 1994). The
opening length of the structure parallel to the bridge face is 64.5 ft. The bridge is supported
by vertical, concrete abutments with spill-through embankments. The channel is skewed
approximately 50 degrees to the opening while the opening-skew-to-roadway is 30 degrees.
The scour protection (spill-through abutments) measured at the site was type-3 stone fill
(less than 48 inches diameter) extending the entire base length and around the ends of the
left and right abutments. 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
Contraction scour for modelled flows ranged from 0.8 to 3.8 ft. The worst-case contraction
scour occurred at the incipient roadway-overtopping discharge. Left abutment scour ranged
from 13.3 to 14.9 ft. The worst-case scour at the left abutment occurred at the 500-year
discharge. Right abutment scour ranged from 4.1 to 6.0 ft. The worst-case scour at the right
abutment occurred at the incipient roadway-overtopping 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
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