Level II scour analysis for Bridge 53 (CHESTH01180053) on Town Highway 118, crossing the Williams River, Chester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHESTH01180053 on Town Highway 118 crossing the 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 20.8-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is predominantly suburban
while the right bank upstream is pasture. There is a house on the right bank downstream and
VT 103 runs parallel to the river along the left bank.
In the study area, the Williams River has an incised, straight channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 64 ft and an average bank height
of 7 ft. The channel bed material ranges from sand to boulder with a median grain size (D₅₀)
of 58.0 mm (0.190 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on September 17, 1996, indicated that the reach was stable.
The Town Highway 118 crossing of the Williams River is a 43-ft-long, one-lane bridge
consisting of one 40-foot steel-beam span (Vermont Agency of Transportation, written
communication, April 6, 1995). The opening length of the structure parallel to the bridge
face is 37.6 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 5 degrees to the opening while the computed opening-skew-to-roadway is 10 degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed at both abutments
during the Level I assessment. Scour protection measures at the site include: type-3 stone
fill (less than 48 inches diameter) along the left bank upstream and downstream and type-2
stone fill (less than 36 inches diameter) along the entire base length of the upstream left
wingwall, at the upstream end of the left abutment, and at the upstream end of the
downstream left wingwall. 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. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 was 0.0 ft. Abutment scour ranged from 5.8 to 6.8
ft at the left abutment and 9.4 to 14.4 ft at the right abutment. The worst-case abutment
scour 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
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.