Level II scour analysis for Bridge 63 (CHESTH00090063) on Town Highway 9, crossing the Williams River, Chester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHESTH00090063 on Town Highway 9 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 eastern Vermont. The 24.0-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is grass with trees and brush along
the immediate banks.
In the study area, the the Williams River has an incised, sinuous channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 64 ft and an average bank height
of 6 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 57.7 mm (0.189 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 18, 1996, indicated that the reach was stable.
The Town Highway 9 crossing of the Williams River is a 45-ft-long, two-lane bridge
consisting of one 35-foot steel-beam span with a timber deck (Vermont Agency of
Transportation, written communication, April 6, 1995). The bridge is supported by vertical,
concrete abutments with wingwalls. The channel is skewed approximately 5 degrees to the
opening while the opening-skew-to-roadway is 0 degrees.
A scour hole 1.8 ft deeper than the mean thalweg depth was observed along the left
abutment during the Level I assessment. The scour hole undermines the left abutment and
extends from 50 ft upstream of the upstream bridge face to 50 ft downstream of the
downstream bridge face. The scour protection measures at the site included type-3 stone fill
(less than 48 inches diameter) under the bridge along the entire base length of the right
abutment and along the right bank from 50 to 88 ft upstream. Type-2 (less than 36 inches
diameter) stone fill scour protection was observed along the downstream left bank from 18
ft to 115 ft, along the downstream right bank from 8 ft to 25 ft and along the upstream left
bank from 50 to 75 ft. 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
computations follows.
Contraction scour for all modelled flows was computed to be 0.0 ft. Abutment scour ranged
from 10.1 ft to 11.0 ft along the left abutment and from 14.1 ft to 15.1 ft along the right
abutment. The worst-case abutment scour for the left abutment occurred at the 500-year
discharge while the worst-case abutment scour for the right abutment occurred at the 100-
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.