Level II scour analysis for Bridge 92 (WSTOVT01000092) on State Highway 100, crossing the West River, Weston, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WSTOVT01000092 on Vermont Highway 100 crossing the West River, Weston, 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 in
south-central Vermont. The 32.7-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover upstream of the bridge is primarily
forest with pasture on the upstream left overbank. Upstream and downstream, the
immediate banks have brush and dense forest cover. Downstream of the bridge is forested.
In the study area, the West River has an incised, sinuous channel with a slope of
approximately 0.006 ft/ft, an average channel top width of 111 ft and an average channel
depth of 3 ft. The predominant channel bed material is very coarse gravel and cobbles with
a median grain size (D₅₀) of 67.7 mm (0.222 ft). The geomorphic assessment at the time of
the Level I and Level II site visit on August 19, 1996 indicated that the reach was laterally
unstable based on the fine bank material, sinuosity of the stream, point bars and cutbanks.
The state highway 100 crossing of the West River is a 113-ft-long, two-lane bridge
consisting of one 110-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 31, 1995). The bridge is supported by vertical, concrete abutments
without wingwalls. The channel is skewed approximately 40 degrees to the opening while
the opening-skew-to-roadway is 25 degrees.
The only scour protection measure at the site was type-2 stone fill (less than 36 inches
diameter) along the entire base length 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 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 0.4 to 2.1 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 8.4 to
30.7 ft. The worst-case abutment scour occurred at the 500-year discharge along the left
abutment. 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.