Level II scour analysis for Bridge 37 (CABOTH00410037) on Town Highway 41, crossing the Winooski River, Cabot, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CABOTH00410037 on Town Highway 41 crossing the Winooski River (also referred to as
Coit’s Pond Brook), Cabot, 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 northeastern Vermont. The 21.4-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is primarily shrub and
brushland while the immediate banks have dense woody vegetation.
In the study area, the Winooski River has an incised, straight channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 53 ft and an average bank height
of 4 ft. The channel bed material is primarily cobbles and boulder with a median grain size
(D₅₀) of 64.5 mm (0.212 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 16, 1996, indicated that the reach was stable.
The Town Highway 41 crossing of the Winooski River is a 29-ft-long, one-lane bridge
consisting of one 26-foot span (Vermont Agency of Transportation, written
communication, October 13, 1995) with four steel I-beams and a wooden deck . The
opening length of the structure parallel to the bridge face is 26 ft.The bridge is supported by
“laid up” granite block abutments with concrete footings. The channel is skewed
approximately 35 degrees to the opening while the computed opening-skew-to-roadway is
15 degrees. The VTAOT computed opening-skew-to-roadway is zero degrees.
The only scour protection measure observed at the site during the Level I assessment was
type-2 stone fill (less than 36 inches diameter) along the entire base length of the left
abutment and upstream right wingwall, along the upstream left bank and along the
downstream left and right banks. 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 and
maximum free-surface flow discharges were determined and analyzed as two other
potential worst-case scour scenarios. 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 0.0 to 2.7 ft. The worst-case
contraction scour occurred at the maximum free-surface flow (with road overflow)
discharge, which was less than the 100-year discharge. Abutment scour ranged from 9.8 to
10.7 ft along the left abutment and from 16.2 to 19.9 ft along the right abutment. The worstcase 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”. Scouredstreambed 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 particlesize distribution.
It is generally accepted that the Froehlich and Hire equations (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.