Level II scour analysis for Bridge 53 (WILMVT01000053) on State Route 100, crossing Cold Brook, Wilmington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WILMVT01000053 on State Route 100 crossing Cold Brook, Wilmington, 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 8.38-mi² drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover predominantly is pasture except for
the immediate channel banks, which are tree covered.

In the study area, Cold Brook has a straight channel with a slope of approximately 0.04 ft/ft,
an average channel top width of 63 feet and an average bank height of 9 feet. The channel is
constructed with stone fill completely covering both banks for 300 feet upstream of the site.
The predominant channel bed materials are gravel and cobbles with a median grain size
(D₅₀) of 66.2 mm (0.217 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 7, 1996, indicated that the reach was constructed.

The State Route 100 crossing of Cold Brook is a 23-ft-long, two-lane bridge divided by a
median strip consisting of one 20-foot concrete span (Vermont Agency of Transportation,
written communication, November 1, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is not skewed to the opening and the opening-skew-
to-roadway also is zero degrees.

The scour protection measure at the site was type-2 stone fill (less than 36 inches diameter)
on the upstream banks, the upstream wingwalls, and 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 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 0.6 to 2.7 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.8 to
10.9 ft. The worst-case abutment scour occurred at the left abutment for the 500-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.