Level II scour analysis for Bridge 49 (WALLVT01030049) on State Highway 103, crossing Freeman Brook, Wallingford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WALLVT01030049 on State Highway 103 crossing Freeman Brook, Wallingford,
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 11.7-mi² drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture with trees and brush on
the immediate banks except for the upstream left overbank which is tree covered. A levee
composed of stone fill was constructed along the upstream left bank in order to keep flow
from reaching the flood plain left (south) of the brook.

In the study area, Freeman Brook has an incised, straight channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 56 ft and an average channel
depth of 6 ft. The predominant channel bed materials are gravel and cobbles with a median
grain size (D₅₀) of 62.9 mm (0.206 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on October 10, 1995, indicated that the reach was stable.

The State Highway 103 crossing of the Freeman Brook is a 54-ft-long, two-lane bridge
consisting of one 50-foot concrete T-beam span (Vermont Agency of Transportation,
written communication, March 15, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 25 degrees to the opening
while the opening-skew-to-roadway is zero degrees.

A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the downstream
end of the left abutment and downstream left wingwall during the Level I assessment. The
scour protection measures at the site included type-2 stone fill (less than 36 inches
diameter) along the entire base length of the upstream left and downstream right wingwall
and type-1 stone fill (less than 12 inches diameter) along the upstream end of the upstream
right wingwall. Type-4 stone fill (less than 60 inches diameter) was found along the
upstream 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 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 0.0 to 1.4 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.6 to
21.4 ft. The worst-case abutment scour was predicted at 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.