Level II scour analysis for Bridge 49 (BENNCYHUNT0049) on Hunt Street, crossing the Walloomsac River, Bennington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BENNCYHUNT0049 on the Hunt Street crossing of the Walloomsac River, Bennington,
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
southwestern Vermont. The 34.1-mi²
drainage area is a predominantly rural and forested
basin. The bridge site is located within an urban setting in the Town of Bennington with
buildings and parking lots on overbanks except for the downstream left bank which is
covered by trees and brush.

In the study area, the Walloomsac River has a straight, incised channel. The confluence of
the Walloomsac River and Roaring Branch is 140 feet downstream. The channel has a slope
of approximately 0.01 ft/ft, an average channel top width of 54 ft and an average bank
height of 6 ft. The predominant channel bed material is cobble with a median grain size
(D₅₀) of 76.8 mm (0.252 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 31, 1996, indicated that the reach was stable.

The Hunt Street crossing of the Walloomsac River is a 51-ft-long, two-lane bridge
consisting of one 49-foot steel span (Vermont Agency of Transportation, written
communication, December 13, 1995). The bridge is supported by vertical, concrete
abutments. The right abutment has a spill-through slope along its face. The channel is
skewed approximately 25 degrees to the opening and the opening-skew-to-roadway is 20
degrees.

Scour countermeasures at the site include type-2 stone fill (less than 36 inches diameter) on
the spill through slope on the right abutment and along the base of the left abutment. Type-
2 stone fill also protects the channel banks upstream and downstream of the bridge for a
minimum distance of 17 feet from the respective bridge faces. 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 computed for all modelled flows ranged from 0.9 to 5.0 ft. The worst-case contraction scour occurred at the 500-year discharge. Computed left abutment scour
ranged from 15.3 to 16.5 ft. with the worst-case scour occurring at the incipient roadway-overtopping discharge. Computed right abutment scour ranged from 6.0 to 8.7 ft. with the
worst-case scour occurring 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.