Level II scour analysis for Bridge 10 (BENNUS00070010) on U.S. Route 7, crossing the Walloomsac River, Bennington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BENNUS00070010 on U.S. Route 7, also known as North 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 30.1-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, parking lots, lawns, and a playground on the overbank areas.

In the study area, the Walloomsac River has a straight channel with constructed channel
banks through much of the reach. The channel is located on a delta and has a slope of
approximately 0.02 ft/ft, an average channel top width of 37 ft and an average bank height
of 5 ft. The predominant channel bed material is cobble with a median grain size (D₅₀) of
96.0 mm (0.315 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 5, 1996, indicated that the constructed reach was stable.

The U.S. Route 7 crossing of the Walloomsac River is a 53-ft-long, two-lane bridge
consisting of one 50-foot steel span (Vermont Agency of Transportation, written
communication, September 27, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The wingwalls are angled in toward the channel because the
widths of the upstream and downstream constructed channel banks are narrower than the
bridge opening. The channel is skewed approximately 5 degrees to the opening and the
opening-skew-to-roadway is 10 degrees.

Scour countermeasures at the site include masonry and stone walls on both the upstream
and downstream 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).
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 0.0 to 0.1 ft. The worstcase contraction scour occurred at the 500-year discharge. Computed left abutment scour
ranged from 5.9 to 6.8 ft. with the worst-case scour occurring at the 500-year discharge.
Computed right abutment scour for all modelled flows was 6.8 ft. Total scour depths for all
modelled flows did not exceed the depth of the abutment footings. 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.