Level II scour analysis for Bridge 42 (BENNCYSCHL0042) on School Street, crossing Walloomsac River, Bennington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BENNCYSCHL0042 on the School 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 and residences on all overbanks.
In the study area, the Walloomsac River has a straight channel with constructed channel
banks downstream of the bridge. 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 6 ft. The predominant channel bed material is cobble with a median grain size (D₅₀) of
132 mm (0.435 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on August 6, 1996, indicated that the reach was stable.
The School Street crossing of the Walloomsac River is a 36-ft-long, two-lane bridge
consisting of one 33-foot concrete span (Vermont Agency of Transportation, written
communication, December 13, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately zero degrees to the
opening and the opening-skew-to-roadway is also zero degrees.
Scour countermeasures at the site include type-2 stone fill (less than 36 inches diameter)
along both upstream banks and upstream wingwalls. Downstream banks are protected by
stone walls extending from the downstream wingwalls to more than 100 feet downstream.
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 was 0.0 ft. Computed left abutment
scour ranged from 9.4 to 10.2 ft. with the worst-case scour occurring at the 500-year
discharge. Computed right abutment scour ranged from 2.7 to 5.7 ft. with the worst-case
scour occurring at the incipient roadway-overtopping 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.