Level II scour analysis for Bridge 81 (NFIETH00PL0081) on Pleasant Street, crossing Union Brook, Northfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NFIETH00PL0081 on Pleasant Street crossing Union Brook, Northfield, 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
central Vermont. The 6.1-mi²
drainage area is in a predominantly rural and forested basin.
The bridge site is located within a suburban setting in the Town of Northfield with homes,
lawns, and pavement on the overbanks. There are trees and brush along the immediate
banks.
In the study area, Union Brook has an incised, straight channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 41 ft and an average bank height
of 4 ft. The channel bed material ranges from gravel to boulders with a median grain size
(D₅₀) of 47.7 mm (0.157 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 24, 1996, indicated that the reach was stable.
The Pleasant Street crossing of Union Brook is a 34-ft-long, two-lane bridge consisting of
one 29-foot steel-beam span (Vermont Agency of Transportation, written communication,
October 13, 1995). The opening length of the structure parallel to the bridge face is 26.6 ft.
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 30
degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the upstream
left wingwall and upstream end of the left abutment during the Level I assessment. The
scour protection measures at the site were type-1 stone fill (less than 12 inches diameter)
along the upstream left bank, the upstream left wingwall, and the downstream left bank, and
type-2 stone fill (less than 36 inches diameter) along the downstream right bank. There is
also a laid-up stone wall in front of 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 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 for all modelled flows ranged from 0.0 to 0.5 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.2 to
13.3 ft. The worst-case abutment scour occurred 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.