Level II scour analysis for Bridge 13 (POMFTH00020013) on Town Highway 2, crossing Barnard Brook, Pomfret, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
POMFTH00020013 on town highway 2 crossing Barnard Brook, Pomfret, 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 study 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 can be found in
Appendix D.
The site is in the New England Upland section of the New England physiographic province
of east-central Vermont in the town of Pomfret. The 7.98-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the surface cover is
primarily field grasses with some brush on the immediate banks.
In the study area, Barnard Brook has an incised, sinuous channel with a slope of
approximately 0.006 ft/ft, an average channel top width of 32 ft and an average channel
depth of 4 ft. The predominant channel bed materials are gravel and cobbles with a median
grain size (D₅₀) of 51.0 mm (0.167 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on September 15, 1994, indicated that the reach was stable.
The town highway 2 crossing of Barnard Brook is a 23-ft-long, two-lane bridge consisting
of one 20-foot concrete span (Vermont Agency of Transportation, written communication,
August 22, 1994). The bridge is supported by vertical, concrete abutments with wingwalls.
The channel is skewed approximately 30 degrees to the opening while the opening-skew-toroadway is 0 degrees.
Scour, 2.5 ft deeper than the mean thalweg depth, was observed along the left abutment
during the Level I assessment. The only scour protection measure at the site was type-2
stone fill (less than 36 inches diameter) along the base and upstream of the upstream left
wingwall. 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 1.5 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 7.2 to
12.6 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.