Level II scour analysis for Bridge 2 (STAMVT01000002) on State Route 100 crossing Roaring Brook, Stamford, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
STAMVT01000002 on State Route 100 crossing Roaring Brook, Stamford, 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 8.26-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of houses with grass lawns,
and trees on the right overbank areas upstream and downstream of the bridge. The left
overbank areas upstream and downstream of the bridge are covered with trees and brush.
In the study area, Roaring Brook has a straight channel with a slope of approximately 0.02
ft/ft, an average channel top width of 56 ft and an average bank height of 5 ft. The channel
bed materials range from gravel to boulders with a median grain size (D₅₀) of 53.7 mm
(0.176 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 aggraded.
The State Route 100 crossing of Roaring Brook is a 44-ft-long, two-lane bridge consisting
of one 42-foot steel-beam span (Vermont Agency of Transportation, written
communication, September 28, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 5 degrees to the opening
and the opening-skew-to-roadway is 5 degrees.
Scour protection measures at the site were type-2 stone fill (less than 36 inches diameter) on
the upstream banks and wingwalls, type-3 (less than 48 inches diameter) on the downstream
wingwalls, and artificial levees made from a variety of materials on the downstream banks.
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 0.8 feet. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 4.2 to
9.3 feet. The worst-case abutment scour occurred at the 500-year discharge at the left
abutment. 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.