Level II scour analysis for Bridge 99 (LUDLVT01000099) on State Highway 99, crossing Branch Brook, Ludlow, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
LUDLVT01000099 on State Highway 100 crossing Branch Brook, Ludlow, 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 15.7-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the channel banks are densely covered by trees and brush.
The overbanks are primarily covered by field grasses.
In the study area, Branch Brook has an incised, straight channel with a slope of
approximately 0.003 ft/ft, an average channel top width of 73 ft and an average channel
depth of 5 ft. The predominant channel bed materials are cobble and gravel with a median
grain size (D₅₀) of 60.5 mm (0.198 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on October 13, 1995, indicated that the reach was stable.
The State Highway 100 crossing of Branch Brook is a 84-ft-long, two-lane bridge
consisting of one 82-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 13, 1995). The bridge is supported by vertical, concrete abutments.
The abutments are set back from the channel edge and have a spill-through slope consisting
of type-4 stone fill (median size less than 60 inches in diameter). The channel skew and the
opening-skew-to-roadway is zero degrees. 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 500-year discharge. Abutment scour ranged from 1.0 to
7.4 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.