Level II scour analysis for Bridge 120 (LEICUS00070120) on U.S. Route 7, crossing the Leicester River, Leicester, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
LEICUS00070120 on U. S. Route 7 crossing the Leicester River, Leicester, 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
west-central Vermont. The 23.0-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of shrubs, brush, and
pasture with some trees except for the upstream left overbank area which is forest.
In the study area, the Leicester River has a sinuous channel with a slope of approximately
0.002 ft/ft, an average channel top width of 52 ft and an average channel depth of 3 ft. The
predominant channel bed material is sand and gravel with a median grain size (D₅₀) of 3.10
mm (0.0102 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on September 18, 1995, indicated that the reach was laterally unstable. Lateral instability
was evident with the presence of some bank material failure and fallen or leaning vegetation
at cut-banks upstream and downstream of this site. Point bars also were found near this site.
The U. S. Route 7 crossing of the Leicester Riveris a 108-ft-long, two-lane bridge
consisting of two 52-foot steel-beam spans (Vermont Agency of Transportation, written
communication, March 13, 1995). The bridge is supported by vertical, concrete abutment
walls with stone fill spill-through embankments on each abutment and one pier. The
channel is skewed approximately 30 degrees to the opening while the opening-skew-toroadway is 15 degrees.
The only scour protection measure at the site was type-3 stone fill (less than 48 inches
diameter) on the spill-through embankments of each abutment. 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 3.8 to 6.1 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 4.0 to
6.7 ft. The worst-case abutment scour also occurred at the 500-year discharge. Pier scour
ranged from 9.1 to 10.2. The worst-case pier 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.