This report provides the results of a detailed Level II analysis of scour potential at structure
ROCHVT01000144 on State Route 100 crossing the White River, Rochester, 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 68.9-mi2
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is pasture with forest on the valley walls.
In the study area, the White River has a meandering channel with a slope of approximately
0.003 ft/ft, an average channel top width of 119 ft and an average channel depth of 4 ft. The
predominant channel bed material is gravel and cobbles with a median grain size (D50) of
72.5 mm (0.238 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on July 22, 1996, indicated that the reach was laterally unstable due to a cut-bank
present on the upstream left bank and wide point bars upstream and downstream in the
vicinity of this site.
The State Route 100 crossing of the White Riveris a 103-ft-long, two-lane bridge
consisting of one 101-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 8, 1995). The bridge is supported by vertical, concrete abutment
walls with spill-through embankments in front of each abutment wall and no wingwalls.
The channel is skewed approximately 10 degrees to the opening while the opening-skew-toroadway is 5 degrees.
The scour protection measures at the site are type-2 stone fill (less than 36 inches diameter)
on the upstream left bank, both abutment spill-through embankments, and the downstream
banks. There also is type-1 stone fill (less than 12 inches diameter) on the upstream right
bank. The stone fill is continuous on both sides of the river in the vicinity of the bridge.
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.
There was no computed contraction scour for the modelled flows. Abutment scour ranged
from 6.9 to 10.9 ft. The worst-case abutment scour occurred at the incipient overtopping
discharge, which was less than the 100-year discharge. Additional information on scour
depths and depths to armoring are included in the section titled “Scour Results”. Scouredstreambed 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 particlesize 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
Additional Publication Details
USGS Numbered Series
Level II scour analysis for Bridge 144 (ROCHVT01000144) on State Route 100, crossing the White River, Rochester, Vermont