This report provides the results of a detailed Level II analysis of scour potential at structure
SHEFTH00440019 on Town Highway 44 crossing Trout Brook, Sheffield, 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 White Mountain section of the New England physiographic province in
northeastern Vermont. The 3.0-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is grass on the upstream and
downstream right overbanks, while the immediate banks have dense woody vegetation.
The surface cover of the upstream and downstream left overbanks is shrub and brushland.
In the study area, Trout Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 45 ft and an average bank height
of 6 ft. The channel bed material ranges from sand to boulder with a median grain size (D50)
of 116 mm (0.381 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on July 31, 1995, indicated that the reach was stable.
The Town Highway 44 crossing of Trout Brook is a 24-ft-long, one-lane bridge consisting
of a 22-foot steel-stringer span (Vermont Agency of Transportation, written
communication, March 28, 1994). The opening length of the structure parallel to the bridge
face is 19.8 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 10 degrees to the opening while the opening-skew-toroadway is zero degrees.
Scour depths and recommended rock rip-rap sizes were computed using the general
guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge was analyzed since it has the potential of being the worst-case scour scenario.
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
Contraction scour for all modelled flows resulted in zero ft. Left abutment scour ranged
from 4.4 to 5.6 ft. The worst-case left abutment scour occurred at the 500-year discharge.
Right abutment scour ranged from 3.6 to 4.8 ft. The worst-case right abutment scour
occurred at the incipient roadway-overtopping 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 19 (SHEFTH00440019) on Town Highway 44, crossing Trout Brook, Sheffield, Vermont