This report provides the results of a detailed Level II analysis of scour potential at structure
WOLCTH00150005 on Town Highway 15 crossing the Wild Branch Lamoille River, Wolcott,
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.
During the August 1995 and July 1997 flood events, the left roadway was overtopped.
Although there was loss of stone fill along the right abutment, the structure withstood both
The site is in the Green Mountain section of the New England physiographic province in north-
central Vermont. The 38.3-mi2 drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the surface cover is pasture upstream and downstream of the
bridge, while the immediate banks have dense woody vegetation.
In the study area, the Wild Branch Lamoille River has an incised, sinuous channel with a slope
of approximately 0.006 ft/ft, an average channel top width of 98 ft and an average bank height
of 5 ft. The channel bed material ranges from gravel to bedrock with a median grain size (D50)
of 89.1 mm (0.292 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on July 17, 1996, indicated that the reach was stable.
The Town Highway 15 crossing of the Wild Branch Lamoille River is a 46-ft-long, two-lane
bridge consisting of a 43-foot prestressed concrete box-beam span (Vermont Agency of
Transportation, written communication, October 13, 1995). The opening length of the structure
parallel to the bridge face is 42 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-to-roadway is zero degrees.
A scour hole 2.0 ft deeper than the mean thalweg depth was observed near the bridge along
the left side of the channel during the Level I assessment. Scour countermeasures at the site
consists of type-1 stone fill (less than 12 inches diameter) along the upstream left bank and
along the left and right downstream banks, type-2 stone fill (less than 36 inches diameter)
along the downstream left and right wingwalls, type-3 stone fill (less than 48 inches
diameter) along the upstream left wingwall and the right abutment, and type-4 stone fill
(less than 60 inches diameter) along the upstream right wingwall and the left abutment.
Additional details describing conditions at the site are included in the Level II Summary
and Appendices D and E.
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 determined and analyzed as another potential 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 was zero ft. Left abutment scour ranged from 7.9
to 23.3 ft. The worst-case left abutment scour occurred at the 500-year discharge. Right
abutment scour ranged from 21.5 to 22.8 ft. The worst-case right abutment scour occurred
at the incipient roadway-overtopping discharge. Additional in formation 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
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