This report provides the results of a detailed Level II analysis of scour potential at structure
WOLCTH00130023 on Town Highway 13 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, collected 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 northcentral Vermont. The 27.7-mi2
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the surface cover is pasture on the upstream right overbank. The upstream
left overbank is brushland. Downstream of the bridge, the surface cover is forested on the right
overbank. The downstream left overbank is pasture while the immediate bank has dense woody
In the study area, the Wild Branch Lamoille River has an incised, straight channel with a slope of
approximately 0.009 ft/ft, an average channel top width of 65 ft and an average bank height of 7 ft.
The channel bed material ranges from sand to boulders with a median grain size (D50) of 85.3 mm
(0.280 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 laterally unstable.
The Town Highway 13 crossing of the Wild Branch Lamoille River is a 41-ft-long, one-lane bridge
consisting of a 39-foot steel girder span (Vermont Agency of Transportation, written
communication, October 13, 1995). The opening length of the structure parallel to the bridge face is
38 ft. The bridge is supported by vertical, concrete abutments. The right abutment has concrete
wingwalls. The channel is skewed approximately 45 degrees to the opening while the opening-skew-to-roadway is zero degrees.
A scour hole 3.5 ft deeper than the mean thalweg depth was observed in the channel during the
Level I assessment. Scour countermeasures at the site includes type-2 stone fill (less than 3 feet
diameter) along the banks, the right wingwalls, the right abutment and the road embankments.
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). 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 1.0 to 2.1 ft. The worst-case contraction
scour occurred at the 100-year discharge. Left abutment scour ranged from 9.1 to 13.2 ft. Right
abutment scour ranged from 15.7 to 22.3 ft. The worst-case abutment scour occurred at the 500-
year discharge for both abutments. 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.
During the August 1995 flood, the Wild Branch Lamoille River overtopped the bridge deck at
structure WOLCTH00130023. Debris also was caught in the upstream I-beam of the structure.
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.
Additional publication details
USGS Numbered Series
Level II scour analysis for Bridge 23 (WOLCTH00130023) on Town Highway 13, crossing the Wild Branch of the Lamoille River, Wolcott, Vermont