This report provides the results of a detailed Level II analysis of scour potential at structure
JERITH00200038 on Town Highway 20 crossing the Lee River, Jericho, 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, obtained from Vermont Agency of Transportation (VTAOT)
files, was compiled prior to conducting Level I and Level II analyses and is found in
The site is in the Green Mountain section of the New England physiographic province and
the Champlain section of the St. Lawrence physiographic province in northwestern
Vermont. The 12.9-mi2
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the surface cover on the upstream and downstream right overbank
is pasture while the immediate banks have dense woody vegetation. The surface cover on
the upstream and downstream left overbank is forested.
In the study area, the Lee River has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 89 ft and an average bank height
of 14 ft. The channel bed material ranges from sand to boulder with a median grain size
(D50) of 45.9 mm (0.151 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 2, 1996, indicated that the reach was stable.
The Town Highway 20 crossing of the Lee River is a 49-ft-long, one-lane bridge consisting
of a steel through truss span (Vermont Agency of Transportation, written communication,
December 12, 1995). The opening length of the structure parallel to the bridge face is 44 ft.
The bridge is supported by vertical, concrete abutments with wingwalls. The channel is
skewed approximately 10 degrees to the opening while the computed opening-skew-toroadway is 5 degrees.
A scour hole 1 ft deeper than the mean thalweg depth was observed in the center of the
channel during the Level I assessment. Scour countermeasures at the site include type-1
stone fill (less than 12 inches diameter) at the downstream left road embankment. Type-2
stone fill (less than 36 inches diameter) protects the upstream left wingwall, the upstream
and downstream right wingwalls and the upstream end of the right abutment. Type-3 stone
fill (less than 48 inches diameter) protects 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. 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 was zero. Abutment scour ranged from 4.9 to 10.7
ft. The worst-case abutment 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
Additional Publication Details
USGS Numbered Series
Level II scour analysis for Bridge 38 (JERITH0020038) on Town Highway 20, crossing the Lee River, Jericho, Vermont