This report provides the results of a detailed Level II analysis of scour potential at structure
JERITH00350031 on Town Highway 35 crossing Mill Brook, 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, gathered 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 15.7-mi2
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the surface cover is forest upstream of the bridge. The downstream
left overbank is pasture. The downstream right overbank is brushland.
In the study area, the Mill Brook has an incised, sinuous channel with a slope of
approximately 0.02 ft/ft, an average channel top width of 117 ft and an average bank height
of 11 ft. The channel bed material ranges from gravel to boulders with a median grain size
(D50) of 81.1 mm (0.266 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 3, 1996, indicated that the reach was laterally unstable.
The Town Highway 35 crossing of the Mill Brook is a 53-ft-long, one-lane bridge
consisting of a 50-foot steel-beam span with a wooden deck (Vermont Agency of
Transportation, written communication, November 30, 1995). The opening length of the
structure parallel to the bridge face is 48 ft. The bridge is supported by a vertical, concrete
abutment with wingwalls on the left. On the right, the abutment and wingwalls are laid-up
stone with a concrete cap. The channel is not skewed to the opening. The roadway is
skewed 10 degrees to the opening.
A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the left
abutment during the Level I assessment. Scour countermeasures at the site were type-2
stone fill (less than 36 inches diameter) at the upstream and downstream left wingwalls, the
upstream and downsteam left channel banks, and the downstream left road embankment.
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).
In addition, the incipient roadway-overtopping discharge is 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 computations follows.
Contraction scour for all modelled flows ranged from 0.4 to 1.3 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 9.9
to 12.4 ft. Right abutment scour ranged from 13.8 to 17.8 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 31 (JERITH00350031) on Town Highway 35, crossing Mill Brook, Jericho, Vermont