This report provides the results of a detailed Level II analysis of scour potential at structure
HUNTTH001007H on Town Highway 1 crossing the Cobb Brook, Huntington, Vermont
(figures 1–10). 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.
In August 1976, Hurricane Belle caused flooding at this site which resulted in road and
bridge damage (figures 7-8). This was approximately a 25-year flood event (U.S.
Department of Housing and Urban Development, 1978).
The site is in the Green Mountain section of the New England physiographic province in
central Vermont. The 4.20-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.
Downstream of the bridge is brushland and pasture.
In the study area, the Cobb Brook has an incised, straight channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 43 ft and an average bank height
of 6 ft. The channel bed material ranges from sand to boulders with a median grain size
(D50) of 65.5 mm (0.215 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 24, 1996, indicated that the reach was stable.
The Town Highway 1 crossing of the Cobb Brook is a 23-ft-long, two-lane bridge
consisting of one 20-foot concrete slab span (Vermont Agency of Transportation, written
communication, June 21, 1996). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 15 degrees to the opening while the
opening-skew-to-roadway is zero degrees.
A scour hole 2.8 ft deeper than the mean thalweg depth was observed along the left
abutment during the Level I assessment. Protection measures at the site include type-1 stone
fill (less than 12 inches diameter) at the downstream right wingwall, type-2 stone fill (less
than 36 inches diameter) at the upstream right wingwall and the downstream end of the
downstream left wingwall, and type-3 stone fill (less than 48 inches diameter) at the
upstream left wingwall. 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
Contraction scour for all modelled flows ranged from 0.2 to 1.3 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge. Abutment scour ranged
from 4.0 to 8.7 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 10. 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