This report provides the results of a detailed Level II analysis of scour potential at structure
HUNTTH00290029 on Town Highway 29 crossing Cobb Brook, Huntington, 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.
The site is in the Green Mountain section of the New England physiographic province in
northwestern Vermont. The 4.16-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest upstream and downstream
of the bridge.
In the study area, Cobb Brook has an incised, straight channel with a slope of approximately
0.024 ft/ft, an average channel top width of 53 ft and an average bank height of 4 ft. The
channel bed material ranges from gravel to bedrock with a median grain size (D50) of 112.0
mm (0.367 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on June 25, 1996, indicated that the reach was stable.
The Town Highway 29 crossing of Cobb Brook is a 36-ft-long, one-lane bridge consisting
of one 30-foot steel-beam span (Vermont Agency of Transportation, written
communication, December 11, 1995) and a wooden deck. The opening length of the
structure parallel to the bridge face is 27 ft.The bridge is supported by vertical, concrete
abutments. The channel is skewed approximately 25 degrees to the opening while the
opening-skew-to-roadway was measured to be 20 degrees. VTAOT records indicate an
opening-skew-to-roadway of zero degrees.
A scour hole 1.5 ft deeper than the mean thalweg depth was observed extending from 12 ft
upstream of the upstream end of the left abutment to 10 ft under the bridge in the center of
the channel during the Level I assessment. Another scour hole approximately 1.2 ft deeper
than the mean thalweg depth was observed along the downstream end of the right abutment
during the Level I assessment. The scour protection measures at the site included type-2
stone fill (less than 36 inches diameter) along the upstream end of the right abutment and
type-3 stone fill (less than 48 inches diameter) along the upstream end of the upstream left
retaining wall. 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 was computed to be zero ft. Abutment scour
ranged from 9.9 to 12.5 ft along the left abutment and from 6.2 to 8.6 ft along the right
abutment. 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