|Abstract:||This report provides the results of a detailed Level II analysis of scour potential at structure
DUXBTH00120037 on Town Highway 12 crossing Ridley Brook, Duxbury, 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
north central Vermont. The 10.1-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, Ridley Brook has an incised, straight channel with a slope of
approximately 0.04 ft/ft, an average channel top width of 67 ft and an average bank height
of 9 ft. The channel bed material ranges from gravel to boulders with a median grain size
(D50) of 123 mm (0.404 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on July 1, 1996, indicated that the reach was stable.
The Town Highway 12 crossing of Ridley Brook is a 33-ft-long, two-lane bridge consisting
of five 30-ft steel rolled beams (Vermont Agency of Transportation, written
communication, October 13, 1995). The opening length of the structure parallel to the
bridge face is 30 ft. The bridge is supported by vertical, concrete abutments with wingwalls.
The channel is skewed approximately 50 degrees to the opening while the measured
opening-skew-to-roadway is 20 degrees.
A scour hole 2 ft deeper than the mean thalweg depth was observed along the right
abutment and downstream right wingwall during the Level I assessment. Scour
countermeasures at the site include type-2 stone fill (less than 3 feet diameter) along the
upstream and downstream left road embankments, and type-3 stone fill (less than 4 feet
diameter) along the upstream right bank and upstream right wingwall. Additional details
describing conditions at the site are included in the Level II Summary and Appendices D
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 0.6 to 1.7 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 5.0
to 8.3 ft, with the worst-case occurring at the incipient-overtopping discharge. Right
abutment scour ranged from 13.1 to 16.7 ft, with the worst-case occurring 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
|Genre: ||USGS Numbered Series
|Citation Author: ||Wild, Emily C.; Ivanhoff, Michael A.
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|Citation Language: ||English
|Citation Larger Work Title: ||Open-File Report
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|Citation Number Of Pages: ||57
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|Citation Phsyical Description: ||iv, 52 p.
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|Citation Publisher: ||U.S. Geological Survey
|Citation Series: ||Open-File Report
|Citation Series Code: ||OFR
|Citation Series Number: ||97-405
|Citation Search Results Text: ||Level II scour analysis for Bridge 37 (DUXBTH00120037) on Town Highway 12, crossing Ridley Brook, Duxbury, Vermont; 1997; OFR; 97-405; Open-File Report; Wild, Emily C.; Ivanhoff, Michael A.
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|Citation Year: ||1997
|Text: ||Level II scour analysis for Bridge 37 (DUXBTH00120037) on Town Highway 12, crossing Ridley Brook, Duxbury, Vermont; 1997; OFR; 97-405; Open-File Report; Wild, Emily C.; Ivanhoff, Michael A.
|URL (THUMBNAIL): ||http://pubs.er.usgs.gov/thumbnails/ofr97405.PNG
|URL (DOCUMENT): ||http://pubs.usgs.gov/of/1997/0405/report.pdf
|Date Other: ||Sat, 1 Jan 1994 00:00 -0600
|Publisher: ||U.S. Geological Survey