|Abstract:||This report provides the results of a detailed Level II analysis of scour potential at structure
RICHVT01050036 on State Route 105 crossing Stanhope Brook, Richford, 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 7.03-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is short grass except for the
upstream left overbank area which is forested.
In the study area, Stanhope Brook has a steep, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 47 ft and an average channel
depth of 5 ft. The predominant channel bed material is cobble with a median grain size
(D50) of 132 mm (0.432 ft). The geomorphic assessment at the time of the Level I and Level
II site visit on June 28, 1995, indicated that the reach was laterally unstable.
The State Route 105 crossing of Stanhope Brook is a 42-ft-long, two-lane bridge consisting
of one 38-foot concrete T-beam span (Vermont Agency of Transportation, written
communication, March 8, 1995). 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 20 degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the downstream
end of the right abutment wall during the Level I assessment. The scour protection measures
at this site were type-2 stone fill (less than 36 inches diameter) along the entire lengths of
the upstream wingwalls, at the corner of the downstream left abutment and downstream left
wingwall and the downstream end of the downstream right wingwall. Additional details
describing conditions at the site are included in the Level II Summary and Appendices D
Scour depths and 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 computations follows.
Contraction scour for all modelled flows ranged from 0.0 to 0.3 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.6 to
9.4 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
|Genre: ||USGS Numbered Series
|Citation Author: ||Boehmler, Erick M.; Ivanoff, Michael A.
|Citation Contributing Office: ||
|Citation Datum: ||
|Citation Day: ||
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|Citation Language: ||English
|Citation Larger Work Title: ||
|Citation LatN: ||45.015684
|Citation LatS: ||44.921965
|Citation LonE: ||-72.535709
|Citation LonW: ||-72.702647
|Citation Month: ||
|Citation No Pagination: ||
|Citation Number Of Pages: ||52
|Citation Online Only Flag: ||
|Citation Phsyical Description: ||iv, 47 p.
|Citation Projection: ||
|Citation Public Comments: ||Prepared in cooperation with Vermont Agency of Transportation and Federal Highway Administration
|Citation Publisher: ||U.S. Geological Survey
|Citation Series: ||Open-File Report
|Citation Series Code: ||OFR
|Citation Series Number: ||96-584
|Citation Search Results Text: ||Level II scour analysis for Bridge 36 (RICHVT01050036) on State Route 105, crossing Stanhope Brook, Richford, Vermont; 1996; OFR; 96-584; Boehmler, Erick M.; Ivanoff, Michael A.
|Citation Start Page: ||
|Citation Volume: ||
|Citation Year: ||1996
|Text: ||Level II scour analysis for Bridge 36 (RICHVT01050036) on State Route 105, crossing Stanhope Brook, Richford, Vermont; 1996; OFR; 96-584; Boehmler, Erick M.; Ivanoff, Michael A.
|URL (THUMBNAIL): ||http://pubs.er.usgs.gov/thumbnails/ofr96584.PNG
|URL (DOCUMENT): ||http://pubs.usgs.gov/of/1996/0584/report.pdf
|Date Other: ||Sat, 1 Jan 1994 00:00 -0600
|Publisher: ||U.S. Geological Survey