|Abstract:||This report provides the results of a detailed Level II analysis of scour potential at structure
ANDOVT00110041 on State Route 11 crossing the Middle Branch Williams River,
Andover, 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
southeastern Vermont. The 12.1-mi2
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is grass on the upstream right
overbank while the immediate banks have dense woody vegetation. The upstream left
overbank and downstream right overbank are brushland. The downstream left overbank is
In the study area, the Middle Branch Williams River has an incised, sinuous channel with a
slope of approximately 0.018 ft/ft, an average channel top width of 71 ft and an average
bank height of 4 ft. The channel bed material ranges from gravel to boulders with a median
grain size (D50) of 85.0 mm (0.279 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on September 10, 1996, indicated that the reach was laterally
unstable due to a cut-bank present on the upstream right bank and a wide channel bar with
vegetation in the upstream reach.
The State Route 11 crossing of the Middle Branch Williams River is a 46-ft-long, two-lane
bridge consisting of a concrete 44-foot tee-beam span (Vermont Agency of Transportation,
written communication, March 29, 1995). The opening length of the structure parallel to the
bridge face is 42 ft. The bridge is supported by vertical, concrete abutments with wingwalls.
The channel is skewed approximately 35 degrees to the opening while the opening-skew-toroadway is zero degrees.
A scour hole 0.8 ft deeper than the mean thalweg depth was observed along the downstream
end of the left abutment and downstream left wingwall during the Level I assessment. Type-
2 stone fill (less than 36 inches diameter) protects the upstream end of the upstream left
wingwall, the downstream ends of the downstream left and right wingwalls and the
downstream right road embankment. Type-3 stone fill protects the upstream end of the
upstream right wingwall and the upstream right bank. 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 was determined and analyzed as
another potential 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 0.0 to 2.1 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 11.1 to
18.7 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
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.; Flynn, Robert H.
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|Citation Language: ||English
|Citation Larger Work Title: ||
|Citation LatN: ||44.5
|Citation LatS: ||44.375
|Citation LonE: ||-73.0
|Citation LonW: ||-73.125
|Citation Month: ||
|Citation No Pagination: ||
|Citation Number Of Pages: ||57
|Citation Online Only Flag: ||
|Citation Phsyical Description: ||iv, 52 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: ||97-771
|Citation Search Results Text: ||Level II scour analysis for Bridge 41 (ANDOVT00110041) on State Route 11, crossing the Middle Branch Williams River, Andover, Vermont; 1997; OFR; 97-771; Wild, Emily C.; Flynn, Robert H.
|Citation Start Page: ||
|Citation Volume: ||
|Citation Year: ||1997
|Text: ||Level II scour analysis for Bridge 41 (ANDOVT00110041) on State Route 11, crossing the Middle Branch Williams River, Andover, Vermont; 1997; OFR; 97-771; Wild, Emily C.; Flynn, Robert H.
|URL (THUMBNAIL): ||http://pubs.er.usgs.gov/thumbnails/ofr97771.PNG
|URL (DOCUMENT): ||http://pubs.usgs.gov/of/1997/0771/report.pdf
|Date Other: ||Sat, 1 Jan 1994 00:00 -0600
|Publisher: ||U.S. Geological Survey