Level II scour analysis for Bridge 25 (CLARTH00100025) on Town Highway 10, crossing the Clarendon River, Clarendon, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CLARTH00100025 on town highway 10 crossing the Clarendon River, Clarendon,
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). Resultsof a Level I scour investigation also are included in Appendix E of this
report. A Level I study 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 Taconic Section of the New England physiographic province in westcentral Vermont. The 19.3-mi²
drainage area is in a predominantly rural basin. In the
vicinity of the study site, the left and right banks are covered by pasture and (or) fields. The
right bank of Clarendon River is eroded due to stream-flow attack immediately upstream of
the bridge.
In the study area, the Clarendon River has a sinuous channel with a slope of approximately
0.007 ft/ft, an average channel top width of 44 ft and an average channel depth of 3 ft. There
are large meanders approximately 100 feet upstream and downstream of the bridge. The
predominant channel bed materials are gravel and cobbles with a median grain size (D₅₀) of
42.4 mm (0.139 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on April 27, 1995, indicated that the reach was laterally unstable.
The town highway 10 crossing of the Clarendon River was a 27-ft-long, two-lane bridge
consisting of one 24-foot steel stringer with a timber deck (Vermont Agency of
Transportation, written communication, March 13, 1995). The deck was removed at the
time of the survey but the analysis was done as if the old deck was in place. The bridge is
supported on the left by a vertical stone abutment and on the right by a vertical, concrete
abutment with an upstream wingwall. The channel is skewed approximately 10 degrees to
the opening while the opening-skew-to-roadway is 0 degrees.
A scour hole 3 ft deeper than the mean thalweg depth was observed along the right bank
extending from 24 to 60 feet upstream of the bridge. No scour prevention measures were
observed at this site at the time of the site visit. Additional details describing conditions at
the site are included in the Level II Summary and Appendices D and E.
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 0.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.7 to
10.6 ft. The worst-case abutment scour also 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
documented herein.