This report provides the results of a detailed Level II analysis of scour potential at structure
CLARTH00010014 on town highway 1 crossing the Cold 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). A Level I study is included in Appendix E of this report. A Level I study provides
a qualitative geomorphic characterization of the study site. Information on the bridge
available from VTAOT files was compiled prior to conducting Level I and Level II
analyses and can be found in Appendix D.
The site is in the Taconic Section of the New England physiographic province in westcentral Vermont in the town of Clarendon. The 36.2-mi2
drainage area is in a predominantly
rural basin. In the vicinity of the study site, the surface cover is primarily pasture, except for
the right bank upstream which is forested.
In the study area, the Cold River has a sinuous channel with a slope of approximately 0.02
ft/ft, an average channel top width of 104 ft and an average channel depth of 3 ft. The
predominant channel bed material is cobble with a median grain size (D50) of 103 mm
(0.339 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. This assessment was due to
the cut-banks and the local anabranching occurring upstream of the bridge.
The town highway 1 crossing of the Cold River is a 80-ft-long, two-lane bridge consisting
of one 77-foot span (Vermont Agency of Transportation, written communication, March
13, 1995). The bridge is supported by vertical, concrete abutments with wingwalls. The left
abutment and upstream wingwalls are protected by type-2 stone fill (less than 36 inches
diameter). The channel is skewed approximately 10 degrees to the opening while the
opening-skew-to-roadway is 15 degrees. 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, 1993). 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 modelled flows ranged from 0.0 to 0.6 ft. Abutment scour ranged
from 17.4 to 23.3 ft. The worst-case contraction and 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, 1993, p. 48). 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