Level II scour analysis for Bridge 5C (CORITH0003005C) on Town Highway 3, crossing Cooksville Brook, Corinth, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CORITH0003005C on Town Highway 3 crossing Cooksville Brook, Corinth, 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 New England Upland section of the New England physiographic province
in east-central Vermont. The 20.2-mi² drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture with a residence on the
upstream right bank near the bridge. The immediate channel banks have some woody
vegetation cover.

In the study area, Cooksville Brook has an incised, sinuous channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 46 ft and an average channel
depth of 8 ft. The channel bed material ranged from sand to cobble and had a median grain
size (D₅₀) of 41.0 mm (0.135 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 5, 1995, indicated that the reach was stable.

The Town Highway 3 crossing of Cooksville Brook is a 39-ft-long, two-lane bridge
consisting of one 37-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 17, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls on the left abutment. The channel is skewed approximately 30 degrees to
the opening while the opening-skew-to-roadway is 0 degrees.

A scour hole 0.5 ft deeper than the mean thalweg depth was observed along the right
abutment during the Level I assessment. The only scour protection measures at the site were
type-2 stone fill (less than 36 inches diameter) at the upstream and downstream ends of the
right abutment and type-4 (less than 60 inches diameter) along the upstream right bank
below the residence. Also, there is a wall along 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 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 2.7 to 3.3 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.0 to
19.0 ft. The worst-case left abutment scour occurred at the incipient overtopping discharge.
The worst-case right 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
documented herein.