Level II scour analysis for Bridge 22 (WALDTH00180022) on Town Highway 18, crossing Coles Brook, Walden, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WALDTH00180022 on Town Highway 18 crossing Coles Brook also known as Joes
Brook, Walden, 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 northeastern Vermont. The 12.5-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is predominantly forested
while the downstream left bank is shrub and brushland.
In the study area, the Coles Brook has an incised, sinuous channel with a slope of
approximately 0.004 ft/ft, an average channel top width of 54 ft and an average bank height
of 4 ft. The channel bed material ranges from gravel to bedrock with a median grain size
(D50) of 124.1 mm (0.407 ft). The D₅₀ was taken from a pebble count in the downstream
channel, because the upstream channel is primarily bedrock. The geomorphic assessment at
the time of the Level I and Level II site visit on August 8, 1995, indicated that the reach was
stable.
The Town Highway 18 crossing of the Coles Brook is a 46-ft-long, one-lane bridge
consisting of one 44-foot steel-beam span with a wooden deck (Vermont Agency of
Transportation, written communication, April 5, 1995). The opening length of the structure
parallel to the bridge face is 41.4 ft. The bridge is supported by a vertical, concrete abutment
with wingwalls on the left and by a vertical, stone abutment with stone wingwalls with a
concrete cap on the right. The channel is skewed approximately 5 degrees to the opening
while the computed opening-skew-to-roadway is 10 degrees.
The only scour protection measure at the site was a stone wall along the upstream left bank
and type-2 stone fill (less than 36 inches diameter) along the entire base length of the
upstream left wingwall, left abutment, and downstream left wing wall. 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)
for the 100-year and 500-year discharges. 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 was 0.0 ft. Abutment scour ranged from 6.4 to 7.9
ft at the left abutment and from 11.8 to 14.9 ft at the right abutment. 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”. Scouredstreambed 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 particlesize 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.