Level II scour analysis for Bridge 25 (REDSTH00360025) on Town Highway 36, crossing the West Branch Deerfield River, Readsboro, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
REDSTH00360025 on Town Highway 36 crossing the West Branch Deerfield River,
Readsboro, 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
south-central Vermont. The 14.5-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture on the upstream right
bank and forest on the upstream left bank. The surface cover on the downstream right and
left banks is primarily grass, shrubs and brush.

In the study area, the West Branch Deerfield River has an incised, sinuous channel with a
slope of approximately 0.02 ft/ft, an average channel top width of 65 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 117 mm (0.383 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 1, 1996, indicated that the reach was stable.

The Town Highway 36 crossing of the West Branch Deerfield River is a 59-ft-long, two-lane bridge consisting of one 57-foot concrete T-beam span (Vermont Agency of
Transportation, written communication, September 28, 1995). The opening length of the
structure parallel to the bridge face is 54 ft. The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 50 degrees to the opening
while the opening-skew-to-roadway is 30 degrees.

During the Level I assessment, a scour hole approximately 2 ft deeper than the mean
thalweg depth was observed along the upstream right wingwall and a scour hole
approximately 1 ft deeper than the mean thalweg depth was observed along the downstream
left wingwall. The scour protection measure at the site was type-2 stone fill (less than 36
inches diameter) at the downstream end of the downstream left wingwall, at the upstream
end of the upstream right wingwall, at the downstream end of the right abutment, along the
entire base length of the downstream right wingwall, along the upstream right bank and
along the downstream left bank. A stone wall was noted along the upstream left 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).
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 0.6 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge. Abutment scour ranged
from 15.1 to 16.3 ft along the left abutment and from 7.4 to 9.2 ft along the right abutment.
The worst-case abutment scour occurred at the incipient-overtopping and 500-year
discharges for the left abutment and at the 500-year discharge for the right abutment.
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.