Level II scour analysis for Bridge 8 (WELLTH00020008) on Town Highway 2, crossing Wells Brook, Wells, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WELLTH00020008 on Town Highway 2 crossing the Wells Brook, Wells, 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 Taconic section of the New England physiographic province in southwestern Vermont. The 14.4-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover on the right overbanks is
predominantly suburban while the immediate banks are vegetated with trees and brush. The
left bank upstream and downstream is predominantly pasture.
In the study area, the Wells Brook has an incised, straight channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 51 ft and an average bank height
of 7 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 48.6 mm (0.159 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 19, 1995, indicated that the reach was stable.
The Town Highway 2 crossing of the Wells Brook is a 35-ft-long, two-lane bridge
consisting of one 32-foot concrete span (Vermont Agency of Transportation, written
communication, March 22, 1995). The opening length of the structure parallel to the bridge
face is 31.7 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 10 degrees to the opening while the opening-skew-toroadway is 5 degrees.
A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the left
abutment during the Level I assessment. The only scour protection measure at the site was
type-2 stone fill (less than 36 inches diameter) along the left bank upstream, and type-5
(placed stone wall) at the upstream end of the upstream left wingwall, at the downstream
end of the downstream left wingwall, and along the downstream 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)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 incipient roadway-overtopping discharge, which was less
than the 100-year discharge. Abutment scour ranged from 5.6 to 10.0 ft at the left abutment
and from 3.1 to 4.2 ft at the right abutment. The worst-case abutment scour occurred at the
incipient roadway-overtopping discharge at the left 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.
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.