Level II scour analysis for Bridge 29 (LONDTH00410029) on Town Highway 41, crossing Cook Brook, Londonderry, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
LONDTH00410029 on Town Highway 41 crossing Cook Brook, Londonderry, 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 6.48-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is shrub on the left bank upstream
and downstream of the bridge while the right bank is forest upstream and downstream of the
bridge.
In the study area, Cook Brook has a straight incised channel with a slope of approximately
0.02 ft/ft, an average channel top width of 80 ft and an average bank height of 8 ft. The
channel bed material ranges from sand to cobble with a median grain size (D₅₀) of 70.9 mm
(0.233 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
August 7, 1996, indicated that the reach was stable.
The Town Highway 41 crossing of Cook Brook is a 25-ft-long, one-lane bridge consisting
of one 22-foot steel-beam span (Vermont Agency of Transportation, written
communication, April 6, 1995). The opening length of the structure parallel to the bridge
face is 20.4 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 5 degrees to the opening while the opening-skew-toroadway is 0 degrees.
A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the upstream
left wingwall and left abutment during the Level I assessment. The only scour protection
measure at the site was type-2 stone fill (less than 36 inches) along the downstream end of
the left abutment and upstream end of the downstream right wingwall. 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 1.5. Abutment scour ranged
from 8.4 to 15.1 ft. 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”. 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.