Level II scour analysis for Bridge 21 (WALDTH00450021) on Town Highway 45, crossing Joes Brook, Walden, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WALDTH00450021 on Town Highway 45 crossing 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 VTAOT files state that the stream is Coles Brook, both the USGS and the
VTAOT maps state that it is Joes Brook.
The site is in the New England Upland section of the New England physiographic province
in central Vermont. The 18.7-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture upstream and downstream
of the bridge while the immediate banks have dense woody vegetation.
In the study area, Joes Brook has an incised, sinuous channel with a slope of approximately
0.01 ft/ft, an average channel top width of 76 ft and an average bank height of 5 ft. The
channel bed material ranges from sand to boulder with a median grain size (D₅₀) of 75.4
mm (0.247 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on July 27, 1995, indicated that the reach was stable.
The Town Highway 45 crossing of Joes Brook is a 35-ft-long, one-lane bridge consisting of
one 29-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 26.2 ft. The bridge is supported by vertical, “laid-up” concrete block
abutments with no wingwalls. The channel is skewed approximately zero degrees to the
opening while the computed opening-skew-to-roadway is 5 degrees.
The scour protection measures at the site included type-1 stone fill (less than 12 inches
diameter) along the upstream left bank and along the entire base length of the left abutment.
There is also type-2 stone fill (less than 36 inches diameter) along the entire base length of
the right abutment. 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 1.5 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 12.4 to 24.4 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”. 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.