Level II scour analysis for Bridge 12 (HUNTTH00010012) on Town Highway 001, crossing Brush Brook, Huntington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HUNTTH00010012 on Town Highway 1 crossing Brush Brook, Huntington, Vermont
(figures 1–9). 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.

In August 1976, Hurricane Belle caused flooding at this site which resulted in road and
bridge damage (figures 7-8). This was approximately a 25-year flood event based on flood-
frequency data contained in the Flood Insurance Study for the Town of Huntington (U.S.
Department of Housing and Urban Development, 1978).

The site is in the Green Mountain section of the New England physiographic province in
central Vermont. The 9.19-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is pasture while the immediate banks have
some woody vegetation.

In the study area, the Brush Brook has a sinuous channel with a slope of approximately 0.02
ft/ft, an average channel top width of 62 ft and an average bank height of 5 ft. The channel
bed material ranges from gravel to cobble with a median grain size (D₅₀) of 100.0 mm
(0.328 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
June 25, 1996, indicated that the reach was stable.

The Town Highway 1 crossing of Brush Brook is a 64-ft-long, two-lane bridge consisting of
one 62-foot steel-stringer span (Vermont Agency of Transportation, written
communication, November 30, 1995). The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 10 degrees to the opening
while the opening-skew-to-roadway is 6 degrees.

Channel scour 2.2 ft deeper than the mean thalweg depth was observed along the upstream
right bank and along the base of the spill-through protection for the right abutment during
the Level I assessment. Scour protection measured at the site was type-2 stone fill (less
than 36 inches diameter) along the upstream left and right banks and in front of all four
wingwalls. In front of the abutments, there was type-3 stone fill (less than 48 inches
diameter) forming a spill-through slope. 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.

There was no computed contraction scour for any modelled flow. Abutment scour ranged
from 1.4 to 2.8 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 9. 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.