Level II scour analysis for Bridge 32 (HUNTTH00220032) on Town Highway 22, crossing Brush Brook, Huntington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HUNTTH00220032 on Town Highway 22 crossing Brush Brook, Huntington, 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
central Vermont. The 5.7-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is forest except on the downstream right
overbank which is pasture.

In the study area, Brush Brook has an incised, straight channel with a slope of
approximately 0.05 ft/ft, an average channel top width of 58 ft and an average bank height
of 6 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 127 mm (0.416 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 22 crossing of Brush Brook is a 36-ft-long, one-lane bridge consisting
of one 34-foot steel-beam span and a timber deck (Vermont Agency of Transportation,
written communication, December 12, 1995). The opening length of the structure parallel to
the bridge face is 35.7 ft. The bridge is supported by vertical, concrete abutments with
wingwalls on the left. The channel is skewed approximately 50 degrees to the opening
while the measured opening-skew-to-roadway is 15 degrees.

A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the left
abutment and downstream left wingwall 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
upstream right 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 0.2 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 6.4 to
10.2 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.