Level II scour analysis for Bridge 34 (HUNTTH00210034) on Town Highway 21, crossing Brush Brook, Huntington, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HUNTTH00210034 on Town Highway 21 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 6.23-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is forest.

In the study area, Brush Brook has an incised, straight channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 43 ft and an average bank height
of 4 ft. The channel bed material ranges from gravel to boulder with a median grain size
(D₅₀) of 90.0 mm (0.295 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on June 26, 1996, indicated that the reach was stable.

The Town Highway 21 crossing of Brush Brook is a 28-ft-long, one-lane bridge consisting
of one 26-foot steel-beam span with a timber deck (Vermont Agency of Transportation,
written communication November 30, 1995). The opening length of the structure parallel to
the bridge face is 25.4 ft. The bridge is supported by vertical, concrete abutments with a
wingwall on the upstream right. The channel is skewed approximately 5 degrees to the
opening and the computed opening-skew-to-roadway is 5 degrees.

A tributary enters Brush Brook on the right bank immediately downstream of the bridge.
At the confluence, the left bank of Brush Brook is eroded and there is a small void under the
downstream end of the left abutment footing which is completely exposed. The right
abutment footing is also exposed. The scour countermeasures at the site include type-2
stone fill (less than 36 inches diameter) along the upstream banks and in front of the right
abutment and type-3 stone fill (less than 48 inches diameter) along the entire base length of
the upstream right wingwall and along the downstream 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)
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 0.7 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 6.9 to 10.9 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.