Level II scour analysis for Bridge 31 (HUNTTH00220031) 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
HUNTTH00220031 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, obtained 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
west-central Vermont. The 5.01-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of trees and brush.

In the study area, Brush Brook has an incised, straight channel with a slope of
approximately 0.06 ft/ft, an average channel top width of 44 ft and an average bank height
of 4 ft. The channel bed material ranges from boulder to gravel with a median grain size
(D₅₀) of 107.0 mm (0.352 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 34-ft-long, one-lane bridge consisting
of one 30-foot steel I-beam span (Vermont Agency of Transportation, written
communication, November 30, 1995). The opening length of the structure parallel to the
bridge face is 31.2 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 15 degrees to the opening while the
computed opening-skew-to-roadway is 10 degrees. The VTAOT computed opening-skewto-roadway is 2 degrees.

A scour hole 1.0 ft deeper than the mean thalweg depth was observed at the downstream
end of the left abutment 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)
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 was computed to be zero ft. Abutment scour
ranged from 7.0 to 10.5 ft. The worst-case abutment scour occurred at the 500-year
discharge for the left abutment and at the incipient-overtopping discharge for the right
abutment. 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.