Level II scour analysis for Bridge 26 (ROYATH00540026) on Town Highway 54, crossing Broad Brook, Royalton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ROYATH00540026 on Town Highway 54 crossing Broad Brook, Royalton, 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 New England Upland section of the New England physiographic province
in central Vermont. The 11.9-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover on the left bank upstream and
downstream is pasture with trees and brush on the immediate banks. The right bank,
upstream and downstream of the bridge, is forested.

In the study area, Broad Brook has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 37 ft and an average bank height
of 4 ft. The channel bed material ranges from sand to boulders with a median grain size
(D₅₀) of 66.3 mm (0.218 ft). The geomorphic assessment at the time of the Level I site visit
on April 13, 1995 and the Level II site visit on July 11, 1996, indicated that the reach was
stable.

The Town Highway 54 crossing of Broad Brook is a 29-ft-long, one-lane bridge consisting
of one 24-foot steel-beam span with a timber deck (Vermont Agency of Transportation,
written communication, March 23, 1995). The opening length of the structure parallel to the
bridge face is 23.3 ft. The bridge is supported by a vertical, concrete face laid-up stone
abutment with concrete wingwalls on the left and a laid-up stone abutment on the right. The
channel is skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is zero degrees.

A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the downstream
end of the right abutment during the Level I assessment. Also, at the upstream end of the
left abutment, the footing is exposed 0.5 ft. The scour protection measures at the site
included type-2 stone fill (less than 36 inches diameter) along the upstream left bank, at the
upstream end of the upstream left wingwall, along the entire length of the downstream left
wingwall, and at the upstream end 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).
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 1.4 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 2.2 to 7.4 ft on the left and from
14.7 to 17.7 ft on the right. The worst-case abutment scour occurred at the incipient
roadway-overtopping discharge for the left and at the 500-year discharge for the right.
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.