Level II scour analysis for Bridge 25 (ROYATH00550025) on Town Highway 55, crossing Broad Brook, Royalton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ROYATH00550025 on Town Highway 55 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 11.6-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture on the upstream and
downstream left overbanks and forest on the upstream and downstream right overbanks.
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 41 ft and an average bank height
of 5 ft. The channel bed material ranges from sand to boulder with a median grain size (D₅₀)
of 58.3 mm (0.191 ft). The geomorphic assessment at the time of the Level I site visit on
April 13, 1995 indicated that the reach was laterally unstable. The stream impacts the
upstream left bank where there is a cut bank.
The Town Highway 55 crossing of the Broad Brook is a 35-ft-long, two-lane bridge
consisting of one 31-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 22, 1995). The opening length of the structure parallel to the bridge
face is 32 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The
channel is skewed approximately 20 degrees to the opening, while the opening-skew-toroadway is zero degrees.
A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the left
abutment and the downstream left wingwall during the Level I assessment. The scour
countermeasure at the site was type-2 stone fill (less than 36 inches diameter) along the
upstream and downstream left banks that extended to the ends of the wingwalls.
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 0.6 to 1.5 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge which was less than the
100-year discharge. Abutment scour ranged from 3.5 to 8.9 ft. The worst-case abutment
scour occurred at the incipient road-overtopping discharge for the left abutment and at the
100-year 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 crosssection 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.