Level II scour analysis for Bridge 86 (VERNVT01420086) on State Route 142, crossing Broad Brook, Vernon, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
VERNVT01420086 on State Route 142 crossing Broad Brook, Vernon, 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 southeastern Vermont. The 23.7-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is primarily forest with the
exception of the downstream left bank which is a wetland.

In the study area, Broad Brook has an incised, meandering channel with a slope of
approximately 0.001 ft/ft, an average channel top width of 132 ft and an average bank
height of 3 ft. The channel bed material ranges from silt to cobbles with a median grain size
(D₅₀) of 80.0 mm (0.262 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 14, 1996, indicated that the reach was vertically and laterally
unstable.

The State Route 142 crossing of Broad Brook is a 98-ft-long, two-lane bridge consisting of
two steel-beam spans with a maximum span length of 47 feet (Vermont Agency of
Transportation, written communication, March 30, 1995). The bridge is supported by
vertical, concrete abutments with spill-through slopes and a concrete pier. The channel is
skewed approximately 30 degrees to the opening while there is no opening-skew-to-
roadway.

A scour hole 2 ft deeper than the mean thalweg depth was observed along the left bank side
of the pier during the Level I assessment. There was also a scour hole 1 ft deeper than the
mean thalweg depth observed along the length of the right abutment. The only scour
protection measure at the site was type-2 stone fill (less than 36 inches diameter) along the
entire base length of the spill-through slopes. 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.

There was no computed contraction scour for any modelled flows. Scour at the left
abutment ranged from 13.2 to 15.9 ft and at the right abutment ranged from 12.0 to 16.3 ft.
The worst-case abutment scour occurred at the 500-year discharge. Pier scour ranged from
12.0 to 16.3 ft. The worst-case pier scour occurred at the incipient-overtopping 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.