Level II scour analysis for Bridge 34 (CORITH0050034) on Town Highway 50, crossing the South Branch Waits River, Corinth, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CORITH00500034 on Town Highway 50 crossing the South Branch Waits River, Corinth,
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 35.9-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture upstream and downstream
of the bridge while the immediate banks have dense woody vegetation.

In the study area, the South Branch Waits River has an incised, meandering channel with a
slope of approximately 0.005 ft/ft, an average channel top width of 63 ft and an average
bank height of 6 ft. The channel bed material ranges from sand to cobble with a median
grain size (D₅₀) of 23.7 mm (0.078 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on September 5, 1995, indicated that the reach was stable.

The Town Highway 50 crossing of the South Branch Waits River is a 56-ft-long, one-lane
bridge consisting of one 54-foot steel thru-truss span (Vermont Agency of Transportation,
written communication, March 24, 1995). The opening length of the structure parallel to the
bridge face is 51.5 ft.The bridge is supported by vertical, concrete abutments with no
wingwalls. Stone fill and bank material in front of the abutments create spill-through
embankments. The channel is skewed approximately 30 degrees to the opening while the
opening-skew-to-roadway is 15 degrees.

A scour hole 2.5 ft deeper than the mean thalweg depth was observed along the left bank
through the bridge 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 left and right banks
extending from upstream to downstream through the bridge. The stone fill under the bridge
creates spill-through embankments. 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 was determined and analyzed as other 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 3.0 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 2.4 to 6.3 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 and HIRE equations (abutment scour) give
“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.