Level II scour analysis for Bridge 39 (RANDTH00730039) on Town Highway 73, crossing the Second Branch White River, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RANDTH00730039 on town highway 73 crossing the Second Branch White River,
Randolph, 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). A Level I study is included in Appendix E of this report. A Level I
study provides a qualitative geomorphic characterization of the study site. Information on
the bridge available from VTAOT files was compiled prior to conducting Level I and Level
II analyses and can be found in Appendix D.

The site is in the Green Mountain physiographic province of central Vermont in the town of
Randolph. The 53.7-mi²
drainage area is in a predominantly rural basin. In the vicinity of
the study site, the overbanks are covered by pasture except for the upstream right bank
which is covered by brush.

In the study area, the Second Branch White River has a meandering channel with a slope of
approximately 0.001 ft/ft, an average channel top width of 44 ft and an average channel
depth of 6 ft. The predominant channel bed material is sand with median grain size (D₅₀) of
0.884 mm (0.0029 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on August 12, 1994, indicated that the reach was laterally unstable. This is because
of severe cut-banks both upstream and downstream where mass wasting and block failure
of bank material is evident. Furthermore, minimal erosion protection is provided by bank
vegetation since woody vegetation cover is sparse.

The town highway 73 crossing of the Second Branch White Riveris a 42-ft-long, one-lane
bridge consisting of one 40-foot span (Vermont Agency of Transportation, written
communication, August 2, 1994). The bridge is supported by vertical, concrete abutments
with wingwalls. The ends of the upstream left wingwall and the downstream right wingwall
are protected by stone fill. However, this stone fill is slumping according to the Level I field
inspection. The channel is skewed approximately 30 degrees to the opening while the
opening-skew-to-roadway is 0 degrees. Additional details describing conditions at the site
are included in the Level II Summary and Appendices D and E.

Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1993).
Total scour at a highway crossing is comprised of three components: 1) long-term
aggradation or degradation; 2) contraction scour (due to reduction in flow area caused by 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 scour depths
for contraction and local scour and a summary of the results follows.

Contraction scour for all modelled flows ranged from 1.9 ft to 4.6 ft and the worst-case
contraction scour occurred at the incipient overtopping discharge. Abutment scour ranged
from 4.0 ft to 22.5 ft and the worst-case abutment scour occurred at the 500-year discharge.
Scour depths and depths to armoring are summarized on p. 14 in the section titled “Scour
Results”. Scour elevations, based on the calculated depths are presented in tables 1 and 2;
a graph of the scour elevations is presented in figure 8 Scour depths were calculated
assuming an infinite depth of erosive material and a homogeneous particle-size distribution.

For all scour presented in this report, “the scour depths adopted [by VTAOT] may differ
from the equation values based on engineering judgement” (Richardson and others, 1993, p.
21, 27). It is generally accepted that the Froehlich equation (abutment scour) gives
“excessively conservative estimates of scour depths” (Richardson and others, 1993, p. 48).
Many factors, including historical performance during flood events, the geomorphic
assessment, and the results of the hydraulic analyses, must be considered to properly assess
the validity of abutment scour results.