Level II scour analysis for Bridge 34 (RANDTH00660034) on Town Highway 66, crossing Second Branch White River, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RANDTH00660034 on town highway 66 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 division of central Vermont in the town of
Randolph. The 51.3-mi²
drainage area is in a predominantly rural basin. In the vicinity of
the study site, the left and right banks are covered by fields with some brush on the
upstream left and downstream right banks and with row crops on the downstream left
overbank.

In the study area, the Second Branch White River has a sinuous channel with a slope of
approximately 0.002 ft/ft, an average channel top width of 60 ft and an average channel
depth of 7 ft. The predominant channel bed material is sand (D₅₀ is 1.34 mm or 0.0044 ft).
The geomorphic assessment at the time of the Level I and Level II site visit on August 11,
1994, indicated that the reach was laterally unstable. Channel scour is evident along the left
half of the channel from about 30 feet upstream to about 20 feet downstream of the bridge.
There is a cut bank with block failures along the left bank upstream of the bridge further
indicating instability of the stream reach.

The town highway 66 crossing of the Second Branch White Riveris a 57-ft-long, one-lane
covered bridge consisting of one 45-foot span (Vermont Agency of Transportation, written
communication, July 29, 1994). The bridge is supported by vertical, concrete abutments
with one wingwall on the upstream left side. The base of the left abutment was protected by
type-1 stone fill (less than 12 inches diameter). The channel is skewed approximately 40
degrees to the opening while the opening-skew-to-roadway is 45 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 6.3 ft to 7.8 ft and the worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 7.9 ft to
20.3 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.