Level II scour analysis for Bridge 54 (RANDTH00BR0054) on Brook Street, crossing Thayer Brook, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RANDTH00BR0054 on Brook Street crossing Thayer Brook, 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 5.39-mi²
drainage area is in a predominantly rural basin. In the vicinity of
the study site, the immediate banks are forested.

In the study area, Thayer Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 60 ft and an average channel
depth of 3 ft. The predominant channel bed materials are gravel and cobble (D₅₀ is 42.4 mm
or 0.139 ft). The geomorphic assessment at the time of the Level I and Level II site visits on
August 3, 1994 and December 5, 1994, indicated that the reach was vertically and laterally
unstable. This assessment was due to the extreme channel misalignment with the bridge
opening and the presence of a drop structure downstream of the bridge protecting against
channel degradation.

The Brook Street crossing of Thayer Brook is a 34-ft-long, two-lane bridge consisting of
one 31-foot concrete span (Vermont Agency of Transportation, written communication,
August 2, 1994). The bridge is supported by vertical, concrete abutments with wingwalls.
Streamflow attacks the upstream right wingwall and has undermined the upstream end of
the right abutment. Type-2 stone fill (less than 36 inches diameter) exists only on the
upstream and downstream sides of the left road embankment. No other protection was
noted. The bank full channel skew at the bridge face is approximately 20 degrees; the
opening-skew-to-roadway is also 20 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 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 1.3 to 2.7 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.3 to
15.1 ft. and the worst-case abutment scour also 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 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, scour
protection measures, and the results of the hydraulic analyses, must be considered to
properly assess the validity of abutment scour results. Therefore, scour depths adopted by
VTAOT may differ from the computed values documented herein, based on the
consideration of additional contributing factors and experienced engineering judgement.