Level II scour analysis for Bridge 51 (RANDTH00SC0051) on School Street, crossing Thayer Brook, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RANDTH00SC0051 on School 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 5.30-mi²
drainage area is a predominantly rural basin. In the vicinity of the
study site, the left and right banks are forested with residences on the left overbanks.

In the study area, Thayer Brook has a sinuous channel with a slope of approximately 0.03 ft/
ft, an average channel top width of 36 ft and an average channel depth of 3 ft. The
predominant channel bed materials are gravel and cobble (D₅₀ is 58.2 mm or 0.191 ft). The
geomorphic assessment at the time of the Level I site visits on August 4, 1994 and
December 8, 1994, indicated that the reach was stable.

The School Street crossing of Thayer Brook is a 39-ft-long, two-lane bridge consisting of
one 35-foot concrete span (Vermont Agency of Transportation, written commun., August 2,
1994). The bridge is supported by vertical, concrete abutments with wingwalls. Type-2
stone fill (less than 36 inches diameter) along the downstream left bank was the only
existing protection. The approach channel is skewed approximately 45 degrees to the bridge
face; the opening-skew-to-roadway is also 45 degrees. Additional details describing
conditions at the site are included in the Level II Summary, Appendix D, and Appendix 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 1.0 to 2.2 ft. with the worst-case
scenario occurring at the 500-year discharge. Abutment scour ranged from 6.2 to 12.0 ft.
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.