Level II scour analysis for Bridge 12 (BRAITH00230012) on Town Highway 23, crossing Ayers Brook, Braintree, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRAITH00230012 on town highway 23 crossing Ayers Brook, Braintree, 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
Braintree. The 18.8-mi²
drainage area is in a predominantly rural watershed. In the vicinity
of the study site, the surface cover of the left and right banks is pasture.

In the study area, Ayers Brook has a meandering channel with a slope of approximately
0.003 ft/ft, an average channel top width of 46 ft and an average channel depth of 5 ft. The
predominant channel bed material is sand and gravel (D₅₀ is 6.15 mm or 0.0202 ft). The
geomorphic assessment at the time of the Level I and Level II site visit on November 16,
1994, indicated that the reach was laterally unstable. Also at the time of the site visit, there
was considerable backwater at the bridge site due to a beaver dam downstream. The beaver
dam was ignored in the analyses.

The town highway 23 crossing of Ayers Brook is a 28-ft-long, one-lane bridge consisting of
one 23-foot span (Vermont Agency of Transportation, written communication, August 24,
1994). The bridge is supported by vertical timber cribwork abutments with wingwalls on
the upstream and downstream sides of the right abutment. The lower half of the right
abutment and wingwalls are constructed of laid-up stone. The right abutment and wingwalls
are also protected by stone fill. The channel is skewed approximately 45 degrees to the
opening while the opening-skew-to-roadway is only 5 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 4.2 to 9.4 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge which was less than the
100-year discharge. Abutment scour ranged from 4.3 to 17.5 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 equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1993, p. 48). 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.