Level II scour analysis for Bridge 23 (CRAFTH00390023) on Town Highway 39, crossing the Black River, Craftsbury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CRAFTH00390023 on town highway 39 crossing the Black River, Craftsbury, 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 assessment is included in Appendix E of this report. A Level I assessment
provides a qualitative geomorphic characterization of the study site. Information on the
bridge, gleaned from VTAOT files, was compiled prior to conducting Level I and Level II
analyses and is provided in Appendix D.
The site is in the New England Upland physiographic province of North-central Vermont in
the town of Craftsbury. The 30.9-mi²
drainage area is in a predominantly rural and forested
basin with some pasture grassland on the valley bottom. In the vicinity of the study site, the
banks have very little woody vegetation coverage except for the downstream left bank,
which has some trees and brush coverage.
In the study area, the Black River has an meandering channel with a slope of approximately
0.0004 ft/ft, an average channel top width of 62 ft and an average channel depth of 3 ft. The
predominant channel bed materials are sand and gravel with a median grain size (D₅₀) of
0.73 mm (0.0024 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on June 6, 1995, indicated that the reach was laterally unstable.
The town highway 39 crossing of the Black River is a 65-ft-long, two-lane bridge consisting
of three spans with a maximum span of 38-feet (Vermont Agency of Transportation, written
communication, August 24, 1994). The bridge is supported by vertical, concrete abutments
and two stone piers with concrete caps. The channel is skewed approximately 10 degrees to
the opening while the opening-skew-to-roadway is 10 degrees in the opposite direction.
The scour protection measures at the site were type-1 stone fill (less than 12 inches
diameter) on the upstream left and right roadway embankments and between the right pier
and right abutment wall. Type-2 (less than 36 inches diameter) stone fill was evident on the
upstream left bank, downstream right bank, between the left pier and left abutment wall,
and streamward of the right pier. 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, 1995).
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 20.1 to 25.2 and the worst-case
contraction scour occurred at the 500-year discharge. Although this bridge has two piers,
the flow through the spans between each abutment and pier is assumed to be negligible.
Hence, abutment scour was computed assuming the forces contributing to scour actually
occur on the main-span sides of each pier in this case. Abutment scour ranged from 8.8 to
10.6 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.
It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1995, p. 47). 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.