This report provides the results of a detailed Level II analysis of scour potential at structure
CRAFTH00550029 on town highway 55 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). Results of a Level I scour investigation also are included in Appendix E of this
report. A Level I investigation provides a qualitative geomorphic characterization of the
study site. Information on the bridge, gleaned from Vermont Agency of Transportation
(VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is
found in Appendix D.
The site is in the New England Upland section of the New England physiographic province
of north-central Vermont in the town of Craftsbury. The 24.7-mi2
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the banks have
woody vegetation coverage except for the upstream left bank and the downstream right
bank, which have more brush cover than trees.
In the study area, the Black River has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 41 ft and an average channel
depth of 5.5 ft. The predominant channel bed material is sand and gravel (D50 is 44.7 mm or
0.147 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
June 7, 1995, indicated that the reach was stable.
The town highway 55 crossing of the Black Riveris a 32-ft-long, one-lane bridge consisting
of one 28-foot span steel stringer superstructure with a timber deck (Vermont Agency of
Transportation, written communication, August 4, 1994). The bridge is supported by
vertical, concrete abutments with wingwalls. The channel is skewed approximately 40
degrees to the opening while the opening-skew-to-roadway is 10 degrees.
A scour hole 2 ft deeper than the mean thalweg depth was evident at mid-channel
immediately downstream of the bridge during the Level I assessment. The only scour
protection measure at the site was type-1 stone fill (less than 12 inches diameter) on the
upstream right bank and road approach embankment. 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 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 0.9 to 1.4 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 12.1 to
15.5 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, 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