Level II scour analysis for Bridge 25 (ALBATH00250030) on Town Highway 25, crossing the Black River, Albany, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ALBATH00250030 on town highway 25 crossing the Black River, Albany, 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 were compiled prior to conducting Level I and Level II
analyses and can be found in Appendix D.
The site is in the New England Upland physiographic province of north-central Vermont in
the town of Albany. The 58.8-mi² drainage area is in a rural, forested basin. In the vicinity
of the study site, the banks have predominantly grass vegetation coverage with a few
shrubs.
In the study area, the Black River has a non-incised, highly meandering channel with a
slope of approximately 0.0005 ft/ft, an average channel top width of 56 ft and an average
channel depth of 4 ft. The predominant channel bed material is fine sand (D₅₀ is 1.68 mm or
0.00551 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
June 5, 1995, indicated that the reach was laterally unstable.
The town highway 25 crossing of the Black River is a 42-ft-long, one-lane bridge consisting
of one 40-foot span steel-beam superstructure with a timber deck (Vermont Agency of
Transportation, written commun., August 3, 1994). The bridge is supported by vertical,
concrete abutments with wingwalls. The channel is not skewed to the opening and the
opening-skew-to-roadway is zero degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed along mid-channel
from 40 feet upstream to about 10 feet under the bridge during the Level I assessment. The
left abutment is slightly undermined at the downstream end. The only scour protection
measures at the site were sparse type-1 stone fill (less than 12 inches diameter) on the
upstream right road embankment, along the left and right abutments, and along the
upstream left wingwall. 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). Scour depths were
calculated assuming an infinite depth of erosive material and a homogeneous particle-size
distribution. The scour analysis results are presented in tables 1 and 2 and a graph of the
scour depths is presented in figure 8.