Level II scour analysis for Bridge 31 (ALBATH00380031) on Town Highway 38, crossing the Black River, Albany, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ALBATH00380031 on town highway 38 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,
gleaned 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 New England Upland physiographic province of North-central Vermont in
the town of Albany. The 47.5-mi² drainage area is in a rural, forested basin. In the vicinity
of the study site, the immediate banks have mainly shrub and brush vegetation. The
overbank areas are pasture, except for the upstream right bank which is forested.
In the study area, the Black River has a meandering channel with a slope of approximately
0.0009 ft/ft, an average channel top width of 57 ft and an average channel depth of 3 ft. The
predominant channel bed material is gravel (D₅₀ is 35.4 mm or 0.116 ft). The geomorphic
assessment at the time of the Level I and Level II site visit on October 5, 1994, indicated
that the reach was stable.
The town highway 38 crossing of the Black River is a 50-ft-long, one-lane bridge consisting
of one 47-foot span concrete T-beam type superstructure (Vermont Agency of
Transportation, written commun., August 3, 1994). The bridge is supported by vertical
abutments with wingwalls. The left abutment is constructed of concrete while the right is
mortared, granite stone blocks. The channel is skewed approximately 30 degrees to the
opening while the opening-skew-to-roadway is 10 degrees.
A scour hole, 1.5 ft deeper than the mean thalweg depth, was observed during the level I
assessment along the left side of the channel under the bridge. The only scour protection
measure at the site was type-3 stone fill (less than 48 inches diameter) along the left bank
under the bridge. 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.