This report provides the results of a detailed Level II analysis of scour potential at structure
IRASTH00050006 on town highway 5 crossing the Black River, Irasburg, 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.
The 91.1-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of
the study site, the left and right banks are forested and a residence is located on the
downstream left overbank.
In the study area, the Black River has an incised, sinuous channel with a slope of
approximately 0.05 ft/ft, an average channel top width of 116 ft and an average bank-full
channel depth of 2.8 ft. The predominant channel bed material is cobble (D50 is 240 mm or
0.789 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 4, 1994, indicated that the reach was stable.
The town highway 5 crossing of the Black River is a 70-ft-long, two-lane bridge consisting
of one 65-foot clear span (Vermont Agency of Transportation, written commun., August 2,
1994). The bridge is supported by vertical, concrete abutments with wingwalls. There is
also a retaining wall along the upstream side of the road embankments. The channel is
skewed approximately 20 degrees to the opening while the opening-skew-to-roadway is 15
A scour hole 3.0 ft deeper than the mean thalweg depth was observed along the right
abutment. The scour hole was 27 feet long, 15 feet wide, and was 2.5 feet below the
abutment footing at the time of the Level I assessment. This right abutment had numerous
cracks and had settled. Additional details describing conditions at the site are included in
the Level II Summary and Appendices D
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.