Level II scour analysis for Bridge 20 (IRASTH00080020) on Town Highway 8, crossing the Black River, Irasburg, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
IRASTH00080020 on town highway 8 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, 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 Irasburg. The 110-mi²
drainage area is in a predominantly rural basin. In the
vicinity of the study site, the left bank surface cover is pasture and row crops and the right
bank is covered by shrub and brush and is adjacent to woods.

In the study area, the Black River has a sinuous channel with a slope of approximately
0.002 ft/ft, an average channel top width of 90 ft and an average channel depth of 5 ft. The
predominant channel bed material is gravel and cobbles (D₅₀ is 49.7 mm or 0.163 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 laterally unstable.

The town highway 8 crossing of the Black River is a 88-ft-long, one-lane covered bridge
consisting of one 80-foot span (Vermont Agency of Transportation, written commun.,
August 2, 1994). The bridge is supported by vertical, concrete abutments with wingwalls on
the upstream and downstream sides of the right abutment. The right abutment has stone fill
protection. The channel is skewed approximately 25 degrees to the opening while the
opening-skew-to-roadway is zero degrees. 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.