Level II scour analysis for Bridge 46 (NORWTH00030046) Town Highway 3 (VT132) crossing the Ompompanoosuc River, Norwich, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NORWTH00030046 on town highway 3, which is also Vermont State Route 132 crossing
the Ompompanoosuc River, Norwich, 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 east-central Vermont.
The 135-mi²
drainage area is a predominantly rural basin. A flood-control reservoir located
approximately 2 mi upstream has 1.66 billion cubic feet of usable storage. In the vicinity of
the study site, the left bank is forested and the right bank is covered by shrubs and brush,
adjacent to woods. The Ompompanoosuc River is parallel to Town Highway 3.
In the study area, the Ompompanoosuc River has a sinuous channel with a slope of
approximately 0.003 ft/ft, an average channel top width of 166 ft and an average channel
depth of 6 ft. The predominant channel bed material is sand (D₅₀ is 0.744 mm or 0.00244
ft). The geomorphic assessment at the time of the Level I and Level II site visit on August
19, 1994, indicated that the reach was stable.
The town highway 3 crossing of the Ompompanoosuc Riveris a 100-ft-long, two-lane
bridge consisting of two steel-beam spans (Vermont Agency of Transportation, written
commun., July 29, 1994). The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 25 degrees to the opening while the
opening-skew-to-roadway is 12 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.