Level II scour analysis for Bridge 26 (BRIDTH00340026) on Town Highway 034, crossing Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00340026 on town highway 34 crossing the Ottauquechee River, Bridgewater,
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 Green Mountain physiographic province of central Vermont in the town of
Bridgewater. The 38.0-mi²
drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the banks have dense woody vegetation coverage except for
the upstream right bank, which is grass covered.
In the study area, the Ottauquechee River has an incised, sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 62.5 ft and an average channel
depth of 4.5 ft. The predominant channel bed material is cobble (D₅₀ is 97.3 mm or 0.319
ft). The geomorphic assessment at the time of the Level I and Level II site visit on
November 8, 1994, indicated that the reach was stable.
The town highway 34 crossing of the Ottauquechee Riveris a 69-ft-long, two-lane bridge
consisting of one 63-foot clear-span steel-pony truss-type superstructure (Vermont Agency
of Transportation, written commun., August 24, 1994). The bridge is supported by vertical,
concrete abutments with wingwalls. The channel is skewed approximately 40 degrees to the
opening while the opening-skew-to-roadway is 45 degrees.
A scour hole 4.5 ft deeper than the mean thalweg depth was observed along the upstream
left wingwall during the Level I assessment. The only scour protection measure at the site
was type-1 stone fill (less than 12 inches diameter) at the upstream end of 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 were computed using the general guidelines described in Hydraulic
Engineering Circular 18 (Richardson and others, 1993). These guidelines provide scour
equations, which assume 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.