This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDUS00040045a on U.S.. Route 4 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 was 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 72.1-mi2
drainage area is in a predominantly rural and forested basin. In
the vicinity of the study site, the overbank areas are lawn or pasture with a few residences.
The immediate channel banks have moderately dense woody vegetation.
In the study area, the Ottauquechee River has a sinuous channel with a slope of
approximately 0.01 ft/ft, an average channel top width of 81 ft and an average channel
depth of 3 ft. The predominant channel bed materials are gravel and cobble (D50 is 54.9 mm
or 0.180 ft). The geomorphic assessment at the time of the Level I and Level II site visit on
October 26, 1994, indicated that the reach was stable.
The U.S. Route 4 crossing of the Ottauquechee Riveris a 172-ft-long, two-lane bridge
consisting of three steel-beam spans supported by spill-through abutments and two concrete
piers (Vermont Agency of Transportation, written commun., August 25, 1994). The
abutment and road approaches are protected by type-2 stone fill (less than 36 inches
diameter). The North Branch of the Ottauquechee River joins the Ottauquechee River
approximately 200 feet upstream of the bridge on the main branch’s left bank. The channel
approach to the bridge has a mild bend with the bridge skewed 15 degrees to flow; the
opening-skew-to-roadway is 30 degrees. Additional details describing conditions at the site
are included in the Level II Summary, Appendix D, and Appendix E.
Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1993). Total scour at a
highway crossing is comprised of three components: 1) long-term streambed degradation;
2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge)
and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is
the sum of the three components. Equations are available to compute depths for contraction
and local scour and a summary of the results of these computations follows.
Contraction scour for all modelled flows ranged from 3.1 to 4.0 ft. with the worst-case
contraction scour occurring at the 500-year and incipient road-overflow discharges.
Abutment scour ranged from 9.3 to 15.2 ft. The worst-case abutment scour also occurred at
the 500-year discharge. Pier scour ranged from 11.4 to 12.4 ft. with the worst-case scenario
occurring at the incipient roadway overflow discharge. The incipient roadway overflow
discharge was between the 100- and 500-year discharges. Additional information on scour
depths and depths to armoring are included in the section titled “Scour Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented in tables 1 and 2.
A cross-section of the scour computed at the bridge is presented in figure 8. Scour depths
were calculated assuming an infinite depth of erosive material and a homogeneous particle-size distribution.
It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1993, p. 48). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection measures, and the results of the hydraulic analyses, must be considered to
properly assess the validity of abutment scour results. Therefore, scour depths adopted by
VTAOT may differ from the computed values documented herein, based on the
consideration of additional contributing factors and experienced engineering judgement.