Level II scour analysis for Bridge 34 (SHERUS00040034) on U.S. Highway 4, crossing the Ottauquechee River, Sherburne, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
SHERUS00040034 on US Route 4 crossing the Ottauquechee River, Sherburne, 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). Results of a Level I scour investigation also are included in Appendix E of this
report. A Level I investigation provides a qualitative geomorphic characterization of the
study site. Information on the bridge, gleaned from Vermont Agency of Transportation
(VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is
found in Appendix D.
The site is in the Green Mountain section of the New England physiographic province in
central Vermont. The 25.8-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is pasture upstream of the bridge while the
immediate banks have dense woody vegetation. Downstream of the bridge, the banks are
forested.
In the study area, the Ottauquechee River has an incised, straight channel with a slope of
approximately 0.028 ft/ft, an average channel top width of 66 ft and an average channel
depth of 5 ft. The channel bed material ranges from gravel to boulder with a median grain
size (D₅₀) of 118.1 mm (0.387 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 25, 1995, indicated that the reach was stable.
The US Route 4 crossing of the Ottauquechee River is a 187-ft-long, two-lane bridge
consisting of three steel-beam spans (Vermont Agency of Transportation, written
communication, March 14, 1995). The bridge is supported by vertical, concrete abutments
above spill-through stone fill (< 36 inches diameter). The channel is skewed approximately
60 degrees to the opening while the opening-skew-to-roadway is 60 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, 1995). 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 was 0.0 ft. Abutment scour ranged from 4.7 to 7.4
ft. The worst-case abutment scour occurred at the left abutment for the 500-year discharge.
Pier scour ranged from 7.5 to 11.4 ft. The worst-case pier scour occurred at the incipientovertopping discharge. 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, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.