Level II scour analysis for Bridge 81 (MARSUS00020081) on U.S. Highway 2, crossing the Winooski River, Marshfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MARSUS00020081 on U.S. Highway 2 crossing the Winooski River, Marshfield, 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 New England Upland section of the New England physiographic province
in central Vermont. The 50.2-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 is
forested with buildings near the bridge on the right bank.
In the study area, the Winooski River has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 83 ft and an average bank height
of 10 ft. The channel bed material ranges from cobble to boulder with a median grain size
(D₅₀) of 64.0 mm (0.210 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 23, 1996, indicated that the reach was stable.
The U.S. Highway 2 crossing of the Winooski River is a 49-ft-long, two-lane bridge
consisting of one 47-foot concrete T-beam span (Vermont Agency of Transportation,
written communication, November 1, 1995). The opening length of the structure parallel to
the bridge face is 44.9 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 10 degrees to the opening while the
opening-skew-to-roadway is zero degrees.
A scour hole 1 ft deeper than the mean thalweg depth was observed near the upstream left
wingwall during the Level I assessment. The scour protection measures at the site included
type-1 stone fill (less than 12 inches diameter) at the upstream end of the upstream left and
right wingwall, the downstream end of the downstream left wingwall, and along the
upstream left and right banks. There was also type-3 stone fill (less than 48 inches
diameter) at the downstream left bank and type-2 stone fill (less than 36 inches diameter)
along the downstream right bank. Additional details describing conditions at the site are
included in the Level II Summary and Appendices D and E.
Scour depths and recommended rock rip-rap sizes were computed using the general
guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995)
for the 100- and 500-year discharges. In addition, the incipient roadway-overtopping
discharge is determined and analyzed as another potential worst-case scour scenario. 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 2.1 to 4.2 ft. The worst-case
contraction scour occurred at the 500-year discharge. Left abutment scour ranged from 14.3
to 14.4 ft. The worst-case left abutment scour occurred at the incipient roadwayovertopping and 500-year discharge. Right abutment scour ranged from 15.3 to 18.5 ft. The
worst-case right abutment scour occurred at the 100-year and the incipient roadwayovertopping 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) give “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.