Level II scour analysis for Bridge 36 (ANDOVT00110036) on VT 11, crossing Middle Branch Williams River, Andover, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
ANDOVT00110036 on State Route 11 crossing the Middle Branch Williams River,
Andover, 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
south-central Vermont. The 5.10-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is pasture on the upstream left bank
and forested elsewhere throughout the reach.
In the study area, the Middle Branch Williams River has an incised, sinuous channel with a
slope of approximately 0.02 ft/ft, an average channel top width of 38 ft and an average bank
height of 2 ft. The channel bed material ranges from sand to boulders with a median grain
size (D₅₀) of 60.1 mm (0.197 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on August 28, 1996, indicated that the reach was laterally unstable due to
a cut-bank on the left bank upstream, side bar formation on the left bank upstream, and a
combination of side bar formation and erosion occurring on the downstream right bank.
The State Route 11 crossing of the Middle Branch Williams River is a 28-ft-long, two-lane
bridge consisting of one 25-foot concrete-beam span (Vermont Agency of Transportation,
written communication, March 28, 1995). The opening length of the structure parallel to the
bridge face is 25.3 ft.The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 30 degrees to the opening and the
opening-skew-to-roadway is also 30 degrees.
A scour hole 0.5 ft deeper than the mean thalweg depth was observed 5 feet upstream of the
bridge during the Level I assessment. Scour protection measures at the site include: type-2
stone fill (less than 36 inches diameter) along the left bank upstream, and type-4 stone fill
(less than 60 inches diameter) along the entire base length of the upstream left wingwall,
and at the upstream end of the upstream right wing wall. 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).
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 0.0 to 2.8 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 9.5 to
13.7 ft. The worst-case abutment scour also occurred at the 500-year 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.