Level II scour analysis for Bridge 125 (MIDBUS00070125) on U.S. Route 7 crossing the Middlebury River, Middlebury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MIDBUS00070125 on U.S. Route 7 crossing the Middlebury River, Middlebury, 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
west-central Vermont. The 46.8-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of row crops on the right
overbank upstream and downstream, and trees on the left overbank.
In the study area, the Middlebury River has a straight channel with a slope of approximately
0.005 ft/ft, an average channel top width of 77 ft and an average channel depth of 4 ft. The
predominant channel bed materials are sand and cobbles with a median grain size (D₅₀) of
59.4 mm (0.195 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on June 18, 1996, indicated that the reach was stable.
The U.S. Route 7 crossing of the Middlebury River is a 202-ft-long, two-lane bridge
consisting of one 91-foot, and two 55-foot steel-beam spans (Vermont Agency of
Transportation, written communication, December 14, 1995). The bridge is supported by
vertical, concrete abutment walls with spill-through embankments. The channel is skewed
approximately 45 degrees to the opening while the opening-skew-to-roadway is 45 degrees.
The scour protection measures at the site were type-2 stone fill (less than 36 inches
diameter) on the spill-through embankments of each abutment and type-1 stone fill (less
than 12 inches diameter) on the right bank upstream. 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 ranged from 0.0 to 1.2 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.5 to
11.0 ft. The worst-case abutment scour occurred at the 500-year discharge at the left
abutment. Pier scour ranged from 8.3 to 15.9 ft. for each modeled discharge. The worst-case
pier scour occurred at the 500-year discharge. In this report, piers are numerically
designated “1” and “2” for the left and right piers respectively. 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.