Level II scour analysis for Bridge 21 (MIDBTH00230021) on Town Highway 23, crossing the Middlebury River, Middlebury, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MIDBTH00230021 on Town Highway 23 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 44.8-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is suburban consisting of single
houses, each with a lawn, trees, and shrubs on all of the overbank areas bordering the river.

In the study area, the Middlebury River has a straight channel with a slope of approximately
0.02 ft/ft, an average channel top width of 87 ft and an average channel depth of 11 ft. The
channel bed material ranges from gravel to boulders with a median grain size (D₅₀) of 152
mm (0.498 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 Town Highway 23 crossing of the Middlebury River is a 52-ft-long, two-lane bridge
consisting of one 49-foot steel-beam span (Vermont Agency of Transportation, written
communication, December 14, 1995). The opening length of the structure parallel to the
bridge face is 42.3 feet. The bridge is supported by vertical, concrete abutments with
wingwalls at each end of the left abutment only. The channel is skewed approximately 10
degrees to the opening. The opening-skew-to-roadway from the VTAOT records is zero
degrees while 5 degrees was computed from surveyed points.

A scour hole 1.0 foot deeper than the mean thalweg depth was observed in the channel at
the upstream bridge face during the Level I assessment. The scour protection measure at the
site was type-2 stone fill (less than 36 inches diameter) on the upstream and downstream
banks and the upstream and downstream left wingwalls. 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) 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 1.2 to 1.8 feet. The worst-case
contraction scour occurred at the incipient overtopping discharge, which is less than the
500-year discharge. Abutment scour ranged from 17.7 to 23.7 feet. The worst-case
abutment scour 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.