Level II scour analysis for Bridge 16 (RIPTTH00110016) on Town Highway 11, crossing the Middle Branch Middlebury River, Ripton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RIPTTH00110016 on Town Highway 11 crossing the Middle Branch Middlebury River,
Ripton, 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 6.6-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover consists of shrubs, brush and trees
except for the upstream left bank which is completely forested.

In the study area, the Middle Branch Middlebury River has an incised, sinuous channel with
a slope of approximately 0.03 ft/ft, an average channel top width of 68 ft and an average
bank height of 5 ft. The channel bed material ranges from gravel to boulder with a median
grain size (D₅₀) of 97.6 mm (0.320 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on June 11, 1996, indicated that the reach was stable.

The Town Highway 11 crossing of the Middle Branch Middlebury River is a 44-ft-long,
two-lane bridge consisting of one 42-foot steel-beam span (Vermont Agency of
Transportation, written communication, December 15, 1995). The opening length of the
structure parallel to the bridge face is 40.2 ft. The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 40 degrees to the opening.
The opening-skew-to-roadway value from the VTAOT database is 20 degrees while 30
degrees was computed from surveyed points.

A scour hole, 3 ft deeper than the mean thalweg depth, was observed along the left
abutment and upstream left wingwall during the Level I assessment. In addition, 1 ft of
channel scour was observed just downstream of the downstream left wingwall along the left
bank. Scour countermeasures at the site included type-2 stone fill (less than 36 inches
diameter) along the upstream left and right banks and along the upstream end of the
downstream left wingwall. 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. 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 0.1 to 0.4 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 7.2 to
8.6 ft along the right abutment and from 11.7 to 13.7 ft along the left abutment. The worstcase 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.