Level II scour analysis for Bridge 17 (RIPTTH00180017) on Town Highway 18, crossing the South Branch Middlebury River, Ripton, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RIPTTH00180017 on Town Highway 18 crossing the South 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 15.5-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest except on the upstream left
bank where it is shrubs and brush.

In the study area, the South Branch Middlebury River has an incised, sinuous channel with
a slope of approximately 0.03 ft/ft, an average channel top width of 86 ft and an average
bank height of 10 ft. The channel bed material ranges from gravel to boulders with a median
grain size (D₅₀) of 111 mm (0.364 ft). In addition, there is a bedrock outcrop across the
channel downstream of the bridge. The geomorphic assessment at the time of the Level I
and Level II site visit on June 10, 1996, indicated that the reach was stable.

The Town Highway 18 crossing of the South Branch Middlebury River is a 61-ft-long, one-lane bridge consisting of one 58-foot steel-beam span (Vermont Agency of Transportation,
written communication, November 30, 1995). The opening length of the structure parallel
to the bridge face is 56.8 ft. The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is skewed approximately 40 degrees to the opening while the
computed opening-skew-to-roadway is 30.

A scour hole 1.25 ft deeper than the mean thalweg depth was observed along the right
abutment and the downstream right wingwall during the Level I assessment. The scour
protection measures at the site include type-2 stone fill (less than 36 inches diameter) along
the left abutment and it’s wingwalls and at the upstream end of the right abutment. Also,
type-3 stone fill (less than 48 inches diameter) is along the upstream right 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. 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 0.1 to 1.1 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 5.6 to
9.0 ft. 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.