Level II scour analysis for Bridge 21 (MORETH00010021) on Town Highway 1, crossing Cox Brook, Moretown, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
MORETH00010021 on Town Highway 1 crossing Cox Brook, Moretown, 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
north-central Vermont. The 2.85-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is predominantly forested.

In the study area, Cox Brook has an incised, sinuous channel with a slope of approximately
0.02 ft/ft, an average channel top width of 23 ft and an average bank height of 4 ft. The
channel bed material ranges from gravel to cobble with a median grain size (D₅₀) of 47.5
mm (0.156 ft). The geomorphic assessment at the time of the Level I and Level II site visit
on July 18, 1996, indicated that the reach was stable.

The Town Highway 1 crossing of Cox Brook is a 29-ft-long, two-lane bridge consisting of
one 27-foot steel-beam span (Vermont Agency of Transportation, written communication,
October 13, 1995). The opening length of the structure parallel to the bridge face is 24.8 ft.
The bridge is supported by vertical, concrete abutments with wingwalls. The channel is
skewed approximately 60 degrees to the opening while the measured opening-skew-to-roadway is 40 degrees.

A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the left
abutment downstream during the Level I assessment. The only scour protection measure at
the site was type-2 stone fill (less than 36 inches diameter) along the left bank upstream.
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-year 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 0.2 to 0.5 ft. The worst-case
contraction scour occurred at the incipient roadway-overtopping discharge, which was less
than the 100-year discharge. Abutment scour ranged from 2.8 to 4.0 ft. The worst-case
abutment scour occurred at the left abutment at the 100-year discharge and at the right
abutment 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.