Level II scour analysis for Bridge 26 (WSTOTH00070026) on Town Highway 7, crossing Greendale Brook, Weston, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WSTOTH00070026 on Town Highway 7 crossing Greendale Brook, Weston, 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
south central Vermont. The 3.13-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest.

In the study area, the Greendale Brook has a sinuous, non-incised, non-alluvial channel with
a slope of approximately 0.015 ft/ft, an average channel top width of 38 ft and an average
bank height of 3 ft. The channel bed material ranges from sand to boulder with a median
grain size (D₅₀) of 64.8 mm (0.213 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on August 19, 1996, indicated that the reach was laterally unstable.
The channel has moved to the right, however, scour countermeasures are in place along the
upstream right bank.

The Town Highway 7 crossing of the Greendale Brook is a 52-ft-long, two-lane bridge
consisting of one 50-foot steel-beam span with a concrete deck (Vermont Agency of
Transportation, written communication, April 07, 1995). The opening length of the
structure parallel to the bridge face is 48.6 ft. The bridge is supported by vertical, concrete
abutments with wingwalls. The channel is skewed approximately 50 degrees to the opening
while the opening-skew-to-roadway is 30 degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the upstream
right wingwall and right abutment during the Level I assessment. Scour protection measures
at the site include: type-2 stone fill (less than 36 inches diameter) at the upstream end of the
upstream left wingwall, along the left bank upstream, at the downstream end of the
downstream left wing wall, and along the entire length of the downstream right wing wall;
type 4 (less than 60 inches) and type-3 stone fill (less than 48 inches) along 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 recommended 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 was 0.0 ft. Abutment scour ranged from 3.9 to 9.9
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). The Hire
equation (abutment scour) is often used when the horizontal length blocked by flow divided
by the depth of flow is greater than 25 (Richardson and others, 1995 p. 49). Although the
Hire equation could be applied to the left abutment more conservative scour estimates were
given by the Froehlich equation on the left abutment. 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.