Level II scour analysis for Bridge 7 (WALDTH00020007) on Town Highway 2, crossing Coles Brook, Walden, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
WALDTH00020007 on Town Highway 2 crossing Coles Brook, Walden, Vermont (figures
1–8). Coles Brook is also referred to as Joes Brook. 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 New England Upland section of the New England physiographic province
in north-eastern Vermont. The 12.8-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the surface cover is predominantly shrub and
brushland.

In the study area, Coles Brook has a sinuous channel with a slope of approximately 0.005 ft/
ft, an average channel top width of 37 ft and an average bank height of 4 ft. The channel bed
material ranges from sand to cobble with a median grain size (D₅₀) of 32.9 mm (0.108 ft).
The geomorphic assessment at the time of the Level I and Level II site visit on August 9,
1995, indicated that the reach was laterally unstable due to cut-banks, point bars, and loose
unconsolidated bed material.

The Town Highway 2 crossing of Coles Brook is a 74-ft-long, two-lane bridge consisting of
one 71-foot steel-beam span (Vermont Agency of Transportation, written communication,
April 5, 1995). The opening length of the structure parallel to the bridge face is 69.3 ft. The
bridge is supported by spill-through abutments. The channel is skewed approximately 35
degrees to the opening while the measured opening-skew-to-roadway is 15 degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed from 60 ft. to 100 ft.
downstream during the Level I assessment. Scour protection measures at the site include:
type-1 stone fill (less than 12 inches diameter) along the right bank upstream, at the
downstream end of the downstream left wingwall and downstream right wingwall; and
type-2 stone fill (less than 36 inches diameter) along the left bank upstream, at the upstream
end of the upstream right wingwall, and along the entire base of the left and right
abutments. 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 ranged from 0.0 to 0.8 ft. The worst-case
contraction scour occurred at the incipient roadway-overtopping discharge. Abutment scour
ranged from 5.7 to 12.9 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). 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.