Level II scour analysis for Bridge 32 (NFIEVT012A0032) on State Route 12A, crossing the Dog River, Northfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NFIEVT012A0032 on State Route 12A crossing the Dog River, Northfield, 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
central Vermont. The 13.5-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover is pasture (golf course).

In the study area, the Dog River has a straight channel with a slope of approximately 0.01 ft/
ft, an average channel top width of 47 ft and an average bank height of 7 ft. The
predominant channel bed materials are gravel and cobbles with a median grain size (D₅₀) of
39.0 mm (0.128 ft). The geomorphic assessment at the time of the Level I and Level II site
visit on July 22, 1996, indicated that the reach was stable.

The State Route 12A crossing of the Dog River is a 35-ft-long, two-lane bridge consisting
of one 32-foot concrete span (Vermont Agency of Transportation, written communication,
October 13, 1995). The bridge is supported by vertical, concrete abutments with wingwalls.
The channel is skewed approximately 30 degrees to the opening and the opening-skew-to-
roadway also is 30 degrees.

A scour hole 1.5 ft deeper than the mean thalweg depth was observed along the upstream
end of the right abutment wall during the Level I assessment. The scour protection measures
at the site were type-1 stone fill (less than 12 inches diameter) on the right bank downstream
and type-2 stone fill (less than 36 inches diameter) on the upstream banks, the downstream
left bank and 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 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 1.5 to 2.9 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge, which was less than the
100-year discharge. Abutment scour ranged from 7.2 to 1 2.7 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.