Level II scour analysis for Bridge 68 (NFIETH00960068) on Town Highway 96, crossing the Dog River, Northfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
NFIETH00960068 on Town Highway 96 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 30.7-mi²
drainage area is in a predominantly rural and forested basin.
In the vicinity of the study site, the surface cover on the left bank upstream and downstream
is pasture while the immediate banks have dense woody vegetation. The right bank
upstream is forested and the downstream right bank is pasture. Vermont state route 12A
runs parallel to the river on the right bank.

In the study area, the Dog River has an incised, straight channel with a slope of
approximately 0.004 ft/ft, an average channel top width of 70 ft and an average bank height
of 7 ft. The channel bed material ranges from sand to cobble with a median grain size (D₅₀)
of 47.9 mm (0.157 ft). The geomorphic assessment at the time of the Level I and Level II
site visit on July 25, 1996, indicated that the reach was stable.

The Town Highway 96 crossing of the Dog River is a 45-ft-long, one-lane bridge consisting
of one 43-foot steel-beam span with a timber deck (Vermont Agency of Transportation,
written communication, October 13, 1995). The opening length of the structure parallel to
the bridge face is 41.5 ft.The bridge is supported by vertical, concrete abutments with
wingwalls. The channel is not skewed to the opening and the opening-skew-to-roadway is
zero degrees.

Channel scour 0.5 ft deeper than the mean thalweg depth, was observed under the bridge
during the Level I assessment. The scour protection measures at the site included type-1
stone fill (less than 12 inches diameter) along the left bank upstream and type-2 stone fill
(less than 36 inches diameter) along the upstream and downstream right banks that extends
partially in front of the right wingwalls. 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 0.8 to 1.2 ft. The worst-case
contraction scour occurred at the 100-year and 500-year discharges. Abutment scour ranged
from 8.5 to 12.2 ft. The worst-case abutment scour occurred at the incipient roadway-overtopping discharge for the right abutment. 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.