Level II scour analysis for Bridge 12 (FFIETH00030012) on Town Highway 3, crossing the Fairfield River, Fairfield, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
FFIETH00030012 on Town Highway 3 crossing the Fairfield River, Fairfield, 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-western Vermont. The 7.34-mi² drainage area is in a predominantly rural basin with
forest on the valley walls and pasture/row crops on the valley bottom. In the vicinity of the
study site, the surface cover is row crops with a few trees on the immediate banks.

In the study area, the Fairfield River has a meandering channel with a slope of
approximately 0.005 ft/ft, an average channel top width of 37 ft and an average channel
depth of 6 ft. The predominant channel bed materials are sand and gravel with a median
grain size (D₅₀) of 32.5 mm (0.107 ft). The geomorphic assessment at the time of the Level
I and Level II site visit on June 16, 1995, indicated that the reach was stable.

The Town Highway 3 crossing of the Fairfield River is a 24-ft-long, one-lane bridge
consisting of one 20-foot concrete span (Vermont Agency of Transportation, written
communication, March 8, 1995). The bridge is supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 40 degrees to the opening. Although
bridge records show an opening-skew-to-roadway of 45 degrees, the skew measured from
surveyed points was 30 degrees.

At the time of the level I assessment, the left abutment had been undermined and settled
into a scour hole at the upstream end. The right abutment footing was exposed but not
undermined. The scour protection measures at the site were type-1 stone fill (less than 12
inches diameter) on the downstream right bank, and type-2 stone fill (less than 36 inches
diameter) along the entire base of the upstream right wingwall, the upstream banks, and
downstream left bank. The type-2 stone fill on the left bank downstream changes to type-1
about 55 feet downstream of the bridge. 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 1.6 to 3.0 ft. The worst-case
contraction scour occurred at the 500-year discharge. Abutment scour ranged from 3.2 to
4.0 ft. at the left abutment and 9.7 to 11.7 feet at the right abutment. The worst-case left
abutment scour occurred at the incipient over-topping discharge, which was less than the
100-year discharge. The worst-case right 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.