Level II scour analysis for Bridge 14 (FFIETH00010014) on Town Highway 1, crossing the Fairfield River, Fairfield, Vermont

This report provides the results of a detailed Level II analysis of scour
potential at structure
FFIETH00010014 on Town Highway 1 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
northwestern Vermont. The 7.84-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site,
the surface cover on the upstream left and
downstream right is row crops. The surface
cover on the upstream right and downstream
left is pasture.
In the study area, the Fairfield River has a sinuous channel with a slope of approximately
0.006 ft/ft, an average channel top width of 33 ft
and an average channel depth of 3 ft. The
channel bed material ranges
from silt to gravel with
a median grain size (D₅₀) of 15.4 mm
(0.0505 ft). The geomorphic assessment at the time
of the Level I and Level II site visit on
July 11, 1995, indicated that the
reach was laterally unstable.
The Town Highway 1 crossing of the Fairfield River is a 26-ft-long, two-lane bridge
consisting of one 23-foot concrete span (Vermont Agency of Transportation, written
communication, March 9, 1995). The bridge is
supported by vertical, concrete abutments
with wingwalls. The channel is skewed approximately 27 degrees to the opening while
there is no opening-skew-to-roadway.
The bridge is located on a sharp channel bend. Th
e left abutment is impacted due to this
bend. A scour hole 1.5 ft deeper than the mean
thalweg depth was observed along the left
abutment during the Level I asse
ssment. Scour protection measures
at the site include type-
3 stone fill (less than 48 inches diameter
) along the immediate upstream banks and along
the base of the upstream left wi
ngwall. Type-2 stone fill (le
ss than 36 inches diameter) was
present along the downstream left wingwall. A
dditional 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 5.4 to 8.0 ft. The worst-case
contraction scour occurred at
the 500-year discharge. Abutment scour ranged from 3.0 to
8.3 ft. The worst-case abutment scour also occurred at the 500-year discharge. Additional
information on scour depths and
depths to armoring are include
d 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.