Indirect Techniques for Erosion Protection
8.2.1 IOWA VANES
The technique called "Iowa vanes" originated from physical model tests performed
by the Iowa Institute of Hydraulic Research for the U.S. Army Corps of Engineers (Odgaard
and Kennedy, 1982). The purpose of the model study was to define a bank stabilization
technique for the Sacramento River which would be both effective and environmentally sound
although the proposed solution was not actually implemented. The first field application was
sponsored by the Iowa Department of Transportation in 1985 on the East Nishnabotna River
near Red Oak, Iowa. Subsequent development of the technique has led to it being patented.
At present, the primary use of Iowa vanes is on bank stability problems on small rivers and
on local sedimentation problems, such as at water intakes, on larger rivers. Results from
these works may in time identify broader applications.
Iowa vanes are fully submerged during high flows, but are above the water level at
low flows. The location and orientation of the vanes with respect to flow is critical to
success. Also, because success depends upon the structures having a precise effect on the
velocity vectors in the bend, stabilization of the upstream bend is recommended if upstream
channel migration is likely to change the flow patterns entering the vane system.
Initial evaluation of the East Nishnabotna installation indicated that flowlines through
the project reach were not affected by the structures (Odgaard and Mosconi, 1987).
8.2.2 BENDWAY WEIRS
Bendway weirs were developed by the U.S. Army Corps of Engineers as a method
to increase channel width in bends on the Mississippi River in order to improve navigation
conditions and reduce maintenance dredging requirements (Derrick et al., 1994). They also
induce deposition in the thalweg of the bend, which should enhance bank stability and reduce
the tendency for scouring velocities in the overbank area during floods. The success of
bendway weirs is based on the premise that the flow over the weir is redirected at an angle
perpendicular to the weir. When the weirs are angled upstream, the water is directed away
from the outer bank and towards the inner bank, or point bar.
The weirs on the Mississippi River are level-crested stone structures angled upstream,
with a crest elevation about 15 feet (4.5 meters) below low water. The design is based on
physical model studies at the Waterways Experiment Station, which indicated that a
pronounced upstream angle was required for the structures to function properly. The first
system was installed in 1990 on the Mississippi River upstream of the mouth of the Ohio
River, and is performing well. That installation and several subsequent ones are being
monitored, and other installations are planned.
Environmental aspects of bendway weirs appear to be favorable. Since they are
submerged well below low water level, the detrimental impacts on esthetics and safety which