Willows were grown in the Colorado State
University to use as part of the research project.
“Some resource agencies such
as state departments of natural
resources tend to discourage, and in
some cases actually prohibit, the use
of rock for streambank protection;
they push for using some kind of
vegetation,” Lagasse said. “But we
feel that using vegetation and hard
material together is best.”
David Reynaud, senior program
officer with the NCHRP, agreed
that more and more entities are
pushing for environmentally sensitive
techniques.
“There’s a movement in the
environmental area to use natural
materials rather than stone or other
man-made materials,” Reynaud said.
Using vegetation provides
environmental and aesthetic benefits,
improves conditions for fisheries and
wildlife, and helps improve water
quality. However, vegetation also has
its limitations. Plants can fail to grow
or can die in drought conditions, they
can get eaten by wildlife or livestock,
and they may require significant
maintenance, to name a few
potential problems. Therefore, the
study recommends that “vegetation
alone should not be seriously
considered as a countermeasure
against severe bank erosion where a
highway facility is at risk” but should
be used in combination with other
“hard” engineering approaches.
TIME FOR TESTING
To conduct their study, the
NCHRP team devised a test in
cooperation with the hydraulics
laboratory at Colorado State
University (CSU) in Fort Collins,
Colorado. First, the team selected
two bank protection treatments
from the original 2005 NCHRP
report to test with the CSU flume,
which is a full-size model of an open
channel that simulates real river flow
conditions. Using a flume to test
bank protection treatments with a
live vegetative component had never
been done before, Lagasse said.
What made the test even more
revolutionary was that the team grew
willows to maturity at real-scale in
a climate-controlled greenhouse at
CSU and then transferred them to the
flume.
“We were able to take the
biotechnical treatments to the big
outdoor flume and subject them to
water discharges to simulate various
levels of river flow, and then see how
the vegetation reacted to the flow,”
Lagasse explained.
The team conducted two
tests. The first test, a biotechnical
A full-size flume at CSU’s campus
allowed the Ayres team to test
streambank protection measures in
real-world conditions.
approach, used a rock toe (base of
the bank) with locally harvested
willows. The second treatment tested
a combination of vegetation with a
natural fabric material configured
in soil lifts with willows inserted
between the lifts, a technique
commonly called vegetated
mechanically stabilized earth (VMSE).
To transport the willows from
the greenhouse, the willow trays
were put on rollers and pulled out
with tractors. A crane put the trays
into place in the flume. For each test,
water ran for four hours through the
flume, and velocity was measured at
various points. Following each test,
changes or damage to the treatment
were examined and documented for
analysis.
“This type of test had not been
done before. No one had this idea
or this type of facility; we were truly
using real-world conditions,” Lagasse
said. “This was a major quantum leap
forward.”
RESULTS REVEALED
In the end, the tests showed
that of the two treatments tested
in the laboratory, one (live siltation
and live staking with a stone toe)
met or exceeded all performance
expectations. The second treatment