Controlling aquatic
vegetation in canal systems
Historically, aquatic vegetation control
measures applied in South Africa have
Aquatic vegetation in the Marico-Bosveld canals.
Filamentous algae (Cladophora glomerata).
Water Sewage & Effluent March/April 2018
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algaecides and herbicides with heavy
metals as the active ingredient were
also tested over the years. But the
problems associated with these products
were many, as their usage resulted
in the build-up of toxic residues in
irrigation soils, were corrosive to canal
infrastructure and irrigation equipment,
and necessitated downtime for the
canals, as well as withholding periods
from crops.
An alternative herbicide, with the trade
name of MAGNACIDE™ H Herbicide
(L8655), has recently been introduced to
the South African market from the United
States. This water-soluble aldehyde,
which comes in liquid form and has
acrolein as its active ingredient, can be
used in operational water conveyance
systems without interrupting irrigation
water deliveries, and is characterised
by superior effectiveness and quick
dissipation without any residue. What
makes acrolein a good prospective is
that the product does not accumulate
in crops, soil, or groundwater, and
treated water can be applied directly
to crops. Furthermore, acrolein (an
organic molecule) dissipates to harmless
carbon dioxide (CO 2 ) and water (H 2 O)
as end products, which makes it more
environmentally friendly.
It is also effective on both algae and
submersed macrophytes and is used
widely in the United States, Australia,
Canada, and other countries with
similar problems.
Scientific trials with MAGNACIDE™
H Herbicide (L8655) have been
successfully completed in Roodeplaat
and Hartbeespoort canal systems.
In the case of Hartbeespoort, water
been symptomatically orientated.
The problem was treated only once it
visibly influences the operation of the
system. Different control options have
been investigated over the years and
implemented with varying levels of
success. As the majority of South African
canals are in operation 365 days per
year, one of the greatest challenges was
finding a control option that takes the
canal out of operation for as short a time
as possible.
In the past, mechanical removal of the
aquatic plant biomass from the canals
had been a popular method; however,
it was expensive, labour intensive,
and time consuming. It has also been
found that in peak summer months, the
biomass cannot be removed efficiently
and fast enough. Long dry periods to do
mechanical cleaning and maintenance
were found to be unproductive.
To date, attempts to control the
biomass mechanically in systems that
continuously supply water seem to be
relatively unsuccessful and uneconomic.
In terms of chemical control methods,
the so-called ‘lower pH method’ was
favoured in the 1980s and 1990s, using
copper sulphate after lowering the pH
of the water with sulphuric acid. In
those years, local herbicides/algaecides
options were limited to the treatment of
aquatic vegetation with mainly copper
and diquat as the active ingredient.
Unfortunately, both substances have
their limitations under local conditions.
The copper products are more
effective on algae than on submersed
macrophytes, while the diquat-
based product offers greater efficacy
on macrophytes than algae. Other
conveyance structures, such as reducing
hydraulic capacity and flow-speed
in affected canals — some to such a
degree that the supplied water cannot
reach the terminal point of the canal
system. Aquatic vegetation can replace
significant volumes of the capacity of a
canal. This results in spillage and water
loss out of the system, as well as crop
losses owing to under-supply of water to
irrigators.
Another operational problem is that
of overestimation of the volume of
water supplied to the user. As aquatic
vegetation biomass replaces water
in a canal, a higher volume of water
is measured at measuring devices,
compared to what is in fact passing at
that specific location. The overestimation
of flow makes the optimal operation of
water supply a difficult task, as less
water is passing at a specific point than
what is measured. To compensate for
these artificially high water levels, more
water is often released into the system,
giving rise to spilling with subsequent
water loss and damage to canal
infrastructure.
Other problems with the presence of
aquatic vegetation in canal systems
include impediment of sluice gates
working at dividing structures, water
logging of long-weirs, structure failure
of concrete-lined irrigation canals due
to flooding, and blocking of irrigation
systems and filters at water purification
works by aquatic vegetation fragments.