data from the National Invasive Alien
Plant Survey, which was conducted in
2007 and 2008. Using 2008 as the base
year, the estimated costs of clearing are
based on the cost of clearing, including
the costs of follow-up clearing.
“The dynamics of alien invasive
plants are such that repeated follow-
up clearing is required to counter the
regeneration of the plants,” explains
Le Maitre, adding that a follow-up fire
within two years of the initial clearing
can kill all the seedlings and minimise
the need for further treatments. It
is important to note that the results
show the estimated effects of the
uncontrolled growth of invasive alien
plants on river flows into the dam,
rather than on yield.
Berg River Catchment
De Hoop Catchment
The De Hoop Dam is located on the
Steelpoort River and is predominantly
invaded by eucalyptus and wattle
species. At the end of 2013, the Working
for Water programme had spent close
to R3-million clearing the equivalent of
180 hectares of dense alien invasive
plants in the catchment.
In the worst-case scenario, as per
the Berg River Catchment, invasion in
the De Hoop catchment would increase
from 7% to between 53% and 55% of the
invadable area of the catchment. River
flows into the dam would be reduced by
between 42% and 44% and the cost to
clear the plants at this point would be
over 700% more.
These results, intended to be
conservative and illustrative rather
than precise, demonstrate the
inevitable outcome of the rapid spread
and densification of invasions in areas
of natural and semi-natural land cover
when control is not effective or not
done at all.
Dams are built to supply a certain
volume of water at a 98% level of
assurance, so they are designed in
such a way that water restrictions
will be required in two out of every
100 years. Allowing invasions to
proceed unchecked would mean far
more frequent water restrictions.
However, these are only two of the
dams whose inflows are at risk from
unmanaged alien plants invasions.
“If these problems are not tackled,
we will see a major water security
problem develop over time across
Located in Franschhoek, the dam is
part of a scheme that provides water to
Cape Town. The government’s Working
for Water programme has been tasked
with clearing invasive alien plants and,
at the end of 2013, spent more than
R90-million clearing the equivalent of
3 600 hectares of dense stands of pines
and acacias in the Berg River Dam
catchment.
In the worst-case scenario, where
the average tree age is 20 years, the
invasion would increase from 3% to
between 49% and 99% of the invadable
area in the catchment. The result
would be that river flows into the dam
would be reduced by up to 50%, with
significant implications for Cape Town
and the irrigation schemes that depend
on water from this dam. To clear the
invasion at this point, would cost over
3 000% more.
At the end of 2013, government spent more than R90-million clearing the equivalent of 3 600 hectares of dense stands of pines and
acacias in the Berg River Dam catchment.
22
Water Sewage & Effluent May/June 2019
www.waterafrica.co.za