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IRRIGATION FEATURE

STORMWATER’S POTENTIAL AS

AN ALTERNATIVE WATER SUPPLY

cientists and engineers from the at 18 Mm3, or 18 000 megalitres. Okedi Cape Water Supply System, which currently
Future Water Institute at the University assessed three demand options for non- provides 98% of Cape Town’s potable
Sof Cape Town (UCT) have been potable water: Sc1 for agricultural irrigation, water, it would necessitate installing a dual
investigating the potential of stormwater Sc2 for residential garden irrigation and reticulation system. Laying all those extra
harvesting for a few years. One such study toilet flushing, and Sc3 for residential pipes to transport the non-potable water
was Dr John Okedi’s WRC-funded research garden irrigation, toilet flushing and would be extremely expensive, not to
project titled ‘The Viability of Stormwater irrigation of public open spaces. He found mention disruptive to everyone.
Ponds in the Zeekoe Catchment as Water that even with the use of RTC, the storage The study found that it would indeed be
Resources for Cape Town’. The catchment in the ponds was only able to supply 40%, more cost-effective to harvest stormwater
has more than 60 stormwater ponds within 60% and 58% of the demands of Sc1, Sc2 for potable water, which is needed
its 89 km2 area, as well as three large but and Sc3, respectively. Much of the mean throughout the year, unlike irrigation water.
shallow lakes – Zeekoevlei, Rondevlei and annual stormwater volume would still Toilet flushing, although year-round, makes
Princess Vlei. overflow from the ponds and be lost as up a very minor portion of total demand.
Through desktop research and a spillage. Also, there are health risks associated with
modelling approach, Okedi assessed the Next, Okedi considered making use of consuming fruit and vegetables irrigated
extent to which water from stormwater the vleis for additional storage. This would with untreated stormwater, which is
ponds could meet the demand for non- allow 22% of the modelled mean annual typically highly polluted, not only through
potable uses in the form of irrigation and stormwater volume to be stored, with faecal contamination but also by heavy
toilet flushing. Given that Cape Town 70%, 79% and 76% of the Sc1, Sc2 and Sc3 metals and hydro-carbons. The findings
lies in a winter rainfall area and water demands being met. showed, however, that it would be difficult
for irrigation is mainly needed in the dry “If you converted the vleis to reservoirs, to abstract water from the vleis, treat it
summer months, creating a mismatch they would provide a significant amount and supply it at a rate that would minimise
between supply and demand, some means of storage, but the issue is practicality,” says spillage.
of storing stormwater would be required. Okedi. “They are used for recreation and A better option would be to take
There is limited space to build new storage have ecological value, so people would advantage of the underground storage
facilities, but real-time control (RTC) offers not be happy if their existing function was potential of the Cape Flats aquifer, through
a way of optimising storage in the existing taken away!” Both Rondevlei and Zeekoevlei managed aquifer recharge (MAR). The
stormwater ponds. form part of the False Bay Nature Reserve, predominantly unconfined aquifer is close
RTC allows water levels in urban drainage which was declared a Ramsar wetland site to the surface here, and is already the main
systems to be dynamically controlled. At its of international importance in 2015, while source of water for the Philippi horticultural
simplest, it might entail somebody being Princess Vlei forms the core of the Greater area, where farming of vegetables and
on site to monitor incoming flow and Princess Vlei Conservation Area and was livestock takes place on approximately
water levels at particular ponds, and then designated a provincial heritage site in 2 000 hectares of farmland, most of it falling
opening or closing sluice gates according September 2020. within the Zeekoe catchment.
to operational rules. Advanced RTC would Thus, the water levels in these permanent Okedi investigated the MAR that could
rely on a network of automatic rain, flow water bodies should not be manipulated be achieved if the stormwater ponds were
and water level gauges, with data fed into on a frequent basis; currently, periodic adapted to promote infiltration into the
predictive models that issue instructions ‘drawdowns’ are done for ecological aquifer. About 70% of the stormwater
for remote control through a SCADA purposes and to lower water levels enough ponds in Cape Town are detention ponds
(supervisory control and data acquisition) for mechanical removal of accumulated – unlined earth basins constructed to
system. Regardless of the complexity, the sediment and reeds. But what if water hold stormwater for short periods during
aim would be to manage the drainage was simply abstracted from the vleis, pre- rainstorm events, after which the water
system so that it fulfils its flood prevention treated and then pumped approximately flows away slowly, evaporates or infiltrates
role, while also ensuring that the maximum 30 km to the Faure water treatment plant, into the ground. The modelling study
amount of stormwater is captured and where it could be fully treated to potable assumed that the detention ponds could be
stored instead of being lost to the sea, standard and then piped through the converted to bio-retention cells, typically
known as spillage. existing water distribution network? composed of a vegetated surface layer, a
The total capacity of the stormwater Because while stormwater harvesting for middle soil layer with organic matter or fly
ponds in the Zeekoe catchment represents non-potable uses such as irrigation and ash mixed with a filter media, and a deeper
about 5.5% of the catchment’s average toilet flushing would theoretically reduce storage layer consisting of crushed stone or
annual volume of stormwater, estimated pressure on the six dams of the Western gravel, with a drain at the bottom.

24 Landscape SA • Issue 129 2023