It is common, much-publicised knowledge that Cape Town is facing an unprecedented water crisis with Day Zero looming.
My intention is not to flog a dead horse but to suggest a source of water supply to Cape Town that will be sustainable - and provide a value-add by reducing traffic congestion.
The crisis has received local and international news coverage and many innovative solutions have been presented and steps are being taken to alleviate the water
shortage.
But these solutions will not be installed and commissioned overnight and significant expenditure will be required to prevent a similar situation in future. Remember that a 1:100 year drought period, that the area has been experiencing, does not mean this weather situation only occurs every 100 years - it could recur in the next 10 years.
In 1995, while working in the Geohydrology Directorate of the then Department of Water Affairs and Forestry, I wrote an article titled “Cape Town Needs Groundwater”, highlighting the water supply potential of the Cape Flats Aquifer Unit (CFAU)
It detailed the benefits of using this source of underground water to reduce the demand on Cape Town’s bulk water supply sources.
The CFAU covers an area of about 750km² and comprises the low-lying sand unit stretching from Table Mountain in the west to Tygerberg in the east, and from the Table Bay coastline in the north to the False Bay coastline in the south.
The water is mostly classified as “fresh”, with low to moderate salinity, generally <1000 mg/L TDS (mg a litre total dissolved solids), falling within the drinking water standards.
To give an idea of the magnitude of the water contained in the CFAU, assuming a conservative aquifer the thickness of 10m over the whole CFAU area, there is a total of 150000ML (megalitres) -150 million m3 - in storage, equivalent to about 60000 Olympic-sized swimming pools.
There is an estimated 30ML/day (30 megalitres a day, or 30000 m3 a day) of fresh groundwater flowing out to the sea along the two coastlines bordering the CFAU; which is significant, representing a daily loss of good quality water into the ocean, and is equivalent to the total rate of groundwater abstraction planned for the Atlantis Aquifer water supply scheme.
So Cape Town needs water desperately. But the city also faces a growing traffic nightmare, with commuters needing more time to get to and from their destinations along transport routes confined by the footprint of the built city area, Table Mountain and the ocean.
Installation of an underground rail system, such as the London Tube, will have an immense benefit to the Cape Town metropole’s transport network and reduce pressure on the existing on-surface transport system.
In addition, an underground rail network can be a vast source of water. Since the water level in the CFAU is shallow (generally 5 to 10m below surface), the underground rail system will be below the water level and located within the aquifer formation.
With engineering design, the concrete tunnels that contain the rail system can be surrounded by a permeable screen or sleeve, enabling the tunnels to act as groundwater collector galleries.
From a hydrogeological perspective, horizontal galleries are orders of magnitude more efficient as a water-harvesting method than conventional vertical boreholes, and are extensively used in the global mining industry to drain water from below large open-pit operations.
If we consider a single underground line from Mitchells Plain to the Cape Town city centre (about 20km), a 5m diameter underground rail tunnel, encased by a 6m diameter permeable sleeve
(typically comprising stainless steel mesh with a wedge-wire design to maximise permeability), could store about 173ML (173000 m3).
In this example, the volume of water contained within the outer permeable screen of a single underground line is more than the daily production rate of the planned groundwater development schemes for the aquifers in and around the greater Cape Town area: ie the Cape Flats Aquifer Unit (80ML/day), the Table Mountain Group Aquifer (40ML/day) and the Atlantis Aquifer (30ML/day).
Obviously this volume represents only the total storage potential of a single underground line from Mitchells Plain to the Cape Town CBD and not all of this stored groundwater could be extracted instantaneously; but sub-level sumps (comprising concrete caissons) could be constructed below each underground station along this line, from which groundwater could be pumped to surface for distribution to the local area surrounding the station.
If potable quality water is required, then water treatment plants can be set up at strategic locations, or the groundwater pumped directly into a bulk “grey water” reticulation system for, for example, irrigation.
The possibilities are endless and this dual method of reducing traffic congestion and providing a source of water can also be applied to other cities with similar hydrological settings, such as Port Elizabeth (which is also experiencing a severe water shortage) and Durban.
While the design, installation and commissioning of an underground will be a massive capital outlay, and will be an engineering challenge to excavate a tunnel network in the Cape Flats Aquifer geological unit largely comprising unconsolidated, saturated sand, the cost to the greater Cape Town metropolitan of running out of water will be billions...
South Africans are not short of hydrological and engineering expertise, with the Gautrain being
a good example of a highly successful underground engineering project. And the benefit is that this engineering project will be self-funded over time through rail ticket sales.
It will also be a safer and more environmentally friendly transport system compared with the current huge daily surface traffic volume that results in road accidents and generates toxic exhaust fumes.
To pacify the naysayers and armchair academics, it is understood that a massive number of scientific, engineering, environmental and economic issues will need
to be considered and a full feasibility study done to make this suggested underground rail system a reality.
Factors that will need to be addressed include water level monitoring to prevent over-abstraction and possible seawater intrusion, water quality monitoring, subsidence prevention etc. But with foresight, commitment, planning and passion it can be done. Let’s go underground to get on top of our burgeoning water and traffic issues.
* Maclear is a hydrogeologist for the African region of a global
mining company