Scientist Gordon Maclear warned more than two decades ago that Cape Town was edging slowly but surely towards a water crisis of epic proportions. He published a scientific report in an academic journal outlining the evidence. Cape Times journalist Barry Streek reported in the mid-1990s on academic concerns that the city was set to run dry within 20 years.
But the information was ignored by city authorities and local politicians, perhaps because the time line was a long one or the scenario implausible. As Day Zero looms, Maclear has stepped forward again – this time not to say ‘I told you so’, but to suggest solutions. Maclear’s ideas for solving the Cape Town water crisis are radical: He reckons installing an underground metro is the answer. Constructing lines beneath the city would also alleviate traffic jams. – Jackie Cameron
It is common and much-publicised knowledge that Cape Town is facing an unprecedented water crisis with ‘Day Zero’ looming in the next few months. My intention is not to flog a dead horse but to suggest a source of water supply to Cape Town that will be both sustainable and provide a value-add by reducing the city’s traffic congestion.
It is understood that the current water crisis has received massive local and international news coverage and that many innovative solutions have been presented and steps are being taken to alleviate the water shortage. It is also understood that these solutions will not be installed and commissioned overnight and that significant expenditure will be required to prevent a similar situation occurring in the future. Remember that a 1:100 year drought period, that the area has been experiencing, does not mean that this weather situation only occurs every 100 years – it could happen again in the next ten years.
In 1995, whilst working in the Geohydrology Directorate of the then Department of Water Affairs and Forestry, I wrote an article entitled ‘Cape Town Needs Groundwater’ highlighting the water supply potential of the Cape Flats Aquifer Unit (CFAU). This article 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 approx. 750 km2 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 <1,000 mg/L TDS) falling within the drinking water standards. To give an idea of the magnitude of the water contained in the CFAU, assuming a conservative aquifer thickness of 10 m over the whole CFAU area, there is a total of 150,000 ML (150 million m3) in storage, equivalent to approx. 60,000 Olympic sized swimming pools. There is an estimated 30 ML/day (30 megalitres per day, or 30,000 m3 per 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.
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 10 m 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.
By way of example, if we consider a single Underground line from Mitchell’s Plain to the Cape Town city centre (a length of approx. 20 km), a 5 m diameter underground rail tunnel, encased by a 6 m diameter permeable sleeve (typically comprising stainless-steel mesh with a wedge-wire design to maximise permeability), could store approx. 173 ML (173,000 m3). In this example, the volume of water contained within the outer permeable screen of a single underground line is more than daily production rate of the planned groundwater development schemes for the aquifers in an around the greater Cape Town area: viz. the Cape Flats Aquifer Unit (80 ML/day), the Table Mountain Group Aquifer (40 ML/day) and the Atlantis Aquifer (30 ML/day). Obviously this volume represents only the total storage potential of a single underground line from Mitchells Plain to 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, e.g. 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 across South Africa with similar hydrological settings such as Port Elizabeth (which is also currently experiencing a severe water shortage) and Durban.
Whilst it is a given that 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… As South Africans we 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 amount of scientific, engineering, environmental and economic issues will need to be considered and a full feasibility study will be required 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 sea-water intrusion, water quality monitoring, subsidence prevention etc.
But with fore-sight, commitment, planning and passion it can be done. Let’s go underground to get on top of our burgeoning water and traffic issues.
By Gordon Maclear. Source: Biz News