GREEN CITIES OR CONCRETE JUNGLES?
In the face of unprecedented population growth coupled with rapid urbanisation rates, South African cities have an increasingly heavy burden of ensuring that all citizens have access to clean water – a basic human right. With large areas of the country plagued by drought, this is no easy task.
So, how do we keep water flowing to and in our cities?
According to Professor Neil Armitage, deputy director of the University of Cape Town’s (UCT) Future Water Institute, stormwater is an important resource that is currently being overlooked and underutilised.
To shed some light on the types of green infrastructure that could harness the power of stormwater, Armitage recently hosted a seminar series in seven cities across South Africa, focusing on permeable interlocking concrete pavements (PICP) and bioretention cells, or “rain gardens”.
He was joined by Professor Ryan Winston, from Ohio State University, who shared his extensive knowledge around the design, operation and maintenance of both PICP and bioretention cells.
With more than 500 professionals from a wide range of backgrounds – including civil engineers, landscape architects, government and municipal staff – having signed up for the seminar series, green infrastructure is clearly a hot topic among urban planners.
Water scarcity in South Africa
South Africa’s population has increased rapidly from 50 million in 2009 to almost 58 million in 2019. Despite population growth gradually slowing down, South Africa is an urbanising population. This means that as the number of people living in our cities increases, so too does the demand for water.
“The problem with this is (that) when you’re a rapidly growing population in a country that doesn’t receive much rain, what happens when it doesn’t rain? Day Zero. This is the destination of the whole of South Africa if we don’t manage our water resources better,” Armitage warned.
But how can this be done?
Drawing from the research of Professor Tony Wong, chief executive of the Cooperative Research Centre for Water Sensitive Cities, an Australian government initiative, Armitage argued that instead of roads being the defining factor in modern cities, drainage systems should instead be viewed as the main integrating element.
In Cape Town for example, when the city receives normal rainfall, the mean annual run-off (MAR) exceeds the annual demand for water.
Currently, however, that run-off water is being wasted as none of it feeds into the city’s water system.
Stormwater as resource
Conventional drainage systems are designed to reduce flood risk by removing stormwater to streams or rivers, as rapidly and efficiently as possible – often through the use of concrete pipes and channels.
Unfortunately, these also raise flood peaks, increase flood volume, pollute receiving waters and destroy biodiversity.
Thus conventional drainage systems are highly problematic and, in many cases, even dangerous to the overall well-being of a city.
The only way to remedy this situation, Armitage argued, is for cities to invest in sustainable drainage systems (SuDS), which attempt to mimic the pre-development situation with regards to run-off quality, run-off quantity, and protecting biodiversity.
Urban areas need to exchange impervious hard surfaces with pervious “soft” surfaces such as green roofs, green walls, rain gardens and infiltration trenches.
Instead of letting stormwater run off, green cities allow stormwater to seep into the ground, recharge the water table and filter out pollutants before being received by new sources.
PICP vs bioretention cells
While there are many ways to harness stormwater more effectively in our cities, Armitage focused on permeable pavements, and bioretention cells.
From sites across Cape Town and Johannesburg, Armitage found that PICP was proving to be a less successful than originally hoped for. Problems include lack of environmental control during construction and lack of maintenance, leading to blockages and poor water quality.
As a result, he established the PICP Working Group in 2018 in an effort to assist in drawing up suitable South African guidelines and standards for the design, construction and maintenance of permeable pavements.
The working group has suggested that alternatives should also be investigated – such as bioretention,
Taking the form of attractively landscaped depressions, bioretention cells capture stormwater run-off from impervious surfaces and, as the water filters through layers of earth and soil, remove harmful pollutants.
Tests conducted at a bioswale in Kraaifontein outside Cape Town revealed over 90% removal of pollutants such as heavy metals and phosphorous.
Apart from this clear advantage, bioretention cells come with the added benefit of providing green spaces in areas that would once have been covered in concrete.
Learning from Singapore
In Singapore, concrete has been pulled up in many areas and replaced with parks, wetlands and gardens. Apart from assisting in creating SuDS, it has also made the city a more pleasant place.
Perhaps most innovatively, Singapore’s old harbour, which is too small to receive modern ships, has been dammed up and turned into a freshwater reservoir. A portion of Singapore’s water supply is thus sourced from stormwater that has been harvested from the centre of the city and cleaned up close to source.
Singapore is leading the way as a water-resilient city and offering a glimpse of how future cities across the world could look.fhttps://www.news.uct.ac.za/article/-2019-06-04-green-cities-or-concrete-jungles
