Green cities or concrete jungles?

In the face of unprecedented population growth coupled with rapid urbanisation rates, South African cities are shouldering 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.

In an effort to shed some light on the types of green infrastructure that could more successfully 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, it’s safe to say that green infrastructure is a hot topic among urban planners.

Water scarcity in South Africa

To illustrate the importance of rethinking the approach cities have been taking to stormwater, Armitage started his presentation by setting the scene: South Africa’s population has increased rapidly from 50 million in 2009 to almost 58 million in 2019.

Even though population growth is gradually slowing down, it’s also important to consider that – in accordance with global trends – 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.

Using Cape Town as an example, he pointed out that 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.

“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.”

Stormwater as resource

Conventional drainage systems are designed to minimise inconvenience and reduce flood risk by removing stormwater to the nearest receiving waters, such as 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.

In other words, 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.

“It means a change in the way we visualise the city,” he said.

In short, urban areas need to start looking less like concrete jungles – covered in impervious hard surfaces – and more like green cities with pervious “soft” surfaces such as green roofs, green walls, rain gardens and infiltration trenches.

Instead of letting stormwater run off, polluting receiving waters as it goes, 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 the seminar series on permeable pavements, and bioretention cells as a possible alternative.

Having investigated various sites across Cape Town and increasingly Johannesburg, Armitage found that PICP was proving to be a less successful form of SuDS than originally hoped for. A few of the problems his research highlighted 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 with the drawing up of 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.

“One of the alternatives that hasn’t been used that much in South Africa is bioretention,” Armitage said.

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 by one of Armitage’s students at a bioswale in Kraaifontein outside Cape Town revealed more than 90% removal of pollutants such as heavy metals and phosphorous.

“One of the alternatives that hasn’t been used that much in South Africa is bioretention.”

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

Armitage used Singapore as an example of how green infrastructure can help build water resilience in cities.

In that city, 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 to navigate and in which to linger.

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.

In its efforts to gain independence from Malaysia as its main water supplier, Singapore is leading the way as a water-resilient city but also offering a glimpse of how future cities across the world could look.