Wadi Hanifah is the normally dry river that runs for seventy kilometres through Riyadh, the capital of Saudi Arabia, and a city of almost 5 million in the middle of the arid Najd plateau. Ten years ago the river bed was a toxic wasteland, used as a dumping ground by municipalities and construction companies, as a traffic detour by frustrated motorists, and as a convenient corridor for power lines and sewage pipes.
As Riyadh's population swelled, the ground water under the city began rising, with water draining down from septic tank systems and rural surface irrigation. To control the rising water table, ground water was pumped into the wadi; so too were increasing volumes of sewage effluent from the city's treatment facilities. Since the 1980s, the mid and lower parts of the wadi have been carrying permanent water flow, even in dry weather, though naturally the water quality is very poor.
Now the wadi has been transformed, the water cleaned up, the river regraded and reshaped, restricted to traffic and opened to residents who can now picnic along its banks. For this, the ArRiyadh Development Authority (ADA), their engineers Buro Happold and planners Moriyama & Teshima won one of the world's most prestigious design awards, the Aga Khan Award for Architecture, in 2010.
Alan Travers, Director of Buro Happold's Water sector, has been leading the engineering team since the project began in 2001. When the ADA tapped his firm, he says they had a "very clear vision they wanted to deliver". The most immediate problem was flooding. When it does rain in Riyadh, it can be "almost monsoonal", and the silt and waste dumped in the river meant the banks would overflow, and runoff would be trapped in stagnant ponds breeding mosquitoes and disease. Over the length of the river, the regrading exercise removed around 1.25 million cubic metres, filling in holes or lowering levels by up to three metres, to make the wadi an effective flood channel as well as dry-weather recreational area.
The second task was to make the water safe for human contact. A large sewage treatment plant in the south of the city releases 500,000 cubic metres of waste water per day, half going into agriculture and half being discharged into the wadi. "The quality of effluent is variable and often extremely poor — the plant is not functioning well," says Travers. He and Buro Happold created a series of interventions along the length of the river to clean the water using sustainable, low-cost and low-maintenance techniques such as micro catchments, swale plantings, rockeries to aerate the water and a large bio-remediation facility which includes fish and other organisms to break down pollutants.
Not that residents may soon be seen swimming in the water. "The objective was not to achieve bathing water quality standards — that would be an unreasonable expectation, and there's no demand for it" — but to ensure that occasional contact with the water, such as children splashing their hands in it, would carry no risk of illness, Travers explained.
Nor will residents be drinking water recycled from the wadi. Cultural regulations forbid the consumption of water derived from human sewage, no matter how clean. Nor are the crops raised on the recycled water consumed by humans, but used only as fodder for livestock, a food cycle that puts the waste water at sufficient distance from consumers to satisfy cultural norms.
However for other cities in arid areas not governed by such rules, the volume of water that can be recycled using these processes offers important lessons for their own sustainability. In the water balance assessment that Buro Happold conducted in 2001, they calculated that 250,000 cubic metres flowing through the wadi could be captured for reuse by the city every day. By 2020, when the city's population is expected to reach over 7 million, that could increase to 1 million cubic metres per day, equivalent to the daily requirements of 3 million residents!
Now, I know from my background research that the Riyadh province receives about 90 millimetres of rainfall per year. Falling over the Wadi Hanifah's mostly rural catchment area of 4,500 square kilometres, this works out to 400 million cubic metres per day that Travers says the wadi can recycle and release for reuse. Does this mean that Buro Happold has created a model where a desert region could sustainably support the water needs for a city of 3 million people?
Travers is quick to correct me: "I can see the logic of what you're calculating there, but there are a lot of reasons why you couldn't interpret it that way."
"Firstly, the average might be 90 millimetres but that varies enormously from year to year. Secondly, the rain never falls over the whole catchment at one time; it might be flooding in the north and bone dry in the south."
"The third point is that if, say, the rain is falling in March or April when temperatures are 30 to 40 degrees, you can expect most of it to evaporate before it ever starts to run off. Finally, when you do get enough water, nevertheless a lot penetrates straight into the ground in rural areas before you can collect it. That's a good thing, because that's how shallow aquifers are recharged. But if you take all of that into account, there's really not a lot of runoff left for the city's residents."
So the 1 million cubic metres of water available by recycling is still predicated on most water being desalinated at Jubail on the Arabian Gulf or drawn from the Minjur aquifer two kilometres below the city. The important lesson is not that the Wadi Hanifah catchment could support a city of 3 million people as a closed system, but that the wadi's sustainable techniques can alleviate the need to draw on those other sources by up to the equivalent of 3 million people's daily needs.
That might not be enough for purists who believe the populations of cities like Riyadh and Dubai should be diverted to wetter regions, but it is an extraordinary improvement on existing models, and an enormous cost and resource saving that other cities' leaders should be learning from.
As Riyadh grows by another 3 million people, its water needs will only grow more intense. Realistically there is no hope of reversing plans to build a new desalination plant by 2018, or of ceasing dependence on the Minjur aquifer's fossil water, which currently provides up to 750,000 cubic metres per day.
Yet the knowledge that the wadi could alleviate future demand on these systems by up to 1 million cubic metres per day is enough to start softening cultural attitudes. National directives are now being issued suggesting that certain recycled water may be used on crops for human consumption, but only for crops that must be cooked beforehand. And changing awareness has reduced the average resident's daily water usage from around 550 to 320 litres per day, which could be brought down further. The government could help by reducing leakage rates within the city's pipes from the current rate of around 20 per cent, and by reducing water subsidies so that consumers are sensitive to the true cost of water.
The success of the Wadi Hanifah has spurred other long term shifts in how urban development is managed. Inevitably the beautification of the wadi has attracted property speculators to build along the river and driven up land prices tenfold in some areas. But to preserve the new environments, the Ministry of Municipal and Rural Affairs has recently released a declaration prohibiting the subdivision (and hence the development) of riparian land, not just on the Wadi Hanifah, but on all wadis throguhout Saudi Arabia. Authorities are acknowledging that waterways and runoff areas must be protected as inalienable public assets.
The ADA has also commissioned Buro Happold to develop flood risk maps for the Wadi Hanifah. As a result of months of hydraulic modelling, Buro Happold has now produced 1-in-50- and 1-in-100-year flood event risk maps for the whole river, which will inform the planning of future land development and the establishment of a flood warning strategy. So the transformation of the wadi, from a dumping ground to a public asset, has brought about the creation of a whole new set of public planning controls and management systems, beyond the gains in sustainable water management originally sought.