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Model answers

Water Buenos Aires flooding

Since 1985, flooding in the Argentinean capital Buenos Aires has killed 25 residents. Dave Parker reports on how computer modeling has highlighted major shortcomings in the city's infrastructure.

Global warming is becoming the usual suspect when a major urban area is hit by increasingly severe and frequent flooding - except in the Argentine capital Buenos Aires. A detailed investigation into the performance of the city's drainage systems has been under way, and, says Halcrow Buenos Aires project manager Rodolfo Aradas, 'you can't blame the flooding on the weather.

'Our computer simulations clearly show that the root cause is a simple lack of capacity in the conveyancing system.'

Halcrow is one of the partners in the joint venture consultancy formed to draw up a masterplan for the Buenos Aires urban drainage system - the other partners are US-based Harza and local consultants ATASO and Latinoconsult.

The project began in May 2001 with a massive data collection exercise that would allow the project team to accurately map the drainage and road networks for the first time. All this data was then integrated into Infoworks CS, a computer modelling tool developed by UK firm Wallingford Software.

Overall catchment area is 28,500ha, made up of 11 separate catchment areas ranging between 190ha and 9,500ha.

With the exception of the central city area, all these catchments have a separate surface water drainage system.

Historically, Buenos Aires was built on land with a well defined natural drainage system.

Towards the end of last century most of the rivers and streams were culverted, some of them in box culverts of impressive dimensions. These were eventually supplemented with a conventional drainage network 'This is a good system in reasonable condition, ' Aradas reports. 'Unfortunately, it's just too small. As the city has grown over the last 50 years the amount of surface runoff has increased until it is now well beyond the system's capacity.'

In extreme conditions overland flows are a crucial factor in the pattern of flooding. Streets and highways can be considered as open channels once water levels reach a critical point. 'One of our main challenges was to come up with ways of predicting water behaviour at street level, ' Aradas explains.

'Each street has unique conveyance characteristics and for us to be able to predict flood levels reliably we have to trace accurately how the flood water flows and what effect it will have on the underground system.'

Some surprises emerged during the study. In places overland and underground flows were travelling in different directions, and one sewer actually linked two otherwise distinct catchment areas. 'Water velocities varied much more between catchment areas than we had expected, ' Aradas adds. 'So some areas are much more vulnerable to flood damage than others.'

For more than 30,000 'parcels' of the city, calculations were made of probable flood depths and durations. These were linked to property and people databases, so that the physical and economic impacts of flood events could be calculated and communities warned in an emergency situation.

At the heart of the project now is the computer model. Development continues, but already it contains around 18,000 nodes: 12,000 on street level and 6,000 underground. Some 25,000 'pipes' are detailed, of which 7,000 represent the actual underground system and the rest model overland flow at street level. For increased accuracy the 11 actual city catchment areas are divided into no less than 7,000 sub-catchments.

'We needed to be able to reflect the real time effect of overland flows, so the software's real time control function was very important, ' says Aradas.

'We can now see what's happening much more clearly and appreciate how different factors impact on each other.'

No quick fix has emerged, unfortunately. When a modern capital city is hit by major flooding the damage runs into many millions if not billions of dollars.

And it is that scale of investment that Aradas believes the study has shown is needed.

'There are some simple things, like increasing the capacity of inlets into the existing system and making sure they're kept unobstructed. But what is really needed is large infrastructure works, such as deep tunnels and local reservoirs.'

Such capital works take years to complete. Until then Buenos Aires and its 3.5M inhabitants will have to live with the flood threat.

In the meantime the joint venture group will continue to refine and maintain the drainage model, which will in due course serve as the basis for the new infrastructure design.

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