A Top-down Approach To Flood Prevention

Gravity offers a simple and cheap way to attenuate stormwater flows – by storing water temporarily on a flat roof. All sorts of causes are being blamed for flooding in the UK; lack of dredging, poor management of catchment areas, construction on flood plains and paving over front gardens are all being mentioned in the press.

A flat roof allows stormwater to be temporarily stored and drain away slowly over a few hours, thereby reducing flood risks.
A flat roof allows stormwater to be temporarily stored and drain away slowly over a few hours, thereby reducing flood risks.

One thing is for sure – we will be paying a lot more attention to the topic given the experience, and the fact that wetter winters are predicted in our changing climate, with a certainty of more extreme events.

We know how to address the problem in new development and how to attain sustainable credits. Large underground storage tanks with specialist flow control regimes allow us to attenuate stormwater flows to minimise their impact on the downstream drainage system.

Sometimes, because of the level of the sewer, we need to pump the stored water – adding to the cost and complexity of the system. And all this is expensive – not a great message for these cash-strapped times. Recent experience has also shown us that it can be ineffective when inundated by a larger flood.


However there is a simple and cheap alternative that makes use of a free resource that we often neglect: gravity. Attenuating roof drains store stormwater on a flat roof, and a depth of only three inches is adequate to provide meaningful attenuation of even a big storm.

The water will drain away slowly over a few hours, without affecting the downstream drainage system. The only additional costs are for slightly higher waterproofing details and a simple insert in the roof drain.

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Worried about flooding into the building? No problem – simply provide extra overflow drains. And can the structure take it? The weight of the water is less than the weight of a heavy snowfall. As most roofs are designed for snow, the roof will be strong enough already.

With modern waterproofing solutions carrying long warranties, roofs can be designed flat. So very little needs to change in the design. This all makes it a very cost effective solution to what can otherwise be an expensive and unappreciated underground installation. It’s also fully compatible with green (sedum type) or brown (biodiverse) roofs, and in fact can improve their appearance and biodiversity by providing a wider range of growing conditions.

If you want to take this solution a little further, then why not use the water as it trickles away down the pipes? The attenuating drains filter the rainwater, so you could pipe it to small tanks for flushing the toilets in the building. This makes it a zero-energy rainwater harvesting system that provides a sustainable urban drainage solution at the same time.

A team of Arup engineers in Bristol have developed these solutions with manufacturers and proven them in the field. The construction industry just needs to start implementing the techniques.

There’s no doubt that finding the full solution to flooding is a complex task. From time to time though it’s very satisfying to find an easy idea that is part of the solution, and saves us money at the same time.


This article originally appeared in Arup.

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