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Flood management: Diversion tactics

Water management professionals are bracing themselves for this spring’s weather cliché, as humourists predict a hosepipe ban by June. But do they have a point?

Engineers and public agencies have battled heroically - and in many cases very successfully - to minimise the human and infrastructure cost of the recent record-breaking rainfall.

Drought and floods are things that we should be looking at as a continuum, not as individual crises

But the situation has been so catastrophic in some parts of the country that the possibility of a summer drought has not been an immediate priority. As ICE water and wastewater expert panel head Michael Norton says, nationally our paradigm is “let’s get rid of this water”.

But, he adds: “Drought and floods are things that we should be looking at as a continuum, not as individual crises. Water security can be seen from the viewpoint of either too little or too much. The public view is: ‘why don’t we store more?’

“The problem with storage is that people see civil engineers and they think we must be thinking grandiose reservoirs. But what we are looking for is much more intelligent ways of doing it. For example, with the Somerset Levels - some of that could have been held back by using different management techniques in the hills nearby.”

Extreme weather

In extreme storm events, such as the UK has seen recently, water run-off from the land increases by up to 1,000-fold, and dealing with such a huge amount so suddenly is virtually impossible without the right mechanisms in place.

As Thames Water has found in Abingdon, Oxfordshire - which was severely affected by flooding - the public is very much against the hard engineering idea of a huge new reservoir (NCE 18 April 2013). Dam-building across rivers and levels has also been unpopular.

But there is hope that soft engineering - encouraging water to stay beneath the top layers of soil, slowing down the passage of water across the landscape - will be more acceptable as people use the lessons of the recent floods to learn more about what is needed.

“There is a lot of scope in the UK for storing more of the excess water, not just during really intense storms, but also in the less extreme conditions that are more common,” says Norton. “We want to divert water so the rocks below the top soil soak it up. In many places at the moment, even that storage capacity is taken, and groundwater is a problem because aquifers beneath the soil are totally soaked and there is nowhere for the water to go.”

The problem in dry spells is that water supply drops because rainfall is not recharging aquifers. Norton explains that sustainable drainage systems (SuDS) can help the aquifers. “It’s all about using storage in very clever ways, using space in the rocks, sands and gravels below the ground to store the water. This is ASR - aquifer storage and recovery - using the aquifer to store more water.”

The ICE’s own report Water: State of the Nation 2012 was clear on this, too. “Multiple, smaller-scale, local solutions provide a significant opportunity to help alleviate current and future water supply deficits,” it says.

“This ‘distributed infrastructure’ is currently underdeveloped in the UK. Constructing medium and small scale storage, such as household and community-scale rainwater harvesting and SuDS, provides an effective way of collecting and managing precipitation at a local level and can help recharge groundwater levels.”

It continues: “ASR also offers a means of storing a proportion of high river flows and of treated wastewater. Recycling of grey water in the home and of treated wastewater for indirect uses offer, in effect, new sources of water to help alleviate the supply deficit.”

The idea of using a variety of different solutions up and down the country is widely accepted as being the best way forward, and the Environment Agency doesn’t have a single “position” on flood water management that it adopts nationally, according to its head of strategy Pete Fox.

“The way we look at trying to tackle flooding is by taking a ‘whole catchment’ approach,” he says. “If you take a partial approach you tend to just shift the problem somewhere else. Water storage is part of the toolbox when you look at catchment solutions.”

When the Agency identifies that a particular catchment has a flooding issue, it steps back to understand why it happens and what the characteristics are - and then decisions are made about how to address it locally.

“In the Pennines, where they get quite steep gradients on rivers, over hard geology, there are rapid response catchments - that’s where you get your raging torrents, roaring floodwaters that can be devastating for several hours - and then the next day you’re thinking ‘where’s it gone?’” says Fox. “But the Thames, with its wide flood plains, has slow infiltration across the catchment - it drains slowly.”

He describes the recent employment of an on stream storage area near central Wigan, where the Agency has incorporated a funnel in the dam that regulates the flow so that it doesn’t flood. And the Leigh Barrier on the River Mole is a good example of off stream storage - it protected Tonbridge at Christmas as the peak flow filled the storage area with 5M.t of water.

No one size fits all

But this is not necessarily the right solution for every catchment, according to Fox.

“The Thames had extended periods of very high flows, so what you need there is more intense use of undeveloped flood plains where the water can spread out without causing too much disruption,” he explains.

“And on the Severn, extended areas of farmland or urban areas of parkland acted as undeveloped flood plains to store water, preventing flooding of buildings.

“A set, standard view of the use of storage as flood attenuation is unhelpful because different areas present such very different needs.”

Using flood reserves is as much an administrative problem as an engineering one

Chartered Institution of Water and Environmental Management policy manager Alastair Chisholm says: “There’s a whole range of things that can be done, and a lot of modelling and evidence. Flood plain is the most effective flood storage mechanism there is - it’s what nature wants to do. Secondary [storage] can free up a fair amount of flood plain and provide second degree protection to communities.”

He adds: “Plan second defences around communities and allow the land around them to flood, ensuring that critical farm infrastructure is protected.”

And he’s all for education. “Perhaps what we’ve just experienced is not actually that severe now in comparison to what we might have to deal with in 50 years’ time. We have to prepare for this kind of thing to happen regularly. It’s very much a risk-based approach. People insure their houses against fire - as a society we have to accept that we want to put into place measures to protect ourselves, our properties, our facilities - and we have to pay the premiums,” he says.

“By definition this is a very long term challenge. It is no use having a short to medium term approach and having to scrap it after 10 years.”

That is a view echoed by Shirel Stedman, who is responsible for bidding for major projects in Europe, the Middle East and Africa at Aecom.

“We need a combination of things. There are two dimensions: one, a whole country strategy; and two, a long term strategy, over 20,30 maybe 50 years,” she says.

Dual strategy

“We need to start thinking about how to attenuate the floods, for example look at perhaps car parks being used as storage areas. In the Netherlands there are car parks that have bunkers underneath them, and at the press of a button all the flood water is diverted underground to be stored in the bunker.

“We have a very old infrastructure in the UK, so of course it’s tricky to do that sort of thing, but it’s something that should be considered as part of the long term plan. But two key plans should be for underground storage and ponds on farmland designed to slow the flow and to store the water.”

This is far from easy, as WSP head of water - UK, Ola Holstrom accepts.

“Using flood reserves is as much an administrative problem as it is an engineering one,” he says. “Farmers are worried about land loss, so it’s easier to use one big source rather than lots of smaller ones.

“We need a combination of things - a more holistic approach, using the science of how these things can be optimised; better ways of compensating people who have to give up their land, so for example people don’t get flooded every year; and educating the public and politicians on what causes flooding and how it has an impact on people’s lives.

“Climate change is societal change,” he says. “We need to educate people about how to deal with it.”

And, says Holstrom, engineers should be at the forefront of that education process: “It’s very much down to engineering companies to make themselves heard, to put their heads above the parapet, because there is such a lack of information directing the policy making of today.”

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