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How to curb urban floods

Minor changes to existing urban features could keep many households free from surface water flooding. Margo Cole reports.

Perhaps unsurprisingly, extreme levels of flooding like those experienced in recent weeks often lead to calls for major engineering solutions.

However, a project that has been under way since spring 2013 is aiming to show that quite a lot can be done to prevent flooding simply by making some minor changes to features like kerbs, road channels and existing open spaces.

The project, “Managing urban flooding from heavy rainfall: Encouraging the uptake of designing for exceedance”, is looking specifically at what can be done to minimise the impact of surface water flooding, which the Environment Agency estimates affects around 3M properties.

“It has been recognised for some time that is unsustainable to keep building drainage bigger and bigger,” says MWH senior principal engineer Chris Digman, who is technical manager for the project.

“Whatever size we build, the drainage will become exceeded at some point, so the challenge is, when this occurs, are we going to manage it or ignore it? Are we going to let that water take its own course so that unmanaged flooding occurs, or are we going to design our urban areas to manage it?”

The concept of “exceedance” - when surface water volumes exceed the capacity of the drainage - is particularly pertinent when it comes to urban surface water drainage.

Finance constraint

Digman says that, not only are finances unavailable to keep building bigger and bigger urban surface water drainage systems, but that, if we did, the drainage infrastructure would be massively oversized for most of the time.

The result is that, as Digman says, almost by default we can expect to see water flowing and ponding on our highways during any rainfall event that is higher than the one in five or one in 10 that it was designed for. The issue, therefore, is, if we know it is going to happen, how do we deal with it?

“Rather than letting the highway manage it without any thought, we can specifically design it to channel those flows,” says Digman.

“We have to find ways to manage that water, because it [exceedance] will happen.”

Left to its own devices, surface water that cannot get into the highway drainage system, or overflows from it, will simply follow the path of least resistance until it reaches somewhere to settle.

Digman is advocating that “safe” paths should be created, so that excess water will instead follow a designated route that diverts it away from vulnerable properties and takes it somewhere it can be temporarily stored until levels in the drainage system have dropped

However, the important ­factor in creating these pathways and storage areas is that it does not involve building new structures, but instead makes use of existing surface features.

Examples might include designating some roads to channel the water, and using existing open spaces - like car parks or green space - to temporarily store water.

“This is not necessarily about having to put in some major, significant pieces of drainage work, but making the best use of what you have on the ground,” explains Digman.

“You can often make minor changes to features that could be very important - for example to divert water right or left by installing a drop kerb, or building kerbs up to channel the flow, or building a speed hump in a particular location.

“One of the key things is that we should not just take an engineering view on this - it needs to be multidisciplinary, because these are features within the urban landscape,” he adds.

Guidance

Guidance has existed since 2006, in the form of Designing for Exceedance in Urban Drainage: Good Practice: C635, published by construction research body Ciria.

The project that Digman has been leading is also being coordinated by Ciria, and aims to improve uptake of the guidance by identifying schemes that have successfully implemented its recommendations.
One really vital factor, says Digman, is to involve local communities, as well as taking a multi-agency approach that includes all the bodies that are affected, such as local authorities, water companies and the Environment Agency.

The current project is being funded by the Agency, with a contribution from Wessex Water and input from other water companies and from local authorities, indicating that there is an appetite to see more of these local schemes to mitigate the impact of surface water flooding.

“We have to try to use a range of ways to engage people and communities, and we have to do the best we can to get people involved,” says Digman.

“When it comes to designing for exceedance, there may not be the challenge of the huge volumes of water you get with river flooding, but it’s just as important to have community involvement to make decisions about where the water will go.”

He acknowledges that this may be a good time to start discussions with some of those affected communities.

“When something’s happened - when there has been some flooding - it is probably a lot easier to do this, because it has captured communities’ imagination, and they want to be part of the solution,” he says.

However, one of the key recommendations of 2010’s Pitt Review into the 2007 floods was that agencies should take more of a risk-based approach to flood defence and mitigation.

New, more accurate flood risk maps are being used to predict which areas are most at risk, with the results being fed into local authorities’ surface water management plans.

As a result, some communities that have not previously experienced flooding may be deemed to be at risk, and Digman says it is more of a challenge to engage these communities in the discussions about exceedance.

According to Digman, there are two types of exceedance design. “The first is fairly simple, when you can quite quickly design a route for water to flow through to a location where you can store it,” he explains.

“This is more typically applied to retrofit.

“The second is more complex, and involves using computer modelling or simulation because there is more of a challenging problem or it is a new development.”

A third area that Digman says must not be ignored is redevelopment.

“One of the opportunities we shouldn’t miss is to manage exceedance in redevelopment or regeneration. You may only get that chance once every 50 years, and this is about making the most of opportunities when they come up,” he says.

The results of the “Managing urban flooding from heavy rainfall: Encouraging the uptake of designing for exceedance” project are due to be published later this year.

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