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A concept design for Green Square, Sydney

To build or not to build on flood plains?

Unlocking flood-prone land for regeneration will help strengthen the resilience of communities vulnerable to climate change, says Fiona Barbour, Mott MacDonald’s global practice leader for rivers and flooding.

The coastal community of Fairbourne in Wales is set to be the first village in the UK to be abandoned as a result of climate change. The local authority has decided it is no longer possible to keep protecting the homes and businesses of the 850 residents. In around 25 years, or sooner, all property and infrastructure in the village will be dismantled and the site will revert to a tidal salt marsh. It’s a fate that will befall other communities as rising sea levels and increasingly intense rainfall elevate the risk of flooding.

In the UK millions of people are living on flood plains and in coastal areas, with some level of protection provided by defences created and maintained at significant cost. The Environment Agency is already committed to spending £2.6bn over six years, delivering 1,500 projects to better protect 300,000 homes.

Simply allowing the land to revert to nature isn’t an option in the great majority of locations. Communities developed around ports and river crossings are commercial hubs and cannot easily be relocated for economic and political reasons.

Where relocation is not possible and relying on higher defences alone for protection is neither effective nor sustainable, the focus must be on increasing resilience, as the Environment Agency recognises in its Draft National Flood and Coastal Erosion Risk Management Strategy for England.

Continuity of services

While there is general consensus new homes should not be built on flood plains that are still ‘green’, there is little agreement on how to increase flood resilience in areas where they have already been developed. The crunch question is how do we help these communities to withstand flood events, maintain continuity of services and recover more quickly. A counterintuitive answer is to allow new development, creating additional infrastructure which can itself play a defensive role.

New or upgraded infrastructure, for example, could reroute flood water away from residential buildings. Sustainable drainage systems that collect and discharge rainwater safely will counteract run-off effects caused by existing hardstanding surfaces. Residential and non-residential properties can themselves be built to be more resilient and adaptable by incorporating concrete floors and flood gates, and installing electrics at waist-high level. In decades to come we could see more houses built on stilts, floating houses moored permanently on the water and amphibious houses able to float when water levels rise.

Risk-based approach

Redeveloping urban communities in flood-risk areas to increase resilience calls for a change of mindset. At present decisions about constructing on flood plains and coastal areas are made in a very black and white way. It is, understandably, an emotive issue.

What’s needed is a more pragmatic, risk-based approach to design, and we should re-examine what is an acceptable flood risk. Instead of striving to provide uniform protection against, say, a 1 in 100 year event, we should focus on different scales of protection that will minimise the localised impacts of extreme weather events.

Planning and building controls should insist on designing for exceedance. We estimate hydrology, flood outlines and levels of blockage, and the impacts of climate change, all to inform the design of assets as best we can. But these are just estimates and can provide a false level of certainty. Putting more effort into designing for exceedance will produce infrastructure capable of functioning even if the estimates prove inadequate and a bigger than anticipated storm comes along – as is increasingly happening with the unpredictable effects of climate change.

If we increase our understanding of where excess water will go if we experience greater rainfall than we are currently designing for, and communities agree to accept there will be water in their streets in extreme weather events (provided it is kept out of their living rooms), we can break the cycle of repeated, disruptive and damaging flooding.

Regenerating communities

Efforts to promote flood-resilient developments are often blocked by policies that do not allow for this risk-based approach and state building on flood plains should be the “absolute exception”, done only in areas of genuine housing shortage where no alternative land is available – as recommended by the seminal 2008 Pitt Review: Lessons learned from the 2007 floods. I believe we should take a different view when the flood plain is already developed. For a start, unlocking flood-prone brownfield sites for further development will ease the pressure to build on greenfield land everywhere. Investment in redevelopment is justified by the economic and social returns as much as the reduced flood risk.

Hull will benefit from one of the biggest flood defence schemes in the country – being delivered by BAM Nuttall and Mott MacDonald – that is designed not just to protect thousands of homes and businesses, but to regenerate the city’s waterfront, create jobs and enhance the environment. In Salisbury, Mott MacDonald provided concept designs to improve the river corridor not just to reduce flood risk but to help rejuvenate the city centre and boost tourism in the wake of the novichok nerve agent attack. At Green Square in Sydney, one of the largest urban renewal projects in Australia, we integrated space for extreme rainfall into the infrastructure and landscape design to minimise land take and impact on utility capacities.

Such schemes emphasise the value of green space in providing routes and storage for excess rainfall during extreme events, providing treatment for improved water quality, and taking rainwater out of overstretched sewer systems. Taking a holistic perspective, and working with all parties involved in water management, will open up opportunities for redevelopment. If denied them, communities in areas at high risk of flooding face stagnation, if not decline. They don’t just become less resilient to flooding in future, they miss out on the economic and societal benefits. Those who can will leave, increasing social deprivation.

Delivering enhanced protection in flood-risk areas provides the confidence required to reinvest in existing buildings and assets. The regeneration effect enhances the value of protected assets further, and makes the economic case for investing in improved defences. It’s an affordable, achievable way to improve the resilience of communities most at risk of flooding, and create happier and healthier places to live and work.

This article first appeared in Flooding and Coastal Change 2019, which is published annually by the Chartered Institution of Water and Environmental Management and showcases the views of recognised thought leaders from the flood and coastal erosion risk management field.

Fiona Barbour

Global practice leader for rivers and flooding

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