How building houses on flood plains could be the ‘best choice’

A recent Guardian article about housing being planned in areas at risk of flooding may raise eyebrows – but an ICE expert explains how it might not be the harebrained scheme it first appears.

Building resilience is another option if there is no option but to build on flood plains. Image credit: Shutterstock
Building resilience is another option if there is no option but to build on flood plains. Image credit: Shutterstock
  • Updated: 25 November, 2021
  • Author: Fiona Barbour, ICE Flooding Community Advisory chair

Concern was raised earlier this week when the Guardian reported that thousands of new houses would be built on flood plains, with the housing crisis being blamed for the decision. It referred to 5,000 homes within flood risk areas in England that have been granted planning permission.

However, the Local Government Association’s housing and environment spokesman, David Renard, said almost 99% of applications were decided in line with the Environment Agency’s (EA) risk advice, while Andrew Whitaker, the planning director of the Homes Builder Federation, stated “development have to meet extremely stringent mitigation requirements”.

What I would like to understand is are these houses now at flood risk as the headline suggested? Have they been built with disregard to the flood risk? Were any of these houses replacing old houses that were already at flood risk?

The reality of each situation may be complex. The EA’s indicative flood maps are very high level, and the more detailed assessment may have identified the flooding as not as severe. The proposed mitigations may have resulted in these new houses having an acceptable level of risk. I would like to see the research, but the article does not cite the source or provider to allow me to do so.

‘We need to acknowledge the housing crisis’

While I agree that we are dealing with legacy issues regarding previous practices of building on flood plains and we should do all we can to reduce this impact and certainly not make it worse, we need to acknowledge the housing crisis. These issues cannot be examined in isolation. As civil engineers, we have a duty to examine the systems of infrastructure required to serve society holistically and respond with pragmatic solutions.

The IPCC identifies that the climate change already locked in is hitting us harder and faster than previously predicted.

We need to look at the resilience of our existing and new development, particularly our housing. We have established communities who may have traditionally been developed around ports and river crossings for trade reasons, and these commercial hubs cannot easily be relocated outside of flood plains for economic and political reasons.

The focus must be on increasing resilience

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.

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.

When infrastructure can improve resilience

New or upgraded infrastructure, for example, could reroute flood water away from residential buildings. Sustainable drainage systems (SuDS) that collect and discharge rainwater safely will counteract run-off effects caused by existing hardstanding surfaces.

This leads to the thought: will allowing redevelopment within existing flood-prone brownfield land be the best way to increase resilience for these communities? If we design for exceedance philosophy, our resilience will improve.

That is, if we design 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.

The more stringent mitigations referred to by Whitaker may mean the houses are constructed with raised electrics and concrete floors, significantly reducing the impact of infrequent flooding and allowing people to return to their homes in an acceptable timeframe. I would prefer this lower impact scenario over an existing house built without these considerations.

The reality of climate change is that whether we are considering new or existing housing, flood risk is increasing and we cannot protect everyone, so increasing resilience is the answer. Therefore, if the process is robust, the correct flood professionals are involved in the decision making and the developments reduce the extent and impact of flooding, then perhaps it is our best choice?

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