The Importance of Designing for Exceedance

As the environment changes – with an increase in frequency of extreme weather events as well as an ever-expanding population – it is crucial to ensure that your surface water management solution is designed to be able to meet the requirements of the ‘new normal’, not just the usual 1 in 30 rainfall.  This was certainly the case earlier this year when we saw a much higher than average frequency and duration of rainfall in many parts of the country but most notably in the Thames area.

Due to the effects of climate change and urbanisation, surface water flooding has increased and the drainage systems in place are being put under stress with excessive amounts of water from extreme events, which unfortunately we’re seeing more and more.

The traditional approach to combat this is to provide underground storage or build bigger pipes, a tactic which has been called by the Environment Agency, amongst others, both unsustainable and unaffordable. Instead, another approach is coming to the fore, managing flood risks in a more flexible way – designing for exceedance.  Almost by definition, at some point rainfall will exceed design capacity, so designing for exceedance is needed throughout the UK and not just those areas that have already experienced serious flooding.

Designing for exceedance, while making best use of existing urban areas, is a great way of dealing with the new, unprecedented levels of water – it’s about taking a more holistic approach to planning, designing and retrofitting drainage systems to cope with excess water. This can be achieved in a number of ways including simple topographical changes such as the use of flood pathways to manage runoff on highways and direct to storage locations; examples of these include swales or flow pathways around properties. Additional storage can also be built on the surface through the use of multifunctional detention basins, making the most of available open spaces. However, there is a potential issue with public perception of rain water not being taken underground ‘immediately’ as ‘flooding’ and therefore a failure of the system.

The idea of designing for exceedance has been endorsed by various organisations, has also been included in the government’s National Planning Policy Framework and was also a focus in CIRIA’s ‘Designing for Exceedance in Urban Drainage – Good Practice’, published in 2006. Despite this, the uptake level is low, in part perhaps because there is no obligation for designers to consider what happens when real rainfall exceeds the return period in the specification they have designed to.

Recent experience clearly demonstrates that the traditional way of managing surface water is no longer sufficient but how do we, in the drainage community, get public understanding that designing for exceedance can benefit them and the wider community?

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