Sustainable Development Today Flooding: a rising threat
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Written by Alex Stephenson

Govtoday's Flooding conference was a timely reminder of one the major natural threats likely to be faced in the UK, with more than 2 million homes and businesses at risk.

According to the latest IPCC report ('Climate Change 2014'), this threat is considered highly likely to increase over coming years, under the twin impacts of changing patterns of rainfall, and a growing population. Counter-intuitively, these same factors are also highly likely to increase the frequency and severity of summer droughts.

These predictions are matched by our own Met Office modelling of future weather patterns, with short-duration intense periods of rain, posing flood risks, being followed by longer drier spells leading to droughts. Whilst being a short-term inconvenience to people, the anticipated change in agro-climatic (growing) conditions that will result are potentially disastrous to the farming industry, and the environment.

Under these circumstances, the need for joined-up thinking is self-evident, to make sure that planned measures taken to alleviate flood risks, do not unintentionally heighten subsequent drought risks. This can happen all too easily if the main aim of these measures is to facilitate the draining of rainfall to the sea as speedily as possible, rather than managing it in a way that preserves it for subsequent use.

Conversely, when planning measures such as rainwater storage to help to alleviate droughts, the design of systems needs to maximise their impact on flood risks, and their cost-effectiveness valued accordingly.

Man-made interventions to reduce flood and drought risks come together in that both can involve attenuating (storing) rainwater temporarily to avoid it overwhelming the national drainage infrastructure and cause flooding and for a longer period to make it available for use in periods of drought.

The first of these requirements has long been well-understood by the development industry, with sustainable drainage (SuDS) being a planning requirement on most projects. The requirement to avoid droughts is less well-appreciated by the industry and has been mainly reflected in building regulation water-efficiency stipulations.

Stand-alone rainwater harvesting (RWH) systems have also been encouraged by the Code for Sustainable Homes and BREEAM, but this encouragement is due to be phased-out.

Meanwhile, our continental counterparts faced with similar flood and drought risks have taken a much more creative and cost-effective approach to the problem by designing combined SuDS/RWH systems.

Typically on a new housing development, for example, this will involve straightforward single property or communal RWH systems serving to reduce mains-water consumption by around 40%.

To deal with exceptional rainfall events the overflow from these is taken to balancing ponds or swales that are an attractive site feature, help meet water-quality and environmental targets, and ease adoption of the overall system.

Alex Stephenson is a Director of Hydro International Stormwater, Chairman of the British Water Surface Water Management working group, and a Director of the UK Rainwater Management Association

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