Surface Water Management and Urban Green Infrastructure - A review of potential benefits and UK and international practices

May 2011

This ROCK, which has about 200 references, considers the breadth of benefits that urban green infrastructure can play for managing surface water with examples from many countries around the world where the authors have seen GI working and/or discussed it with people who have implemented GI for surface water management.

In the past the trend has been to get rainwater away as quickly as possible and into underground pipes (out of sight, out of mind) – grey infrastructure. Many sewer systems are combined, i.e. they carry sanitary drainage as well as rainwater, sewers are designed to flow by gravity but almost always the sewage has to be pumped at some stage and it has to be treated before the water can be released back into the environment. More rainwater in sewers means more to pump and to treat, i.e. more climate change effect. Another trend has been to “seal” our urban areas with roads, car parking, roofs, etc. from which water runs off rapidly. This means large peak flows (m3/sec), inevitably these exceed the capacity of the underground pipes sometimes and the dirty water overflows into properties, which is very distressing and also expensive to remediate. Climate change is predicted to increase the frequency and severity of heavy rainfall events, which means the grey infrastructure is going to flood more often unless its capacity is increased or the size of the peak flows is reduced.

Green infrastructure has been implemented successfully in many cities (but not often enough in the UK) to reduce the total volume of water entering the grey infrastructure and also to reduce the size of the peak flows. It does this by using soil and vegetation to achieve a combination of absorbing some of the rainwater, evapotranspiring some of the water, and slowing the rate of runoff of the rest. Some monitored sites have measure an 80% reduction on total annual runoff.

Green infrastructure has been implemented as retrofit in heavily developed city centres, where land is always at a premium, but there are always roofs and some planting, e.g. street trees that can be adapted to green infrastructure. Cities exhibit the urban heat island effect (e.g. the centre of London was 5 ºC hotter than Richmond Park in the extreme heat of 2003) evapotranspiration from green infrastructure reduces the urban heat island effect and thus reduces the need for air-conditioning. Green infrastructure is aesthetically more pleasing and less stressful than hard surfaces; it makes cites more liveable. It obviates the necessity of expanding the capacity of the grey infrastructure and it is more resilient to extreme weather than grey infrastructure. The grey infrastructure needs to be maintained and preserved to sanitary drainage. The frequency of property flooding is less.

This ROCK includes reference to models to monetise the multi-faceted benefits of using green infrastructure for rainwater management. Using green infrastructure for rainwater management requires a more sophisticated approach than grey infrastructure because cooperation is needed between a wider range of parties but as this ROCK summarises, the outcome is better in every way, financial, quality of life, climate change and resilience.

Copies of the report are available from the Foundation, price £15.00, less 20% to FWR members.

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