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The proliferation of paved gardens is putting the UK's towns and cities at greater risk of flooding and, by 2080, the UK's urban drainage system could be overwhelmed by ‘runoff' equivalent to the volume of up to 100 Olympic-sized swimming pools.  

The potential impact that paved gardens could have on urban flooding in Edinburgh, Exeter, Manchester and London by 2050 and 2080 was examined by Dr David Kelly, associate professor in Heriot-Watt's Water Academy. 

Many gardens in the UK have been paved by homeowners who want low maintenance gardens or off-street parking. Covering traditional gardens with hard paving, and the subsequent loss of green vegetation, reduces the amount of rainfall that can be dealt with naturally and significantly increases the rate and volume of runoff flowing to surface water drainage systems. 

Dr Kelly applied projected rainfall intensities for each of the four UK cities to simulated front gardens that reflect the trends for paving. 

Calculating the runoff contribution from new and existing paved gardens will help planners and policy makers identify areas of risk in their town and city – and decide whether to call for homeowners to depave. Additionally, data of runoff from individual paved gardens could highlight the need for behaviour change, and help encourage homeowners to take action themselves by depaving their gardens and enhancing green vegetation.

Dr Kelly, assistant professor in Heriot-Watt University's Water Institute said: “Domestic front gardens cover almost 30% of our urban space and play a vital role in managing surface water runoff in towns and cities. 

“Existing urban drainage systems will be inadequate to cope with the level of increased runoff from paved front gardens. With runoff from all impermeable surfaces, including paved front gardens, likely to increase in future due to urban densification, the risk of urban flooding will increase unless substantial efforts are made to minimise runoff.

Homeowners and policymakers need to focus on depaving gardens across the UK.

Dr David Kelly

Dr Kelly's research showed that the collective runoff by the 2080s from front gardens alone is expected to increase by substantial amounts during extreme storm events due to climate change. 

“In Edinburgh, considering only gardens that are currently at least three-quarters paved, during just one storm, runoff could increase to 29,000m3 across the city, equivalent to 12 Olympic swimming pools, by 2080.  

“In London, the volume of runoff could increase up to as much as 278,000m3 (equivalent to 100 Olympic swimming pools).  

“But, if all of these gardens were depaved and had zero impermeable cover, then the runoff could almost be eliminated, particularly if combined with enhanced green vegetation solutions.”
 
Dr Kelly's research paper, “Impact of paved front gardens on current and future urban flooding” has appeared in the British Journal of Environment and Climate Change and the Journal of Flood Risk Management

Image: Google Street View ©, 2015