Driveways pose flooding risks

12:28pm Wednesday 16th April 2008

This month the rules changed and parking tickets can now arrive courtesy of CCTV. In October, another amendment will be introduced and it is to do with parking spaces in the front gardens of private properties.

Many houses built in the middle of the last century were not expected to have car owners, and so garages were not provided. With the explosion of road traffic since 1950 there was the problem of where to leave the car when it was not in use.

The last ten years have seen a marked increase in the paving-over of the front gardens of many properties, sometimes with brick blocks, and often with tarmac or concrete, and the new hard standings have been protected with walls and gates.

In London, almost two thirds of properties now have front gardens converted to hard standings.

The result is an increase in the immediate surface run-off, into the drains, particularly with the more intense rainfall following the climate change warming of the atmosphere. Not all drains can cope.

A typical 20-house road with block paved fronts adds 500,000 litres of water to the drains each year, immediately, rather than allowing it to percolate slowly into the soil.

The 2007 floods in Hull, Toll Bar and Tewkesbury caused £3 billion worth of damage and the floods in two thirds of the 55,000 houses were due to surface run-off, and not overflowing rivers.

Each sewer flood costs around £39,000 and the total cost is enormous as the Environment Agency reckons there were 2,062 such floods last year.

The aim is to reduce such flooding, particularly as a one per cent drop in run-off means a nine per cent decrease in the sewer overflows and this is when the planners enter the story.

From October, planning permission will be required for an impermeable surface, but permeable material will still be permitted development and will not need a planning application.

It is expected that most people will go for the permeable material to avoid the delays associated with a planning application, and the cost (£150 currently), and this will mean using gravel, or porous blocks or those with small cavities that hold soil and grass.

It may be slightly more expensive, and the building industry will need to develop the skills, but if it helps the 80,000 properties at significant risk from surface water flooding it will be worth it. Additional benefits are the increase in soil moisture levels for street trees and the increased CO2 absorption level.

A weakness of the proposal is that it is limited to domestic dwellings and it should also apply to the car parks at supermarkets, schools and commercial centres.

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