Micro Drainage senior partner Aidan Millerick is convinced that the way to cut down flooding is to slow the run-off from hard areas such as roads, car parks and house roofs. 'Source control involves increasing storage upstream to slow down the delivery of rainwater to rivers and streams further down the system, ' he says.
'Increasingly, that extra storage is being provided by soft alternatives - everything from water butts to catch roof run-off and soakaways in back gardens, to porous carparks and large scale infiltration systems.'
These soft, sustainable source control options could also be used to increase the capacity and effectiveness of existing drainage networks, Millerick says. After 20 years in the drainage design software business, Micro Drainage is about to launch a new package which will encourage this type of approach.
The company's well established WinDes package already has a source control module.
Millerick believes the increasing use of source control marks a sea change in drainage design philosophy.
'The look and feel of drainage systems has to change, ' Millerick insists. 'Hard drainage - pipe systems - will still play an essential role, especially in heavy clay soils, but we must use more soft, sustainable solutions.'
Source control is particularly important where run-off eventually finds its way into combined sewers. 'Pollution caused by combined drainage storm overflows is a major worry at the moment, ' says Millerick. 'And it is rainfall that usually causes the problem. Even taking roof runoff out of the pipes and into soakaways can have a dramatic effect on the frequency of overflows.'
The WinDes source control module helps designers of new housing schemes and industrial parks to convince planners and environmentalists that run off from newly hardened sites will not overload existing drainage systems. Its ability to perform complete real time simulations of the effect of extreme events enables different combinations of soakaways and other infiltration systems, porous pavements, storage ponds and pipe sizes to be evaluated very quickly. But in isolation it offers no quick fix for inadequate drainage.
However attractive source control sounded, it was impossible to calculate which of the possible combinations of new infiltration systems, storage ponds and porous car parks would offer the best combination of performance and capital cost.
Until now, that is. Later this year Micro Drainage will launch CASDeF, an expert system package which will allow users to assess the effects of adding extra storage at key points in the network. This could mean no more than taking advantage of unused storage capacity already in the system.
'Some of the Victorian combined sewers have enormous unused capacity which can be mobilised simply by restricting the flows through them, ' says Millerick. 'Another simple move is to issue water butts to householders and let any overflow run into flowerbeds.'
However, he warns that infiltration systems - basically large scale soakaways - are not always the answer, even in granular soils. 'If there is arsenic in the subsoil you obviously cannot install an infiltration system near a potable water supply.
'But on the whole source control using soft drainage helps reduce pollution from run-off, especially hydrocarbons. Letting the bacteria on the soil surface and in the storm drains do their job is vital.'