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A drain on resources

Following our earlier article on topographical modelling, we examine the rise of flood tides, risk and mapping

Last month saw the launch of the Environment Agency's Flood Map, which is intended to give homeowners and prospective purchasers an insight into the flood risk of particular areas - to literally ascertain the lie of the land.

On the 20th anniversary of the completion (and 30 years since the start) of one of the most impressive flood defence projects, the Thames Barrier - cited by some as second only to the Channel Tunnel in late 20thcentury infrastructure provision - it is as good a time as any to assess the state of flood prevention knowledge and technology.

Approved Document H: 2003, 'Drainage and Waste Disposal', sets out the requirements for dealing with rainwater disposal. It states that adequate provision shall be made for rainwater run-off from roofs and paved areas to discharge, in order of priority, into either a soakaway, a watercourse or, where these are 'not reasonably practicable', into a sewer. That is to say, that providing rainwater drainage into a non-natural means of disposal is the least favoured option. There are no plans to upgrade this section of the Building Regulations at present.

Even though the test of reasonable practicability is often a get out of jail card for many pieces of legislation, in this instance it is equivalent to guilt until proven innocent. Essentially, both higher priority solutions - soakaways and watercourses - must be explored and proven to be inappropriate before taking rainwater to a run-off sewer will be accepted - and proving that something is 'not reasonably practicable' is not easy.

I well remember having to install rainwater drainage from a housing scheme to the nearest watercourse that was 75m away - across somebody else's land. Extortionate fees to the landowner of the ransom strip and the need for a 4m-deep trench to accommodate the falls were not sufficient excuse. After all, Approved Document H: 2002 states that methods of drainage 'other than connection to a public surface water sewer are encouraged where they are technically feasible'.

This demand results from the fact that existing surface-water sewers and outfalls across the country, as well as combined sewers and cesspit or other disposal facilities, are currently inadequate and in desperate need of upgrading. The Approved Document legislative mechanism therefore seeks to avoid the necessary infrastructural improvements and passes the liability on to the developer. Moreover, natural drainage badly done can be akin to burying the problem only to have it literally resurface some time later;

exacerbating the very problem that it seeks to resolve. Local sustainable disposal can often add to the groundwater impact in a particular area. Streams and rivers that have evolved due to the gentle percolation of water through the soil now have to deal with major additional flows.

Sewer-flow restrictors and made-up environmental impact assessments are not sufficient to limit the impact.

The growth in sustainable urban drainage systems (SUDS) argues that while built-up areas need to be drained to remove surface water, traditional underground pipe systems do this by 'conveying the water away as fast as possible', which, says construction industry research association CIRIA, causes an 'alteration of natural flow? [leading] to problems elsewhere in the catchment'.

The objective of SUDS is to manage run-off rates but equally to provide habitat for wildlife, protect water quality and, where appropriate, to encourage natural groundwater recharge. To this end, local controls on run-off quantities and qualities have to be managed as close to source (ie the run-off point) as possible. Unsurprisingly, CIRIA's first piece of advice is to limit the run-off by recycling, water butts, etc - 'good housekeeping' measures that are reminiscent of the Thatcherite values of old. It suggests that pollution can be kept to a minimum 'by keeping paved areas clean' and recommends that 'maintenance measures such as sweeping hard surfaces regularly will reduce pollution'.

It continues: 'Litter and animal faeces can be kept out of drainage systems by education and the provision of bins'.

A list of CIRIA's top tips for sustainable drainage includes:

l Filter strips and swales that are vegetated surface features (grassy slopes or ditches respectively), which 'mimic natural drainage patterns' by allowing vegetation to filter the flow and pollutants as the water seeps away.

l Infiltration devices including soakaways, which are below ground and have been calculated to seep filtrated water through to surrounding ground at a given rate in normal circumstances. As CIRIA notes, 'limitations occur where the soil is not very permeable, the water table is shallow or the groundwater under the site may be put at risk'. Soakaways are alleged to 'enhance the natural ability of the soil to drain water', which is simply a way of saying that they give rainwater a larger surface area through which to pass than simply through a piped point discharge.

l Filter drains or permeable surfaces - which are effectively exposed soakaways - whereby rainwater, etc, can be 'stored' under or alongside the hardsurface and slowly percolate through to neighbouring soil.

l Basins, another word for ponds.

These 'store water at the ground surface, whether as temporary flooding of dry basins and flood plains, or permanent ponds'. Basins, then, are open soakaways or flooded swales.

With 30-year plans for a reservoir for Kent being revived, it seems more appropriate to start providing piped treated rainwater drainage to fill it, as well as other reservoirs that are undoubtedly needed. The need for centrally managed domestic supply might then take precedence over local survivalist storage or allowing it to filter through into the soil. Nature's way is to provide droughts and floods with equal ferocity (see 'Real flood risk'). It might be well worth our while exercising humanity's revenge on nature, as has been shown by the Thames Barrier, by taming it and directing it to our beneficial ends.

Discuss.

l For the article on topographical modelling, see AJ 7.10.04.

CIRIA is hosting a series of seminars on SUDS at its offices in London in December and January. For more information, please visit www. ciria. org. uk

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