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Weather report

Drainage Climate change

Climate change is here to stay with hotter, wetter summers and sodden winters. But what does that mean for drainage engineers? Diarmaid Fleming investigates.

Climate change is likely to have a massive impact on the environment of future generations. The UK water industry and engineers have begun examining the effect that changes in rainfall patterns are likely to have on the UK's drainage infrastructure.

Already, the industry's research body UK Water Industry Research (UKWIR) is commissioning a number of major studies to examine the implications. One project, already a year under way, is a root and branch examination of the effect of climate change on the hydraulic design of sewerage systems.

While some sceptics may still doubt the existence of global warming at all, engineers who design drainage systems with a George Bush attitude to the phenomenon will do so at their peril.

Engineering of drainage systems is guided by the government's planning policy guidance document PPG25 and the water industry's Sewers for Adoption 5th Edition guidelines. PPG25 starkly warns drainage engineers to adopt the 'precautionary principle' of 'taking action now to avoid possible environmental damage when the scientific evidence for acting is inconclusive but the potential damage [resulting from inaction] could be great.

'It is particularly relevant to dealing with the hazard of flooding on the wider scale. Its application acknowledges the uncertainty inherent in flood estimation, but at the same time, enables more open and betterinformed decisions to be made.

The use of a precautionary approach can reduce the environmental impact and improve the safety of people and property.' Put simply, even though we cannot predict accurately what global warming may mean, we should prepare. To underline this, the guidelines also say that 'prudent action can be taken in advance of scientific certainty'.

The recently published 5th Edition of Sewers for Adoption also contains significant extra requirements. Sewers should be designed for a one in 30 year storm as before, but now engineers must make further checks, themselves likely to be as complex as executing the original design.

The guide states: 'During extremely wet weather, the capacity of the surface water sewers may be inadequate, even though they have been designed in accordance with the guide.

Under such conditions, sewers may surcharge and surface water may escape from those manhole covers which lie below the hydraulic gradient. Checks should be made to ensure that an adequate level of protection against flooding of properties internally is achieved and the design adjusted where the required flooding protection is not achieved.'

A leading hydrology engineer says that combined, these spell out radical changes for the design engineer. 'Engineers may not be aware but they are now duty bound to take into account climate change in their designs, ' says Aidan Millerick, managing director of hydrology and hydraulics software specialist Mirodrainage. 'This means that climate change can no longer be put on the back burner.'

While these guidelines are not mandatory under the law, ignoring them would have serious consequences for the engineer, he says. 'Engineers have a legal duty of care to others, and this would of course include avoiding the risk of flooding properties. Then ignoring the recommendations of Sewers for Adoption would mean that water companies might not adopt the sewers the engineer has designed, ' says Millerick. 'If an engineer's design fails and causes flooding, and they have not followed these principles, then they would have no defence.'

But what are the practical implications for the engineer?

'The workload of design engineers for a particular system goes up ten-fold, ' Millerick matter-of-factly states. Compared to 20 years ago when you would design for a limited set of circumstances, now you must design for twelve winter storms, twelve summer storms, and failure modes, plus carry out sensitivity analysis for new sites, ' he says. Work includes three-dimensional flood flow path models to examine the effect of flooding on properties.

The reason for the significant increase in design input is in part due to the uncertainty in predicting what is likely to happen. Sensitivity analysis involves making predictions based on increments up to 40% increased rainfall. But it is not just a matter of increased volume of water: intensity and storm durations all have to be examined.

Predicting these is a huge scientific challenge. The most complex weather prediction tools in the UK are run by the Hadley Centre for the Meteorological Office. One, the Regional Climate Model, provides a detailed model of Europe working to a 50km grid, while the other, the Global Circulation Model, views the world on a 300km grid.

'The Hadley models are extraordinarily complex and involve huge levels of data. But there are duration issues - seasonal information is important for those involved in water resource management but in drainage we work in terms of one or six hour storms, ' says consultancy HR Wallingford's principal engineer Richard Kellagher. Water authority Severn Trent has engaged the Met Office to help obtain data for a smaller grid for its region, but as yet providing accurate predictions for engineers is a long way off.

Ultimately, prediction tools will have to take into account variables such as the amount of CO 2or CFCs in the atmosphere, Kellagher says.

'With such complex models needed, we have to ask how confident we are with the numbers that are produced. Flooding causes billions of pounds of damage, yet how can we decide whether it's worth spending huge sums if the science behind our predictions is not entirely accurate? Perhaps we should be looking at mitigation measures, rather than going for hard solutions, ' he adds.

Rainfall information for designers from the mid-1970s was provided by the Institute of Hydrology's 1975 Flood Studies Report. This has been superseded by the Centre for Ecology & Hydrology's Flood Estimation Handbook, produced in 1999.

The effect of climate change is dramatically seen here: while regional differences of up to 80% can be found, in broad summary the recent handbook shows that some rainfall levels have increased by 40% since the mid1970s.

These increases will be matched in coming decades, climate modelling predicts. 'There is a range of papers showing winter rainfall increases of between 20% and 40% by 2080, and predictions of 20% to 30% increases in summer rainfall intensities.

Studies commissioned by Severn Trent suggest that duration of rainfall intensity will increase by up to 25%, ' says Millerick.

While summers will be warmer, intensity of summer rain will be greater, winters will be much wetter, and return periods of design storms shorter.

'This is the worst of both worlds. Where we have fairly simple urban drainage systems without much storage, they'll be sensitive to summer rainfall. The tendency over the last 20 years was to focus on storage: these systems will be much more susceptible to flooding with increased winter rainfall, ' says Millerick.

Kellagher says that the physical effects of such rainfall increases are very significant: A 40% increase in rainfall can translate into a twofold increase in flood risk. Storage capacity will have to be vastly increased in proportion to rainfall to prevent catastrophic sewer overflows, he warns. He also warns of increased pollution risk as the frequency of discharge from combined sewers increases.

Close examination of existing systems will be called for to gauge whether they will be able to cope with increased storm water flows. Many will require remodelling to provide extra holding capacity. But to the likely relief of water companies, attention to the design of roads, verges and landscaping can help delay surface water run-off, sparing some of the huge expense of larger pipes and storage tanks, says Millerick.


PPG25 can be found at: www.planning. odpm. gov. uk/ppg25 Sewers For Adoption is at: www.

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