SUDS, floods, tunnels and solutions
No one can be in any doubt that the UK's storm water drainage system is insufficient to meet the current and future demands. What is less certain is the best way to expand capacity. Alex Stephenson, David Schofield and Dr Ian Pallett of British Water's SUDS Focus Group examine issues that are holding back progress.
We are over half way through the year and stormwater has become a hot topic, making the national news almost daily. But this time it is not only the effect on our urban areas that has become apparent. The consequences for many agricultural regions have also become clear.
Following a hot, dry April, extreme and unseasonal rainfall events in June and July caused severe flooding in many parts of the UK. This has highlighted the potential effects on services and utilities, as well as on individual lives.
This apparent suddenness is partially due to a disbelief that it could happen in the UK. Our vulnerability and apparent inability to minimise – let alone control – its impact have raised it up the political agenda.
Flooding’s potential to spread contamination and disease has been worsened by the recent foot and mouth outbreak. Although the connection is far from being proven.
Apart from localised incidents, London has largely been unaffected. But the publication of the report into how to reduce contaminated stormwater flowing into the Thames has caused its own storm – at least in engineering circles.
Writing in New Civil Engineer in May this year, Professor Richard Ashley argued that the proposed Thames Tideway Tunnel may be an expensive white elephant.
On the other hand, Chris Binnie, former chairman of the Thames Tideway Strategy Steering Group, claims there is no realistic alternative.
While the experts debate, Londoners just want something done to reduce flooding and the dangers associated with sewage discharges into the Thames.
The problems of rural and urban flooding, contaminated stormwater, the management of surface water, whether normal or exceptional in volume, are becoming more apparent.
As is the inability of our drainage and sewerage infrastructure to cope with increasingly unpredictable quantities of rainfall.
Human activity is also making the situation worse, as developments continually increase the area of impervious surfaces.
Across the world, urban development is encroaching onto flood plains. And the countryside is also threatened by agricultural sprawl, as pressures for increased agricultural production grows. Low-lying land has been drained, rivers canalised and the sea excluded, all in the name of increased and more efficient agriculture.
Looking back, could we have developed our cities and countryside in a better way?
Should we plan developments on flood plains? After all, they are called flood plains for a reason.
If it is imperative that we continue such developments, then we should alleviate local stormwater problems by using contemporary principles of source control and sustainable urban drainage systems (SUDS).
Even without the effects of climate change, our countryside has periodically been unable to absorb the water from storm events. Exacerbated by increased development and changed farming practices, we have interfered with natural processes and how surface water dissipates.
Covering large areas with impermeable surfaces has removed the natural sponge – trees and grassed areas.
And channelling watercourses has also increased the frequency and seriousness of flooding.
In London, greater housing density and larger hard surface areas overwhelm the Victorian drainage infrastructure with increasing frequency.
The Thames Tideway Strategic Study selected an option, now given the go-ahead by government. Whether it is Prof Binnie’s “only realistic solution” or Prof Ashley’s “white elephant” remains to be seen.
The solution is a huge tunnel 80m below the River Thames. It will be capable of keeping 1,500,000m3 of combined sewage and rainwater out of the Thames until the treatment plants can treat it.
The sewage/rainwater will be pumped back to the surface, treated and discharged into the Thames.
The debate as to whether it is the best option will doubtless continue, even beyond its completion. And similar solutions may be considered for other conurbations.
It is ironic that the carbon emissions produced in constructing, operating and maintaining this type of solution will further exacerbate the reason for it’s construction – climate change.
But, of course, while we continue to mix sewage and stormwater, treating large volumes will always be required.
No one could claim that in the short-term, retrospective application of SUDS would completely remove the increasing problem of contaminated stormwater discharges in London and other urban areas.
It is clear, however, that a broader uptake of contemporary sustainable drainage and source control principles – dealing with stormwater as near as possible to where it falls – would undoubtedly help prevent floods in new developments.
It must also be acknowledged that retrofitting within established urban environments is fraught with problems. This is especially true if the geology is unfavourable to infiltration and other SUDS solutions.
The DTI recently funded a technical mission to the US to investigate sustainable drainage. British Water visited a range of locations where a variety of sustainable drainage systems had been installed, with very positive results.
The sites included established extensive urban areas, as well as new developments, many of which had significantly degraded nearby sensitive environments.
The local populations had been involved with project planning and implementation.
Flooding was reduced and the environments successfully regenerated, so the public willingly accepted the extra charges incurred.
Portland, Oregon, had a Big Pipe project – similar to the Thames Tunnel – to deal with unsatisfactory discharges from combined sewage overflows (CSO).
The pipes’ sizes could be significantly reduced due to a 50,000-dwelling program of source control (downpipe) disconnection, that reduced the quantity of stormwater entering the system.
Developers were encouraged to install green roofs – in one instance an extra storey was added in return for a green roof.
Of course, the geology contributed to the success of the downpipe disconnection programme.
It was even suggested that, had more sustainable drainage systems been introduced earlier, construction of the pipes could have been avoided.
The overall impact on the environment was enormous, and valued by local
people and businesses.
Similar achievements, as well as massive water quality improvements in estuaries and rivers feeding into them, were obtained in Seattle and Boston by applying sustainable drainage principles.
Boston replaced its single large CSO interceptor project with 29 smaller local projects.
They still include some storage and improved treatment facilities, but these are linked to what are described in the US as BMPs – Best Management Practices, or, in UK parlance, SUDS.
The US Environmental Protection Agency actively promotes BMPs via its Center for Watershed Protection. (www.oti.globalwatchonline.com/online_pdfs/36635MR.pdf to download the mission report).
The adoption by drainage authorities of approved natural and proprietary systems is another reason why sustainable drainage principles have been successfully applied in the US.
One of the barriers in the UK to increased use of SUDS, especially proprietary systems, has been the failure to remove the legislative hurdles.
Developers and designers have to overcome significant legislation-based problems to gain approval for innovative, sustainable solutions.
Recent consultations by the Scottish Executive on construction standards and vesting conditions, and Scottish Water on Sewers for Scotland, resulted in constraints on the options available to developers for sustainable drainage solutions.
The recently published CIRIA Manual C697 covers in detail many natural and proprietary SUDS solutions that are suitable for different situations. But the Scottish proposals limited which of these options could even be considered.
The longer-term maintenance of all drainage solutions, both natural and proprietary, requires consideration, as does any infrastructure development. But this should not preclude their consideration for applications in which they would be particularly suitable.
There are many examples where effective technical solutions have been unacceptable due to the failure of legislation or lack of charging mechanisms.
Legislation may exclude their use overtly, by implication, or simply by being
prescriptive rather than performance-based.
This has contributed to the blinkered approach to sustainable drainage in the UK that was not evident in the US. Unless we can collectively adopt a more joined-up approach to stormwater management, and learn from the lessons of others, we will continue to build large, unsustainable solutions and miss opportunities to develop our infrastructure in a sustainable manner .
There is a huge selection of contemporary sustainable drainage solutions available, (see CIRIA C697), but unless they can be used in a challenging and holistic fashion, their full potential will not be realised.
The most likely outcome will be repeated and increasing urban and rural flooding by stormwater.
This is a prospect that the industry could influence and significantly reduce and which the public will not accept.
Alex Stephenson is Stormwater director at Hydro International; David Schofield is an associate at Arup; and Dr Ian Pallett is technical director at British Water.
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