The Importance of Designing for Exceedance

As the environment changes – with an increase in frequency of extreme weather events as well as an ever-expanding population – it is crucial to ensure that your surface water management solution is designed to be able to meet the requirements of the ‘new normal’, not just the usual 1 in 30 rainfall.  This was certainly the case earlier this year when we saw a much higher than average frequency and duration of rainfall in many parts of the country but most notably in the Thames area.

Due to the effects of climate change and urbanisation, surface water flooding has increased and the drainage systems in place are being put under stress with excessive amounts of water from extreme events, which unfortunately we’re seeing more and more.

The traditional approach to combat this is to provide underground storage or build bigger pipes, a tactic which has been called by the Environment Agency, amongst others, both unsustainable and unaffordable. Instead, another approach is coming to the fore, managing flood risks in a more flexible way – designing for exceedance.  Almost by definition, at some point rainfall will exceed design capacity, so designing for exceedance is needed throughout the UK and not just those areas that have already experienced serious flooding.

Designing for exceedance, while making best use of existing urban areas, is a great way of dealing with the new, unprecedented levels of water – it’s about taking a more holistic approach to planning, designing and retrofitting drainage systems to cope with excess water. This can be achieved in a number of ways including simple topographical changes such as the use of flood pathways to manage runoff on highways and direct to storage locations; examples of these include swales or flow pathways around properties. Additional storage can also be built on the surface through the use of multifunctional detention basins, making the most of available open spaces. However, there is a potential issue with public perception of rain water not being taken underground ‘immediately’ as ‘flooding’ and therefore a failure of the system.

The idea of designing for exceedance has been endorsed by various organisations, has also been included in the government’s National Planning Policy Framework and was also a focus in CIRIA’s ‘Designing for Exceedance in Urban Drainage – Good Practice’, published in 2006. Despite this, the uptake level is low, in part perhaps because there is no obligation for designers to consider what happens when real rainfall exceeds the return period in the specification they have designed to.

Recent experience clearly demonstrates that the traditional way of managing surface water is no longer sufficient but how do we, in the drainage community, get public understanding that designing for exceedance can benefit them and the wider community?

Another step forward

ACO floodingIn the 2014 budget, revealed yesterday, the Government announced that an additional £140million would be spent following the damage caused by the recent floods.  This will take the total spend for repairing and rebuilding UK infrastructure up to £340million – £200million to fix potholes and the additional £140million will be used to repair flood defences.

As we are all too aware, the devastation caused by the recent flooding has been significant and it is good to see that more is being done to help avoid this in the future.  In some part, we have already seen the positive results of investing in flood defences and surface water management solutions.  If you compare the recent flooding to that of 2007, there has actually been significant progress. Then, ten times the number of properties were flooded than in recent months and the Environment Agency has estimated that over 800,000 additional homes have been protected by flood schemes this time round.

Funding is therefore just part of the story, it is also essential that there is an exhaustive examination into what has and hasn’t worked and how these situations can be avoided in the future.  We should also be using our experiences to identify best practice – not only in the design solutions but also in terms of the ways of working that have made a difference.

Now, let’s seek collaboration; where planners, designers, manufacturers, policy makers, budget holders, engineers and the general public input into an integrated approach, to ensure that we adopt best practice and spend effectively, within a timely framework of guidance and legislation.

To see how improvements have been made since 2007, take a look at our infographic.

Flood prevention plans delay – new controversy?

With the profile of flood prevention high on the agenda it seems to be the right time to be talking SUDS.

News on the subject has been building gradually over the last week or two and the article by Roger Harrabin on the BBC article makes for interesting reading

Central Europe on Flood Alert

Current weather patterns are causing serious problems throughout Europe. According to a recent BBC news piece, Central Europe is on high alert with emergency operations under way in Austria, Germany and the Czech Republic in order to deal with record levels of rainfall and the evacuation of thousands of homes. The German cities of Passau and Rosenheim have declared a state of emergency and in Austria, the meteorological service said that the country has witnessed two months worth of rain in just two days.

Now may be the time to highlight the importance of water management at every level in order to help protect our environment. As Central Europe experiences on-going urbanisation, each country’s population continues to grow and migrate towards larger settlements, with towns and cities continuously expanding. As a result, the natural landscape is replaced by hard, man-made surfaces leading to potential problems with water quality and safety, as pollutants mix with the rainwater run off as well as altering the natural drainage patterns. Coupled with the ever-changing weather pattern, a more extreme climate rainfall is expected to increase year on year according to the Met office.


This increased risk of flooding during high rainfall will have a significant and widespread impact on people, businesses and essential services. Addressing these flooding issues will inevitably not be easy, and it is likely that numerous different initiatives will be required to help alleviate the dramatic and disastrous effects impacting people’s lives and livelihoods. However, by turning to more considerate and innovative approaches to drainage we will have a better chance of over-coming the challenges of rapid climate change now, as well as in the future.

Since the introduction of the Pitt review and the Flood and Water Management Act, we are seeing greater urgency to introduce more sustainable solutions on the micro level that will help manage surface water in a macro context. The SuDS concept is one that looks to deliver effective management of quality and quantity whilst also recognising some of the real positive aspects of water – sustaining biodiversity, improving local amenities, the re-use of water and of course the replenishment of groundwater.  As urbanisation increases so does the urgency for drainage technology to address and support natural processes; ACO are working hard to deliver solutions that integrate with effective natural elements to best manage surface water. Years of experience in dealing with surface water all over the world has put us in a favourable position to provide product and advice with regards to managing surface water.

Global warming will mean more rain!

A report from Australian scientists declares significant statistical proof that global rainfall is on the increase based on a century of records. The report links global warming with this increase in precipitation and predicts that continued intensification of rainfall events will lead to more frequent flooding throughout the world.

The research, using data from 8,326 observing stations which had collected at least 30 years of record over the period from 1900 and 2009, found the intensity of rainfall was statistically associated with temperature. It showed that increases of between 5.9% and 7.7% in rainfall occurred for each degree of temperature rise.

According to NASA the average temperature has increased by 0.8°C since 1880, with over two-thirds of that rise occurring since 1975. If this trend continues we will see a rise of around 0.15°C per decade.

Picture courtesy of The Independent

Picture courtesy of The Independent

A BBC report in January stated days of particular heavy rainfall in the UK have become more common since 1960, mirroring the increases seen in other parts of the world. It will be no surprise to hear that last year extreme downpours in the UK occurred on average once every 70 compared with the normally expected once every 100 days. If global temperatures continue to rise, however, our air will hold increasing amounts of moisture which statistically means yet more rain and, if we do nothing, the misery of the floods that go with it.

This information only serves to strengthen our viewpoint that the need for sustainable drainage systems (SuDS) is increasingly important. To help in managing more intense rainfall, their implementation must be seriously considered by all local and national governments worldwide. If predictions about our climate are right, then we should maintain the drive for sustainable drainage to help manage the increased rainfall in our built environment. The need to engineer long term solutions that protect environments against the now all too common floods is recognised in the Floods and Water Management Act and we should welcome the careful consideration and subsequent implementation of SuDS in 2014 in England.

What happens when the ground is full?

With the second highest rainfall on record in a year which started with drought, 2012 has certainly bought a lot of comment about changing weather patterns and the challenges we face in dealing with the increase in flooding we have been experiencing. The BBC recently reported on how the ground is completely saturated with water and has nowhere else to go underground. The feature also comments on how the groundwater levels have risen faster than any other time since records began in the 1830s, leaving us in unknown territory as to how the groundwater will respond.

Forecasting our weather has always been a topic of much discussion and it has been particularly difficult to predict over the past few months. Milder air holds more moisture and has the propensity to produce much more intense rainfall. When the ground is already full, however, water simply runs along the surface – causing the excessive flooding we have been seeing. The big concern is that these sorts of conditions become the norm and we see these events occurring more frequently year on year. So what can we do for the future?

The requirement for sustainable drainage schemes (SuDS) provision in Schedule 3 of the Flood and Water Management Act 2010 is scheduled to be enforced in 2014. Where we have continued to urbanise our landscape, issues arise with fast water runoff through conventional piped drainage which overwhelms the sewer capacity – a situation we have seen all too much of during the past 12 months. SuDS require that drainage mimics runoff from the site in its natural state using techniques such as retention and infiltration along with linear channel drainage to provide an effective conveyance mechanism to transfer surface water. The aim is to reduce the rate of rainwater runoff and therefore reduce the risk of flooding.

A SuD needs to be an adaptable and flexible solution to suit individual site requirements. Combining ‘hard’ (or engineered) and ‘soft’ (or natural) components to best effect will certainly help to counter the effects of urbanisation. However in an environment where any system is struggling to cope with volume, will the forthcoming legislation really be effective in delivering all it seeks to? Can it really deliver Quantity management, Quality Management and Quality amenity?

So Much Weather!

The weather has certainly hit the news more frequently over the past months. The extreme weather with droughts and hosepipe bans in March followed by excessive and multiple flooding across the UK has left a lasting impression of 2012.

Our whole planet appears to be seeing a shift in weather patterns with more extremes in heat, cold, rain and wind. After a hot summer in Russia with the raging forest fires, Yakutsk in Eastern Russia has recently being seeing temperatures nearing extreme lows of -50°C, the coldest in decades. 2012 saw severe droughts in many places including the USA and a dry monsoon season in India. 2013 has started with rampaging bush fires in the exceptionally hot Australian summer and here in the UK the wet but very mild end to 2012 has been followed by snowfall across much of the country. However it falls, more wet weather is probably not what you have been hoping for.

Courtesy of

Extraordinary weather seems to be becoming more of the norm and with many UK businesses and homes suffering badly from the floods of 2012 we have to prepare ourselves to handle the extreme weather for the future – protecting our environment and livelihoods. 2012 was the second wettest year on record for the UK. The question we don’t know the answer to, as yet, is whether this is a long-term change to our climate, but many seem to think so.

Sustainable drainage is important for managing the excessive rainfall we have been seeing. As we build for the future we have to plan to manage surface water without loss to business, keeping transport routes safe and protecting our homes. Here at ACO we want to help to make the best provision for this whatever the weather throws at us.

Read the full NY Times article by Sarah Lyall