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Developing optimum solutions to flooding

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Courtesy of Innovyze

Developing optimum solutions to flooding

With a very long period of wet weather in the summer overlaid by intense rainfall, 2007 was the wettest summer in the UK on record. This unusual weather pattern resulted in very high localised runoff, with three events in England exceeding the 1 in 100 year return period over that time – some of the worst flooding that has ever occurred in Britain.

Since then, the water industry and its consultants have been working exceptionally hard to develop its strategy. Some of the flooding was not a surprise – some urban areas, despite various flood relief schemes over the years, continue to experience problems. One of the reasons is that the problems in these areas genuinely are difficult to resolve, and the same is true of much urban flooding.

The biggest surprise in 2007 was the extent to which key infrastructure was affected. Tewkesbury, which featured prominently in news bulletins, was flooded extensively. Aerial photographs showed the church still dry on the high ground surrounded by floodwaters, proving that our forefathers had perhaps a better understanding of flood risk management.

The problem for Tewkesbury was that not only did the town flood but the local water treatment works, which supplied around 80,000 people, was also inundated. The treatment works was out of action for over two weeks, so there was no water available to clear up the mess, and no water for drinking or cooking, which turned an emergency into something of a disaster. Indeed, the area almost lost its electricity supply as well.

Lessons learned

This impact on key infrastructure was a key lesson that was learned. Because water flows downhill and accumulates in low spots, so the artificial drainage systems – the conventional sewerage systems – were overwhelmed early in the storm with the bulk of the floodwaters being conveyed above ground.

What was typically found was that in urban areas the roads, pathways and spaces between buildings are not designed or developed to allow the free passage of floodwaters. That often leads to extensive flooding and flood damage in urban areas and also that flooding is indiscriminate – who and what is flooded is decided by pure chance. Buildings are not designed to withstand floods, and because in many areas urban flooding is a fairly short timescale event there was little warning and the flooding came as a complete surprise.

Industry research

The common theme that emerged from the news coverage was that the flooding genuinely was an extreme event, unprecedented and unexpected. But should it have been? Back in 2004, the water industry teamed with HR Wallingford, the Met Office and Imperial College to do an extremely important study for UKWIR that looked at the forward effects of climate change on the performance of urban drainage systems. This was a groundbreaking piece of work, because in part it took the output from the climate models and produced downscaled rainfall data that could be used to predict the performance of urban drainage systems in 2080.

This was such an important piece of work that the industry progressed from that to work with a wider group of universities and other organizations for the Government’s Foresight project on climate change, flooding and coastal defence. In that project, the team evaluated flood risk for 115,000 properties across the UK against the background of the predicted rainfall. What the rainfall downscaling showed was that for the intense rainfalls that tend to cause urban flooding, there would be a 40% increase across wide swathes of the UK by 2080, leading to a consequent 40% increase in flows to urban drainage systems. At that point the linearity ends and non-linear effects occur: there would also be a consequent 100% increase in surface flood volumes, a 130% increase in the numbers of properties affected, and a 200% increase in flood damage – a threefold rise.

That work was also coupled with a wider study looking at rivers and coastal defences. This showed the annual average flood damage, comparing the present day to four possible future scenarios. These took into account various predictions of how the world may develop in terms of economics, socially and technically, to the year 2080. These were:

  • The World Markets model, which assumes each major economic power looks after its own interests autonomously;
  • The National Enterprise model, which assumed a sensible agreement from the Copenhagen climate talks in December;
  • The Local Ownership model, which postulates a semi-feudal system of government and economic management;
  • The Global Sustainability model, which assumes that an environmentally-focused power (such as the various Green Parties) is elected in every major economic power.

The likely scenario is thought to be somewhere between the World Markets and National Enterprise model positions, which is of concern because there are a number of important resultant features. First, the increase in flood-related damage is huge, and it is particularly important to consider this in the context of the recent global recession and countries’ ability to spend money on dealing with this future risk. A second important aspect is that urban flooding increases disproportionately.

Cost issues

Looking back at the 2007 flooding and its impact, it is clear that there could be significant problems to cope with in the future. One of the real issues is that if this prediction is translated into an examination of the cost of dealing with flooding as it is resolved today, the costs rise to somewhere between four and eight times the current costs, excluding any effects of urban growth and other measures that might be necessary to cope with related environmental pollution. This means, for example, that there are no figures for the cost of meeting the requirements of the Water Framework Directive, a separate but parallel challenge for the industry.

The predictions show that the conventional solutions of coastal defence works, river defences and increasing the capacity of urban drainage systems will become completely unsustainable in the long term. It will be necessary to find a completely new way of dealing with this risk. The first element in solving any testing problems is understanding the needs and the challenges that the industry faces. Only by understanding the needs, and the problems that are generated out of the basic performance of the drainage systems, and flooding, and natural systems, will it be possible to plot a sensible way forward.

Glasgow and Wembley flooding

This can be illustrated by examining an earlier flood event in 2002 in Glasgow. This was also somewhere between a 75 and 100 year return period, and caused major urban inundation. What is of interest is the analysis of the flooding. This detailed analysis for one location showed that there was a division between the three major causes of flooding – that the flooding was not just caused by under-capacity in the public sewerage system, it was also caused by overspill and flooding from local watercourses. Also, around a third of the flooding was due to direct overland flows from runoff that never entered a drainage system before it flooded properties. This demonstrates the importance of understanding the interactions between the various components of urban flooding.

Looking at the work undertaken in Wembley, a video of the actual flood event was compared side-by-side to an InfoWorks CS 2D model of the event running simultaneously. The InfoWorks model showed the detail of the sewers that overloaded and surcharged, and the overland flows generated in the 2D mesh. This exercise demonstrated an extremely close correlation between the extent of flooding predicted by the model and that seen in the video footage.

This completely integrated model, running in InfoWorks CS, integrated three piped sewerage systems, the local watercourse system and overland flows. All of the components seen in the Glasgow study were brought together in that model, which was key because it allowed the interactions between the different systems to be fully understood. Without this knowledge, it would be very easy to invest in the wrong sort of solution.

The model showed that river water backing up into the sewer system was one of the major drivers for the flooding. It is critical that such mechanisms are understood because simply upsizing the sewerage system would not resolve that flooding problem. It would in fact probably be much more cost effective to deal with some of the fundamental issues on the river system even though this achieves its target for flood management. This underlines the importance of being able to understand the interactions that take place.

Flood mechanisms and measures

It is also important to remember that the actual flooding mechanisms of property are quite complex. It is not only overland flows that are important, but also below-ground mechanisms. An integrated model that allows both mechanisms to be grasped is key to understanding the needs.

What measures might be used in future to tackle these needs and address the issue of sustainability? To examine this it is necessary to structure the country’s drainage systems into three components: source, pathway and receptor.

To look at the first of these, the industry has begun, in some new drainage systems, to work to limit the flows that enter such networks. For the second component, the pathway, it is necessary to determine how best to effectively retrofit existing urban networks and maximise the capacity of the drainage system. The industry cannot afford a wholesale upgrade, but it can invest in removing pinch points and dealing with isolated issues of capacity and flooding within the drainage infrastructure. Managing above-ground flows is critical, especially for extreme events.

Finally, it is necessary to look at receptors and their resilience to flooding, both the urban fabric – the buildings and other features, particularly key urban infrastructure - and the resilience of the urban communities.

Example schemes

Illustrative schemes from recent years from Holland include a project in which a series of source control ponds and bioretention areas have been installed among new housing on an area cleared of previous urban development, to deal both with local runoff to reduce flooding, and to improve water quality. These features are designed into the landscape to create a pleasant urban area.

A recent project in Delft involved retrofitting an infiltration basin into an existing 100-year old housing area. The basin is designed to capture roof runoff that currently drains into the combined sewer system. The runoff will be stored in a tank beneath a small urban park, and from there it will drain into a deep aquifer via a borehole. This option was chosen to avoid waterlogging the shallow aquifer, which could cause local groundwater flooding issues.

Another project for a large urban development in The Hague utilizes both source control, via storage basins, and local wetlands. A point of note is the integration of the architecture of the flow control facilities with the general architecture of the redeveloped area. Another new housing development on a regenerated area demonstrates a particular feature of surface water management at source, retaining the surface water at ground level. Here, the roof runoff drains through rainwater pipes into channels that discharge onto the highways – there is no below-ground separate surface water drainage system, retaining the flows on the surface is considered part of the drainage.

Future approaches

Maximising drainage capacity will, in future, be targeted at flood hotspots rather than the wider inundation caused by more extreme events. Work to increase capacity is disruptive in urban areas, which is one of the key challenges in dealing with these hot spots, but the projects are likely to be focused around localised groups of what is known as ‘DG5 flooding’ – flooding that occurs frequently enough that the properties are on water companies’ flooding registers.

In taking such approaches – managing flows above ground, and improving the layout of urban areas – engineers have begun to work much more strategically with planners, architects and others to integrate urban drainage into urban design as a whole. Some of the examples from The Netherlands may be seen more frequently in the UK in future – a current example is a swale in a new development, where the area to either side, above the normal water level, is designed as an emergency flood channel for managing extreme events. The buildings are set back from this central green corridor, which provides a good amenity area under normal conditions and a flood channel with considerable capacity in extreme events.

For surface areas, it might be possible to utilize completely different profiles for local roads, a current example showing the surface sloping down towards the centre of the carriageway where a central channel conveys the surface water to an outfall. Flooding overflows onto an infiltration area, and when its capacity is exceeded flows enter a wooded area that acts as a temporary sacrificial flood area during extreme events. This approach is a different philosophy in terms of how to deal with surface runoff and flood management.

Protecting critical infrastructure, and identifying where it is located, will be a key element of future strategies. Location is not always easy to determine as utility companies are not always eager to share their information, but it is very important to have their buy-in when managing urban flood risk.

In addition, architects are now looking much more seriously at building design and the urban fabric with a view to ensuring buildings are much more flood resilient. Possibilities such as homes raised on stilts are not favoured by architects because they do not necessarily create a pleasant urban environment, so there is still some way to go, working with architects to design residential buildings for flood risk areas that genuinely create a pleasant urban landscape but it is an issue that is currently attracting a great deal of research.

Localised measures are also being adopted, such as a tilting dam in the Sheffield area that rises when the adjacent river Don overflows its banks to complete the floodwall around local buildings to protect them. The city’s giant Meadowhall shopping complex flooded twice, and then the owners acted to retrofit rising floodgates in the car parks that rise from the ground and link with the flood embankment that surrounds the development.

Conclusion

When examining future challenges, and how to adapt to climate change, it becomes clear that conventional approaches to flood defence will be simply unsustainable and it is necessary to undergo a shift in thinking from the provision of flood defences to building resilience to flooding.

There is a need to understand the complexity of urban flooding, in order to develop solutions that are robust and that actually work. To achieve this, it will be necessary to use integrated models to understand what must be done. It is necessary to structure interventions to provide holistic and cost-effective solutions, and to achieve that it will be necessary to ensure that the various stakeholders are fully engaged and working with the industry on developing them.

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