Last summer was the wettest in England and Wales for a century. It was a dramatic turnaround after two dry winters and a drought that saw 20 million people issued with a hosepipe ban. In fact, the recovery from the drought experienced in 2012 was unprecedented, according to a report published by the Centre for Ecology and Hydrology.
As a result of all the rain, widespread flooding was reported across the country in the autumn. People lost their lives and the floodwaters caused hundreds of thousands of pounds worth of damage. More than 5.5 million people in the UK live or work in properties that are at risk of flooding, says the Environment Agency. The jury is still out as to whether these types of weather patterns will persist in the future, but the increase in flooding has got some engineers looking at ways in which towns and cities can shore up their flood defences.
David Butler, professor of water engineering at the University of Exeter, says: “We are already seeing potential climate change variability. Whether you call it climate change is another matter, but we are already seeing extremes of rainfall.”
As a result of climate variability, more frequent storms of greater magnitude are likely to come in the years ahead, accompanied by more frequent flooding, he says. “We have to start to prepare. This is not going away in the short term. This is the new normal as far as we can tell,” says Butler.
Preparing for a more flood-prone future will involve work across a variety of areas, he says. Local authorities need to be more reactive when a flood happens, forecasters at the Met Office need to prepare people better by warning of potential floods, engineers need to try to stop or reduce flooding in the first place, and make existing buildings more flood resilient.
It is this last point that interests Butler most, and he has recently been awarded a £1.5 million fellowship by the Engineering and Physical Sciences Research Council to develop a new approach for managing water in cities. There is already an issue with drainage systems in urban areas, he explains. “Look at the flooding we have had in the UK just recently. A lot of that tends to be in cities, or indeed caused by cities.”
Much of a city is given over to hard standing and impervious areas, which distorts the natural hydrology. Rainwater cannot be absorbed by the ground and so it runs off into the drainage system. During storm conditions, when excessive rain falls, the runoff overloads the drainage system, causing the rainwater to collect in the streets.
In late November, the Great Western railway flooded. The line between Exeter and London was damaged, and services in Devon and Cornwall were disrupted for weeks. Butler explains that, although railways have built-in drainage systems, these are only designed for a certain amount of rainfall. Once rainfall exceeds this level, as happened recently, the system is unable to cope.
“All our infrastructure needs to be thinking about increasing its resilience to these events,” says Butler. He admits that it will be tough to boost the flood resilience of established cities that already have drainage systems in place. Storing rainwater in the event of a storm in underground tanks so that it does not overload the drains is one solution that has been used for many years.
If the climate is becoming more variable, more storage capacity will be needed. “In a city, building underground storage is expensive and it can be difficult to find space for it,” says Butler.
To get around this, a project in north London is under way that will store rainwater up high. The concept, dubbed the “blue roof,” sees the water held on the flat roof of a new housing development. Thanks to tight waterproofing, the water is stored on the roof and slowly released into drainpipes by a system of flow restrictors. By slowing down the flow of water, the engineers behind the blue roof hope to alleviate the pressure on the city’s drainage system in times of stress.The developers of the site, Zenith House in Barnet, were limited in the depth that they could excavate for a stormwater tank. As part of the planning conditions, the authorities required the developers to attenuate 600m3 of stormwater, with a maximum discharge rate to the sewers of 9 litres/sec. So storing some of the stormwater on the roof offered a solution.
Around 150m3 of water will be stored on the roof with a maximum hydraulic head of 150mm. This water will drain through 16 outlets at a rate of 4 litres/sec. The remaining water will be stored in the basement and pumped to the sewers at a rate of 5 litres/sec.
Andrew Lee and his team at Alumasc Exterior Building Products have been crunching the numbers to model and test water flow through outlets of different diameter that could meet the requirements of a blue roof. The resulting technology comprises a set of 100mm-diameter metal gullies fitted with flow restrictors. These devices comprise a clamp with adaptor that can house a 10-50mm flow restrictor, and are fitted with a removal handle and vent pressure pipe with protector. Over the top of the device sits a grating that helps to stop the system from becoming blocked.
Lee says: “The smaller hole sizes could block up more easily, so filtration is important.” Lee is also working on a heated gully system.
The system has overflows and in the event of a roof leak or major catastrophe the restrictors can be removed manually and the roof will drain in the conventional way. The structural load of the water does not pose an issue, says Lee. “The roof is already capable of holding a snow load or indeed the volume of water that we want to hold,” he says.
Butler says that the blue roof could have potential and is a sustainable drainage solution. “From a flooding and urban drainage point of view, blue roofs could do a really good job.”
Retrofitting the technology to existing flat-roofed buildings could be an option, he says. But Butler stresses that waterproofing the roof to prevent leaks could pose a challenge, and the extra load of the water needs to be accounted for in the design process.
Butler and his team have been looking at systems that store water high up in the building rather than underground so that the need for a pump, and its associated energy use, is eliminated. Work is also ongoing to develop porous pavement materials that can hold rainwater.
Looking ahead, Butler foresees a new paradigm for water management. “Today’s infrastructure and drainage systems have been designed to be safe, but what I am arguing is that they need to be sure as well. By sure I mean sustainable and resilient.”
He says this can be achieved by making the systems flexible and interconnected rather than rigid and separated. With these new qualities, systems will have multiple options so that, if one part fails, another part can provide back-up.