Urban planning engineers explore anti-flood options

The UK is considering new techniques to deal with the recent rise in localised flooding.

It rains in Britain in the summer; persistently and, until relatively recent years, predictably. The summer weather systems would see periods of good weather interspersed with weather fronts sweeping from Cornwall up towards the north-east coast, trailing curtains of rain.

That changed, unexpectedly and catastrophically, in June and July 2007. Rather than a procession of showers, a new type of rain emerged — localised storms, dropping a lot of water in one place over a short period of time. Villages and towns were overwhelmed.

In Boscastle, a river burst its banks and swept cars down the steep valley into the heart of the village, where they formed a dam and allowed water to inundate the town and harbour. Tewkesbury’s historic abbey was surrounded by floodwater on a newly-formed island. The civil and military emergency services estimate that the rescue efforts were the biggest ever launched in peacetime.

It has become obvious that the water management infrastructure in many areas was inadequate to manage this type of rainfall. Urban planning engineers are now starting to turn to other countries for advice; countries where localised heavy rainfall is more common. Odd though it may seem, the chilly, damp UK is looking to the tropics. ‘We are looking to take lessons from wherever we can,’ said Scott Steedman, vice-president of the Royal Academy of Engineering.

Wales, with its particularly soggy reputation, is looking closely at all options. ‘We’re well aware that we were lucky in 2007,’ said Roger Falconer, professor of water management at Cardiff University. ‘If the weather systems had been a little to the west, we would have borne the full brunt.’

Currently, urban flood management in most towns is limited to the Victorian infrastructure of drains feeding into the pipes under the streets. ‘Almost every major city in the UK is facing similar problems,’ added Falconer.

Other countries employ many techniques to control floodwaters, some of which may be more applicable to the UK than others. The best method, Falconer said, is ‘to create room for the water’. This involves planning areas of parkland in the zone where flooding is likely to occur, and diverting water into these when appropriate. This technique was used in Cardiff around the new Millennium Stadium.

Asian cities tend to have large open drain channels at the roadside, with further drainage down the middle; a technique which could be applied in UK cities, said Falconer. ‘This is something we might have to look at to contain surface waters.’

South American cities such as Buenos Aires accept that floods will happen and divert surface water down designated streets away from commercial and residential areas. ‘That is something that can be considered for new developments,’ added Falconer.

Other urban planning techniques include porous pavements placed on top of drainage channels and reservoirs underneath car parks to receive floodwaters. ‘It does represent an investment, but if you look at the £3bn that the 2007 floods cost, in damages and rescue efforts, it would be worth it.’

But although many techniques could be imported, UK engineers are also looking at the problem, with one new technology about to be trialled in the Midlands.

Devised by John Greenwood of Nottingham Trent University, the SELOC (Self-Erecting Low-Cost Barrier) is, he admits, not a high-tech solution. The barrier itself, originally made of wood covered with a waterproof membrane on the side facing the flood-risk area, lies flat on the ground most of the time, hinged along its bottom edge. As the water rises, it floats, with its top edge rising with the water level. A restraint stops it when it reaches the vertical.

‘The system’s strength comes from the membrane,’ Greenwood explained. ‘Before you install the barrier, you dig a trench along where it’s going to be. Most of the membrane forms a liner for the trench, leaving just the part that covers the inner side of the barrier itself. You then backfill the trench, and that’s what makes the system withstand the pressure of the water.’

Although it seems simple, SELOC provides a reliable automatic solution to flooding. ‘I looked at some of the other systems around, such as temporary barriers, but these systems were all “hard-engineered” and needed manpower to put them up. That takes time and effort, and while the barriers aren’t in place, the water is coming in. The SELOC system rises with the water and it doesn’t need any intervention to activate it.’

Since the original development of SELOC, Greenwood has teamed up with GA Geotechnical, a company specialising in membrane-based technologies, to commercialise it. ‘We’ve made it more robust,’ said Peter Atchison, GA director.

The first market for SELOC is likely to be protecting areas such as music festival sites and sporting arenas, particularly low-lying cricket pitches and golf courses, added Atchison. ‘But we’re also talking to local authorities, and it could be used to protect areas of villages and towns.’

Greenwood’s original vision for the project, however, was for countries such as Bangladesh, which are often hit hard by flooding. ‘That’s still a goal,’ Atchison confirmed. ‘For that, we’d strip it back to the wooden barrier and the membrane forming the hinge; just the simple, effective solution. It’s great seeing something such as this in action, and realising it can make a difference.’

Road works: Kuala Lumpur’s smart traffic solution

 

Flood prevention in Malaysia involves some impressive civil engineering. In Kuala Lumpur, one single — and mostly hidden — structure takes care of two of the sprawling city’s most pernicious problems: frequent flash-floods, and terrible traffic.
The city has expanded hugely in the last 15 years or so, and the urbanisation of the landscape has led to flooding becoming an annual event. And ‘flash’ is an apt description. The Klang river, which used to flood at 148m3/s before the city’s expansion, now rises at almost 450m3/s; and the floodwaters rush straight towards the commercial centre. A typical flood lasts for three to six hours.

Tasked with coming up with a solution, civil engineering firm Gamuda called upon a local consultant engineering firm, SSP, and Mott MacDonald in the UK to develop a tunnel linking holding ponds at either end to divert floodwaters underneath the city. For part of its length, the tunnel could also act as a traffic bypass.

The result is unlike any traffic tunnel in the world. The SMART (Stormwater Management and Road Tunnel) is 9.3km long, and for its central 3km stretch, can carry both cars and water.

Between the two service gates at either end of the traffic section, the tunnel diameter is divided into three by two car decks, one on top of the other. The storm channel runs below the bottom deck. The service gates consist of three barriers, one of which — the emergency gate — blocks both traffic decks, while the other two block one deck each.

In normal operation, all three gates remain closed under their own weight. The Stormwater Control Centre monitors the point where water is diverted from the Klang into the upper holding pond, and uses information on rainfall and the height of the river to predict the magnitude of floods.

If the Control Centre predicts a flood where the flow past the pond intake is likely to be less than 70m3/s, then the pond capacity will be sufficient to contain the flood, and the tunnel will remain dry. In floods predicted to be between 50m3/s and 70m3/s, the gates at the top and bottom of the tunnel will open, and water will flow down, filling the tunnel at either end, butrunning underneath the traffic in the central section, maintaining the flow of traffic into and out of the commercial district, but keeping the water flow away.

Major floods, where the water is predicted to flow at above 150m3/sec, only occur once a year on average. But in this case, one or both decks can be used to carry the water. The upper gates close, and the traffic is monitored until the tunnel is empty. At that point, the emergency gates and service gates for one or both tunnels are opened to let the water through.

Restoring the road after a flood takes two days. Water both drains and is pumped out of the traffic section, floating debris is removed from the trash barriers that prevent it entering the road section, and the road deck areas are washed down and sprayed with disinfectant.