What tropical countries can teach the U.K. about flood management

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 realizing it can make a difference.”

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 fifteen years or so, and the urbanization of the landscape has led to flooding becoming an annual event. “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 toward the commercial center. 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 United Kingdom 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.

Two-way street

The result is unlike any traffic tunnel in the world. The SMART (Stormwater Management and Road Tunnel) is 9.3 km long, and for its central 3 km 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 Center 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 Center 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, but running 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.

Heavy usage

Major floods, where the water is predicted to flow at above 150m3/sec, only occur once a year on average. In this case, however, 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.