Flood tunnels are a component of a flood management system usually found in built-up urban areas where there is no space for surface canals or stormwater channels. A flood tunnel is a larger order of stormwater pipe and the term sometimes encompasses additional components such as retention basins, temporary storage tanks and associated pumps and infrastructure.
There are a number of interesting examples of flood tunnels around the world. We have decided to run a series of article highlighting a few selected examples of flood tunnels, some completed, some in development and others only being proposed at this stage. Given the growing problem of urban flooding throughout the world, we may well be seeing more discussion surrounding the use of flood tunnels in the future.
To start our series we take alook at the SMART Tunnel project in Kuala Lumpur, Malaysia.
Malaysia’s SMART (Stormwater Management And Road Tunnel) Tunnel project, as its name suggests, is a unique multipurpose tunnel which was completed in 2007. In this article the main points of the project are briefly outlined to give an idea of the way it operates.
Over 7 million people live in the greater Kuala Lumpur metropolitan area, an area that has witnessed rapid population and economic growth since the early 1980s. Accompanying this growth has been an increase in the severity of flooding events as river courses became constricted by urban development, especially the Klang and Gombak rivers, which merge in the centre of the city. The Malaysian government has monitored the situation since the early 1970s and found that the average annual flooding for the Klang River has increased nearly 300 percent, from about 148 cubic meters (39 100 gallons) per second before 1985 to 440 cubic meters (116 200 gallons) per second since then.
The government attempted to control flooding by increasing river channel capacity and creating holding ponds, but, owing to lack of available space in the city centre, this has had only limited success.
In 2001, the government called for proposals for a solution to prevent disruption to the city’s centre during a typical flood event with a duration of three to six hours. Initially, the intention was for a tunnel to divert and store stormwater, but the idea progressed into the concept of a multi–purpose tunnel that would allow traffic flow when the tunnel was not being used to convey stormwater. A major factor leading to this concept was that, for liability reasons, the tunnel needed to be below government-owned land, causing planners to consider locating it beneath road routes.
Thus was born the idea of integrating a motorway tunnel into the project. By designing part of the tunnel as a toll-road, private sector participation in a Public-Private Partnership (PPP) could be secured, reducing the costs of the project for the Malaysian government.
The result was the Stormwater Management And Road Tunnel (SMART), the longest stormwater tunnel in Southeast Asia at 9.7km (6 miles), and the second-longest in Asia as a whole. Part of the tunnel includes a 4km (2.5 miles) long double-decker roadway beneath the city centre, inserted above the stormwater channel.
In case of minor flooding, the stormwater flows in the section of the tunnel beneath the road. When it becomes apparent that the full volume of the tunnel will be needed for diverting floodwater during a major storm event, the entrances to the tunnel are closed to traffic and the road tunnel is cleared of vehicles. Stormwater is then permitted to flow through the roadway section of the tunnel, so that the full 11.83m (38ft) internal diameter of the tunnel is available for diverting floodwaters away from the city centre. After the threat of flooding has passed, the roadway can be cleaned and cleared for traffic within 48 hours, sometimes in as short a period as 8 hours after closure.
The tunnel begins just before the confluence of the Klang and Ampang Rivers, so that flood water can be diverted away from the Klang River, eliminating flooding in the city centre. The tunnel is part of a larger flood mitigation system, the Kuala Lumpur Flood Mitigation System. The whole project provides storage for 3 million cubic meters (792.5m gallons) of water, sufficient to prevent most flooding in the city centre.
Construction of the tunnel, which cost RM1.887bn (US$514.6m) started in November 2003 and the stormwater function of the tunnel commenced at the end of January 2007, with the official opening of the whole tunnel on 14 May 2007. It was first used in its full flood diversion mode only three weeks after being officially opened, and within three years had already prevented seven potentially disastrous flash floods in the city centre.
Other cities considering the SMART Tunnel concept include Bangkok in Thailand, see > for more.