Since last year's floods inundated Bangkok, city and government officials have set about the task of dredging close to 1,000 canals to try and prevent a repeat of the catastrophe in the capital.

But they can offer no assurance that the city is flood-proof as it has a limited capacity to cope with the dual threats of heavy localised rainfall and the damaging effects of torrents of overflowing water from northern dams. Bangkok Metropolitan Administration (BMA) officials say that the capital's flood prevention strategy is centred around coping with water from rainfall and not the exceptional circumstance of additional water streaming from the North, as happened last year.

''The capacity of our flood management system in the city is limited. That's a matter of fact,'' said a former senior official with the BMA's Drainage and Sewerage department.

After more than two weeks of flooded sois and water-logged roads, Bangkokians are again this weekend bracing for heavy rains as tropical storm Gaemi heads to Thailand after progressing west from the South China Sea. To compound the situation, another storm, Phrapiroon is expected to follow in its wake.

THE BANGKOK PLAN

According to the Drainage and Sewerage department, Bangkok's principal flood management strategy is a polder system _ in which dykes are built around the city and floodwaters are directed to the sea by pumps, water drainage canals and tunnels _ to cope with rainwater-induced flooding.

The system comprises 1,655 canals (2,606km in total length), drainage tunnels (7,939 km), water pumps and stations, and barriers which intercept water before it is pumped through the network into the Chao Phraya River.

A total of 15 sub-polders are mostly located close to either side of the river. Once they receive the excess rainwater, small permanent or temporary pumping stations pump it from the existing drains to the drainage canals or the river.

Bangkok is located in a low-lying area, where water cannot flow south naturally toward the sea. Therefore water pumps and tunnels play a major role in the city's flood management system. The BMA has so far constructed seven giant drainage tunnels, 15m to 22m underground, to help suck the excess water out at a maximum rate of 100 cubic metres per second.

To cope with water run-off from the North, Bangkok relies on a dyke running along the Chao Phraya River and the level road on the eastern side of the city.

Since the flood of 1983 which caused 6.6 billion baht in damage, several construction measures have been implemented including submersible pumps. But more importantly, the King's Dyke on the eastern Bangkok, about 72km in length, was constructed from the north through the east and south with installations of water control gates.

After severe flooding in 1995, the BMA built another barrier along the Chao Phraya River, about 77km in length, to prevent overflow from the river.

Bangkok deputy governor Vallop Suwandee says that the existing system can cope with flooding caused by heavy rainfall as long as it does not exceed 60mm. If rainfall does exceed that level, Bangkok might experience local flooding for a few hours until excess water is sucked out and drained into the river.

The deputy governor said that since last year's flooding, the BMA has undertaken major dredging and clearing of the city's canals and tunnels. The BMA is responsible for 638 canals and the government 317; the total length of all 955 canals slated for the clean-up is 1,648km. The work is 93% complete.

More water pumps have been installed, while barriers have been reinforced.

The deputy governor conceded that to prevent flooding from the northern run-off, water must be diverted to the eastern and western sides of the city.

''We have agreed with the government,'' said Mr Vallop. ''It's the only way that we can prevent water entering Bangkok.''

He insists that the measure would not mean shifting the flood burden to those living on the city's edges.

He said that on several occasions the city has had to receive excess water to help relieve the burden of those living in outlying areas. But he argued that Bangkok is Thailand's transport and economic centre, and letting the city flood severely would put the entire country in a difficult situation.

''People need to understand this point too,'' said Mr Vallop. ''In addition, they need to understand that we must live with water because many locations are in low-lying areas, and because the natural environment has changed a lot. It's not just Bangkok that is facing a flood threat. Other major cities around the world face similar threats and are trying to cope with them.''

NORTHERN RUN-OFF THE REAL THREAT

The Drainage and Sewerage department official said Bangkok has a fairly effective system to cope with flooding from heavy rainfall. He said any major city in the world would have difficulty coping with rainfall of 100mm per day, which is what Bangkok has been contending with in recent weeks. Flooding, then, is to be expected in some locations.

However, he said, so far officials responsible for flood management have only focused on minor issues, such as drainage capacity, and have ignored the overall picture.

''They are pointing fingers at one another over the failure to dredge canals. Is this really worth long discussions in a time of crisis?'' asked the official, referring to recent heated exchanges between the government and the BMA. He said the situation is highly politically charged.

The official said that while the BMA can cope with rain-induced flooding, it needs to seriously look into the threat posed by northern run-off.

To deal with that threat effectively he said a more holistic plan was needed than those currently in place.

By its very nature, he said, water cannot be divided by area because it's all part of the same fluid mass. Bangkok is at the southern end of the more than 150,000 sq km Chao Phraya Basin. Flood management plans need to cover the entire basin, from one end to the other, not just Bangkok.

However, this complicates the issue greatly as city officials would have to deal not just with localised flooding but also with other agencies and areas beyond their control.

The official said there is an urgent need for greater dialogue between the government and the BMA.

''Unfortunately, there is an absence of cooperation and creativity in politics, where people point their fingers at every minor mistake the other side makes,'' said the official. He said he was uncertain if a real dialogue is even possible in the current political environment.

Sutat Weesakul, a member of a government-appointed flood task force who has been working on water management solutions since 2006, agreed that Bangkok needs to work with other provinces in the Chao Phraya Basin to come up with an effective flood management infrastructure.

He said the BMA should be commended for taking some new approaches to flood management, such as its suggestion to create water retention ponds in suburban areas to help relieve pressure on the city, and also for its scheme to divide the city into zones to cope with floods in a decentralised manner. But he said the BMA may need to improve its polder system, especially the drainage capacity from tunnels to canals. Connections between the two systems have been found to be generally poor, lowering drainage capacity.

More importantly, said Mr Sutat, the city needs to seriously consider measures to deal with the northern run-off as currently there is hardly any infrastructure or administrative organisation in place. What Bangkok desperately needs are floodways to help discharge water from north to south, he said.

''The existing system in Bangkok can be adjusted to help cope with the threat from the northern run-off, but to carry out mega-projects like floodways will definitely require a nationally coordinated effort.''