Saturday, October 29, 2011

Drainage system not up to task

Bangkok Post


Bangkok has been struggling to divert floodwater out of the city because its water drainage system was developed mainly for handling localised flooding caused by heavy rainfall, not massive run-offs from the North, said former senior officials of the Bangkok Metropolitan Administration (BMA).

Bangkok invested heavily in its water drainage infrastructure after 1983 when major flooding hit the city.

Since then, canals and tunnels have been dug, retention ponds designated, and pump stations constructed to help drain the water.

The water drainage system is based on a polder system, where dykes are built around the city, and floodwaters are directed to the sea by pumps, water drainage canals and tunnels.

Polder systems have been developed mainly in inner Bangkok, the western side of the city, and the eastern side.

In inner Bangkok, a large polder has major roads and railways cutting through the Ramkhamhaeng and Phetchaburi areas, acting as its main dykes.

The polder is divided into more than 10 sub-polders where drainage canals and tunnels, pump stations and pumps help drain water out from the protected areas inside them.

The western side has a network of dykes along the Chao Phraya, and Mahasawas and Bangkok Noi canals preventing flooding from flowing into the protected areas inside the dykes.

In addition, a major water retention pond in the South can hold up to six million cubic metres of water.

On the eastern side, His Majesty the King's dyke, which runs north to south, and around 20 retention ponds help retain up to six million cubic metres of water before it is pumped out and drained to the sea. Seven giant tunnels have also been installed to help speed drainage.

Phichit Rattakul, former Bangkok governor, said the water drainage system was developed to fight flooding caused by rainfall and run-off from the North which generally head to the eastern side.

The main protection measures against run-off are HM King's dyke and Pasak Cholsid dam in Saraburi further northeast of Bangkok, which can help hold up to 800 million cubic metres of water.

The rest of the run-off usually goes to the Chao Phraya and Thachin rivers. But another mass of water is travelling to Bangkok from the North, and the city has hardly any infrastructure to cope.

Massive run-off above Bangkok needs floodways to travel through the city, but man-made objects block its path.

Suvannabhumi airport, for instance, is built below several major canals in the east, blocking water from flowing further down to the sea.

Mr Phichit said the city must develop more floodways from which run-off from the North can travel down to the sea.

Mr Phichit has proposed the idea of a second ''Chao Phraya''.

The idea was proposed some years ago but scrapped due to lack of support. ''Water needs to travel past Bangkok before going to the sea. Water can travel through Thachin or Bang Pakong rivers, but it will also travel past Bangkok.

''We need more ways to allow water to travel past the city,'' he said.

Mr Phichit said infrastructure should be developed to deal with heavy floods.

Mr Phichit, who is also executive director of the Asia Disaster Preparedness Centre, a regional non-profit organisation, said the city also needs to come up with disaster risk assessments, hazard maps, and better disaster warnings.

Chanchai Vitoonpanyakij, former deputy director of the Drainage and Sewerage Department, agreed Bangkok needs new infrastructure to cope with flooding from the North, especially more floodways to allow water to pass to the sea.

Some canals running through Bangkok come under the responsibility of the Irrigation Department, and Bangkok officials must ask for its help in diverting water away from the city.

A source at the government's Flood Relief Operations Command (Froc), which is handling the present flooding, said BMA officials started to join the team at Froc only in the second week.

The centre opened as flooding spread in Ayutthaya early this month.

Officials bickered as they had differing viewpoints on how to manage water. The Irrigation Department initially paid attention mainly to floodwaters in the rivers and tried to manage them, even as fields were left inundated.

''They are getting along now, but it's a bit too late,'' said the source.
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