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Sunday, 17 February 2013

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Mitigation program for landslide-prone areas:

NBRO warns residents



A subsided location in the Thotagamuwa junction.


Another subsided site in the Thotagamuwa junction area.

Landslide Studies and Services Division Head, NBRO R.M.S. Bandara.

The National Building Research Organisation (NBRO) plans to implement a mitigation program in locations prone to landslides and along the A-1 Road jointly with the Road Development Authority, Landslide Studies and Services Division Head, NBRO R.M.S. Bandara said.

He said the NBRO has held discussions with the Japanese International Cooperation Agency (JICA) in regard to the proposed program and JICA will provide technical and financial assistance.

Bandara was confident that the proposed mitigation program will commence before the end of the year.

He said it is an opportunity for our mitigation workers to study the Japanese landslide mitigation technology. The NBRO will be able to implement mitigation programs without the assistance of others in the future.

The mapping of landslide and subsidence prone areas is nearing completion. Over 90 percent of the work is over now. The next focus of the NBRO is to mitigate identified landslide sites. A proposal has also been submitted to the relevant authorities in regard to this program.

"The NBRO plans to conduct landslide as well as subsidence awareness programs targeting executive range officers and people in the grassroots level", Bandara said.

"Necessary arrangements have been made to give early warnings to people living in areas vulnerable to subsidence or landslides. Automated rain gauges will be set up at suitable locations. People will be given instructions and timely warnings in case of any landslide or subsidence threat due to heavy rains. The assistance of the Meteorology Department will be obtained in the process", the Landslide Division Head said.

"The recent landslides claimed 22 lives in the Matale district. Subsidence is the other major problem in the district. The NBRO conducted a survey in the Matale district in 2008 in connection with subsidence and has mapped out locations prone to subsidence", Bandara said.

The maps have been given to local authorities such as Pradeshiya Sabras and Divisional Secretariats. The people who live in areas vulnerable to landslides or subsidence have been instructed to be vigilant and refrain from building houses or other structures in such areas, the Landslide Division Head said.

"The NBRO has identified a number of subsidence prone locations in the Thotagamuwa junction, Matale. Two subsidences were reported in the area in December 2012, accompanied by heavy rains", Bandara said.

"The NBRO conducted a thorough study in the area and found there were nearly 15 to 20 cavities underground", he said.


Alapatha landslide.

A stretch of the A-1 Road.

A subsided place in Palapathwala, Matale.

"Cavities are formed due to the pressure of water. A limestone layer is seen underground. Due to the water fluctuation there is a possibility of limestone erosion. As the limestone is eroded a cavity is formed in the rock. When these cavities grow bigger the earth will go down or sink. This process is called subsidence", Bandara said.

Subsidence may occur even in flat lands. It is a sudden occurrence and is dangerous. It can occur overnight if the water fluctuation is high underground, Bandara said.

He said after last year's subsidence in the Matale district, no incidents were reported in the area. The Matale District is vulnerable to both landslides and subsidence, he said. Areas with castaic marble formation are vulnerable to subsidence. Any place in the world with this structure is no exception to subsidence.

Residents in subsidence prone areas will be relocated to safe areas, Matale District Secretariat sources said. They said residents in various areas in Matale such as Rattota and Ukuwela Division who are at risk will be resettled in safer locations, and the relocation program is being conducted with the guidance of the NBRO.

"There are subsidence prone locations in South Africa as well as Australia. If there is a marble formation underground those areas will definitely become vulnerable to subsidence. Even big houses may sink underground. In other countries houses have also sunk, but not so in Sri Lanka", Bandara said.

The NBRO has cautioned the people to pay maximum attention to the condition of land before commencing buildings or other structures. The NBRO has deployed its officers to the relevant areas and they are kept in vigil round the clock to help the residents. People have also been asked to inform the NBRO about cracks appearing on the walls of houses and changes occurring in the ground.

Landslide is a different phenomena when compared with subsidence. Landslides may occur due to the fall of cuttings, and bad land practice. Falling of rock, debris, slope angle, heavy rainfalls, geological conditions, water conditions and drainage systems are the causes of landslides.

The sinking of the earth structure is known as subsidence. Due to various reasons such as gem mining subsidence could occur, especially in the Ratnapura district. Subsidence is possible in places where people engage in mining.

There could be cavities in rocks. Houses and giant structures are constructed on rocks. But people are not aware of the underground condition. If there is a cavity in the rock, it may go down or sink any time. People who construct houses and other structures should conduct a thorough study on the condition of the underground.

"Subsidence is concerned with soil thickness, water conditions and cavities. Mitigation work is not possible against subsidence. If there are many cavities in the rock sending soil or applying concrete layers are not possible. Mitigation works cannot be carried out to counter natural subsidence. Some countries use the identified subsidence areas for crop and vegetable cultivation", the Landslide Divison Head said.

Land subsidence occurs when large amounts of ground water have been withdrawn from certain types of rocks, such as fine-grained sediments. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rock falls in on itself. You may not notice land subsidence too much because it can occur over large areas rather than in a small spot, such as a sink hole Bandara said.

It doesn't mean that subsidence is not a big event. States such as California, Texas and Florida have suffered damage to the tune of hundreds of millions of dollars over the years, due to subsidence.

The reaction of water in the marble bands underground is the technical reason for land subsidence. It will take place in certain areas, but landslides may occur in any place. Landslide is also very dangerous. Already we have mapped out subsidence locations and given it to relevant local authorities", Bandara said.

The NBRO has created awareness among the public about subsidence and landslides through the local authorities, but the people forget everything in due course. They start talking about it only when an incident takes place", the Landslide Division Head said.

The Landslide Research and Risk Management Division of NBRO has implemented a landslide hazard zonation mapping program within the 10 landslide prone districts of Kalutara, Galle, Hambantota, Nuwara Eliya, Matale, Kandy, Kegalle, Ratnapura, Matara and Badulla.

 

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