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Sunday, 29 May 2016

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Coming to grips with nature's fury

In the aftermath of the worst disaster in a decade, Sri Lankan experts began forecasting the future, including the island's vulnerability to flooding, cyclones and landslides.

Trees uprooted in a landslide in Aranayaka
Pic: Nirosh Batepola

Sri Lanka mainly experiences flash floods and fluvial or river floods. Flash floods are caused by heavy rainfall. Independent of an overflowing water body, river floods occur when excessive rainfall over an extended period of time causes a river to exceed its capacity.

Unfortunately, the island experienced both types of flooding a fortnight ago, affecting many urban and semi urban areas in the Gampaha and Colombo Districts, when the country experienced the worst flood since 1989.

According to Prof. P.K.S. Mahanama, Department of Town and Country Planning, Moratuwa University, recent floods were mainly the result of improper town planning. "We must understand the river function," he said, adding that development should not hamper the natural flow of a river, as such disturbances may cause river flooding. He noted, while there are many riverfront cities in the world, they remain safe as long as encroachment on a river's command area has not taken place.

Landslide vulnerability

Prof. Mahanama said, the estuary of a river should be free of any settlements. "The British developed the Colombo city as a port. We have mindlessly developed on either side of the Kelani River, and narrowed the estuary area."

Colombo needs 1,000 acres of land for water retention but has only 600 acres at present, he said. Filling marshlands for development is another contributor to urban flooding with floods turning severe due to Colombo's inability to absorb flood waters due to land-filling.

Landslides, according to R.M.S Bandara, Head of the Landslides Research and Risk Management Division of the National Building and Research Organization (NBRO), occur largely due to a combination of factors. "Landslides are the result of various physical, morphological and geological causes," he said, adding that bad land use practices, improper water or drainage management and non-engineered constructions were the leading man-made causes, while deforestation, mining, vibrations caused by excavation and pollution also contribute.

Identifying the exact reasons for the Aranayake landslides is still to be determined conclusively, he said, while geological research is being carried out.

The NBRO issues clearance certificates, handles rain gauges, landslide hazard zone mapping, creates awareness programs, issues landslide warnings and research. It is empowered to take action against constructions without NBRO certification in designated areas.

"There are various constructions in landslide-prone areas that lack proper retaining structures, he noted, adding that people in areas of landslide risk should adhere to best practices in rainwater management.

Meanwhile, unknown to the general public, the city of Colombo still relies on the flood water control system put in place by the British in 1925. "This system still works and the Colombo city is well-protected from river flooding," said G.K. Pathmakeerthi, Director of Irrigation, Western Division.

He explained the flood control system divisions as major and minor systems. "The major flood control system protects Colombo, Kelaniya and Jayawardenepura."

Affected areas in Colombo such as, Sedawatta are meant to be the Kelani River's estuary and therefore, not protected by these systems, he added. According to him, areas upstream such as Meethotamulla and Kaduwela are supposed to be covered by the minor flood control system. However, during a major rainfall, minor flood control systems fail due to the loss of paddy fields and marshlands that absorb excess water. "We had an agriculture-based system and depended on paddy fields for water retention," he explained. Rapid urbanization has changed the character of urban areas and this requires urgent upgrading of systems.

Flood control

He added: "Major flood protection systems need to be improved to include unprotected areas. But, it will require occupation of a large amount of land. People living in these unprotected areas have become a major concern today," he noted.

In Prof. Mahanama's view, land should be occupied for flood mitigation in low-lying areas. "An ingenious solution is needed to control flooding. We need a proper drainage plan. A bund or barrier should be built along the Kelani River to withstand critical flood levels, ideally about 9.2 feet in height," he proposed.

"We can use this bund for transport and it can even be a solution for easy travel from Kaduwela to Colombo. Pumps could be installed along the bund to pump out excess water in a controlled manner," he added.

Relocation of families

However, for this plan to work, relocation of families in these areas to perhaps apartment complexes is a necessity. "The Gin Ganga is a good example and it now protects the Galle city from flooding," he explained.

According to Pathmakeerthi, the Department of Irrigation is now looking at options to mitigate flooding. "President Maithreepala Sirisena has requested a report in two weeks on the matter," he said, adding that a study will be done of the Kelani basin, subsequently.

NBRO's Senarath Bandara holds the view that mitigation is possible through proper water table management, providing ground support to landslide risk areas such as ground nailing and soil binding. During mitigation, landslide-prone areas should be identified and preventive measures taken to restructure the soil mass.

"The NBRO does a lot of landslide mitigating work through geometric, hydrological, chemical and mechanical methods, and has successfully mitigated a large number of landslides in risky areas, using new technology," he said. "Methods such as construction of retaining structure and surface drainage, installation of automatic rain gauges, extensometers and strain gauges are all part of that, while other non-structural methods too are used," he said.

Experts also believe hazard mapping and increasing forest cover in such areas can save lives.

"Landslides occur across the world and despite mitigation efforts, they can occur in hazardous areas, as prevention is impossible sometimes against natures' fury," Bandara said.

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