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Danger of landslides looms large



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When the soil layer becomes saturated as rainfall exceeds 1 inch per hour, the rain ‘lubricates’ the surface of the bedrock, and all of the soil, trees and rocks pulls away and slides down the hillside.


By Thushara Dissanayake


(Chartered Civil Engineer)


South-west monsoonal rainy season has started and heavy downfall can be expected. Rains will be most welcome following a long spell of drought. Ongoing power crisis can be tackled to some extent if the hydro reservoirs receive enough rains. On the negative side, landslide risks in the western slope of the central hills will escalate. Hence, the people have to remain vigilant as we have lost many lives and property due to natural disasters in the past.


Landslide is a general term for the results of rapid movement of earth or rock mass downward from one place to another that would occur with or without human activity. At its simplest, the reason for this scenario is the inability of the earth or rock mass to remain stable under the gravity force. However, landslides are determined by a combination of factors including geology (nature and strength of materials), geometry (slope steepness), and moisture content. Heavy rainfall, steepening of slopes through human activities and the removal of stabilizing vegetation trigger landslides. Besides, factors such as vibrations due to an earthquake, rock blasting or heavy vehicle movements may also cause landslides.


Landslides are commonly identified as socio-natural hazards, and the districts of Badulla, Nuwara Eliya, Ratnapura, Kegalle, Kalutara, Kandy, and Matale are most prone to landslides in the country. As a whole, nearly one-third of the land area of the country is prone to landslides. Percentage wise, when all types of disaster events are concerned, it is about 8 %. In particular, aforementioned regions consist of steep slopes and heavy rainfall increases the risk of landslides. During 2014-2017, more than 160,000 people were affected by landslides, while the death toll stood at 202 according to National Disaster Relief Services Centre.


As stipulated by the Disaster Management Act, National Building Research Organization (NBRO) is the mandatory organisation to deal with landslides. The identification of landslide prone areas and hazard zonation mapping, issuing early warnings, instructing the public on landslide mitigation measures, and enforcing necessary regulations in development works are the main functions of the NBRO.


In Sri Lanka, landslides are mostly initiated by rainfall and disturbance of slopes by construction works. Landslides in undisturbed mountain areas are due to excessive rainfall. Meeriyabedda, Samasara Kanda and Meethotamulla garbage dump are recent locations of massive landslides, which caused many deaths in addition to property damage. Meanwhile, a huge increase in the number of events is visible in the recent years.


Landslide warning signs


The commonest signs of imminent landslide are objects such as progressively leaning trees, fences, retaining walls and utility poles in the area. Apart from that following are other signs, which need attention and regular monitoring. However, it is noteworthy that the list does not include every possible sign of an impending landslide. Further, it should be noted that things mentioned here will not immediately lead to a landslide, but, initial warning signs which have to be ascertained with the help of an expert.


In buildings new cracks appear in plaster, tile, brick, or foundations, and sometimes, gradually widening.


Slowly developing, widening cracks or unusual bulges appear on the ground or on paved areas such as roads or sidewalks.


Water comes through the ground surface in new locations.


A sudden decrease in stream water levels, even though rain is still falling or has just recently ceased.


Protecting your properties


There is a higher tendency for recurrent landslides in areas, where there are records of past incidents. If you are living in a landslide hazard prone area as broadly demarcated by the relevant authorities, learn about your local geology and the potential for landslides in your premises consulting an expert. A ground assessment; a construction clearance as done by NBRO, will be important before doing any development works. It is not suitable to undercut a steep bank, build near the top or base of steep slopes, fill on steep slopes, and allow water flow down steep slopes. Planting ground cover on slopes and building retaining walls, where necessary to make the ground stable, is also important.


If you are indoors when a landslide hits


If you are indoors when a landslide occurs and unable to escape, unfortunately, only few things can be done to protect yourself. You have to find cover in the part of the building that is furthest away from the approaching landslide and take shelter under a solid piece of furniture like a strong table. Then curl up into a tight ball and protect your head until all movement has ceased.


If you are outside when a landslide hits


Landslides move very fast and trying to outrun a landslide is impossible. Instead, move quickly away from its likely path, keeping clear of embankments, trees, power lines and utility poles. Stay away from the area as additional slope failures can occur even after few days.


How to reduce landslide Hazards


The best option to reduce landslide damage is to avoid the most landslide-prone areas altogether during development work.


One of the important measures to reduce landslides is keeping cut slopes geometries during constructions works at suitable safe angles. Removing unnecessary loads acting on the slopes, planting suitable vegetation for stabi


lization and maintaining drainage paths appropriately are other steps.


As policy measures, though some are already in place, imposing strict regulations and controls on development in the area at risk can prevent


 


damage and loss of life. If adequate data on rainfall are available in the pre-identified risk zones early warnings will also help reduce the risks.


 
 
 
 
 
 
 
 
 
 
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