How to Prepare for Landslides
By Eng. Thushara Dissanayake
Yet another rainy season has reached its peak and heavy downfalls are being experienced. Rains will be most welcome by those involved in the agriculture sector and hydropower generation. On the negative side, landslide risks in hilly areas especially in the mid-country, and flood risks in low lying areas will escalate. Hence, people have to remain vigilant as we have lost many lives and property due to natural disasters in the past. This article discusses some technical aspects of landslides and imperative safety precautions against them.
Landslide is a general term for the results of the 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 stabilising 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 eight per cent. In particular, the 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 the National Disaster Relief Services Centre.
As stipulated by the Disaster Management Act, National Building Research Organisation (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 dumps 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 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 by 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 a 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 a 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 construction works at suitable safe angles. Removing unnecessary loads acting on the slopes, planting suitable vegetation for stabilisation, 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.