Challenges to Overcome: An Overview of Koslanda Landslide

What has happened at Koslanda?

Landslides are one of the prevalent and frequent natural disasters in Sri Lanka.  Although the area of influence is small, the extent of damage caused to the lives and property by a landslide is immense. The Koslanda landslide in the Koslanda estate occurred in 29th of October at about 7.45am.  Rainfall data about the Poonagala gauging station revealed that the amount of precipitation poured into the area for three consecutive days from 26th to 29th exceeded 500 mm. This has caused to destabilize an area in Koslanda located above six state line houses, which was already identified by NBRO as a landslide risk area.  This severe catastrophe caused 12 deaths and 23 disappearances, completely wiping out all the state line houses.koslanda landslide

Causes of landslides

Landslide occurs due to various causes either natural or man-made. But the decisive triggering factor for Sri Lankan landslides is the high intensity rainfall. The slopes of the Uva province receive rainfall mainly from the second inter-monsoon from October to November and the north eastern monsoon from December to February. The other main landslide causative factors are bedrock geology (including degree of weathering and the nature and intensity of defects), slope angle, landform, overburden soil cover, drainage pattern and land use patterns. First four factors are static, whereas the last two are dynamic mainly due to human interventions. Although all of these six factors satisfy favorable conditions for a landslide occurrence, failures do not take place until the threshold limit of main decisive triggering factor is achieved. Therefore, it is a very well-known fact that even in well developed countries the prediction of landslide disasters to near precision is very difficult. But, geologists can give the probability of landslide occurrence to greater precision by evaluating all available causative factors with local and regional geomorphology.

Geomorphological setting of K area

Geomorphologically, the area is a gently inclined talus slope, where a thick, loosely compacted colluvium deposit is observed at the foot of the near vertical rocky scarp and is situated at the middle part of the slope. The lower area shows a fairly steep surface as well. The composition of the colluvium deposit includes a randomly arranged mixture of weathered clayey and sandy products and organic material that can act as a sponge with high water content. The area was an abandoned tea cultivated land in which the properly maintained surface drainage system has been neglected. This colluvium deposit is underlain by garnet biotite gneiss bedrock, which is highly foliated and jointed.

Mechanism of Koslanda landslide

Due to the destruction of the existing drainage system and vegetation cover, infiltration of rain water into the soil overburden has enhanced tremendously in the area during the recent past. As a result, an underground water pool has developed increasing the weight of the soil mass in a short period of time. This may have weakened the inter particle forces prevailing among soil particles, loosing the shear strength of the material leading to create slip surfaces along the weaker planes. At the same time, internal water pressure in joint planes can also be developed by percolating water into them. As a result, these joint planes can be expanded by creating an additional force onto the soil mass. This also helps to satisfy the appropriate conditions to loosen the shear strength of the soil material. The ultimate consequence is the downward movement of all available material on the slope along a surface of rupture under the gravity to attain a static condition with low potential energy. This whole phenomenon is known as a landslide. The Koslanda mass movement is a deep seated rotational landslide, which is incorporated with several minor slip surfaces apart from the main one.koslanda landslide

Do landslides show pre signals?

Generally, almost all landslides show pre signals; such as appearance and expansion of tension cracks on the ground and floors and walls of buildings located in upper slope areas, disappearance of springs and drying up of wells located on the upper slopes, appearance of springs and increase of water levels of wells located at lower slope areas.   Muddy water outpouring from springs at lower slope areas, ground subsidence of the upper slopes, tilting of poles and trees, etc., and prior to the main movement except some rapid sudden slope failures. In the case of Koslanda landslide, some of these features like appearance and widening of tension cracks on the upper estate road, floor and ground cracks of line houses, small local ground subsidence happened at times in the middle part of the destabilized area, water seepages more or less throughout the year revealing high elevated ground water level, etc., have been observed prior to the incident.

What is NBRO? And why people did not move from Koslanda in time?

Victims of the Koslanda landslide disaster are estate workers residing in line houses. Since there were a large number of people in those line houses, it is impossible to resettle them without the help of estate officials and the government. National Building Research Organization (NBRO), which had initiated landslide studies in back in the late eighties and still continuing with greater development, has identified the high potentiality of the K area in the year 2005 and informed the severity to relevant government authorities. Due to the service rendered to the nation in terms of landslide disaster management, NBRO has been recognized as the national focal point by the government, public and private sector institutes for landslide warning. But, NBRO has not yet been established and empowered by a Parliament act giving the due recognition and legal provision enables it to carry out its activities, effectively.  Therefore, NBRO is lacking a required mandate in case of resettlement of identified vulnerable population or adopting mitigatory measures to such areas. Due to lack of adequate lands to the government also hinders the resettlement activities. In addition, some people do not want to vacate their original places, though they have been informed about their vulnerability and also received a new place to live, since immediate threat is not exhibited on the ground because, landslide warnings are issued based on the higher probability.koslanda landslide

How to minimize such tragedies?

The basic need is to build a genuine necessity and realize the importance of resettlement of vulnerable people, creating awareness among all related parties, including the government, the public authorities and especially the affected communities. It is real fact that, with the population growth, people encroach marginalized lands such as reservations and moves onto unstable sloppy lands. In most cases, these people are under privileged groups whose income level is far below the required to survive. Therefore, they have no other option than the occupation of such areas, exposing to various types of hazards.  It is an urgent necessity to formulate a national policy or Standard Operating Procedures (SOPs) to optimize these activities in the country. Accordingly, relevant institutes must undertake their due responsibility earnestly.

NBRO is the responsible agency for the identification of vulnerable areas and providing guidelines to the relevant authorities. Mitigation proposals are also being prepared. However, proper mechanism has not still been put in place to apply mitigation or precautionary measures on such identified unstable slopes. Government is directing NBRO during the last few years to mitigate some such places with priority basis. Somehow, it is a need of today to attend such unstable areas after they have been identified. Furthermore, the government introduced a circular in year 2010 making it mandatory to obtain a suitability certificate from NBRO for all constructions and development activities carried out within declared landslide prone areas in view of minimizing ad hoc interventions, which may lead to enhance the vulnerability. For undertaking these mandated activities effectively and efficiently, the capacity and resources of NBRO must be upgraded. Being a self funded organization, which is not dependent on the treasury, it is very difficult to cope this national requirement effectively with available facilities.

However, it is a must now, to apply short term mitigation measures such as surface drainage control, application of erosion control measures, dewatering of high elevated ground water level after identification. Mechanism and legal provision must be established with relevant agencies. At the same time if the risk is high, evacuation of vulnerable community must be done and proper temporary shelters must be provided to them immediately. Then, immediate steps need to be taken for permanent resettlement with a social assessment along with technical considerations. More importantly, the new place must be provided with more facilities than the original place. If such a system is available any community will not deny settling in a more comfortable location.  Landslide Research & Risk Management Division (LRRMD) of NBRO is directly involved in landslide disaster management activities in Sri Lanka. Geologists of NBRO are playing a vital role in those activities. Lack of required number of geologists is one of the main challenges facing NBRO, which still failed to initiate a mechanism to retain experienced geologists and to encourage new recruits. Requirement of geologists is felt only during rainy seasons and totally neglected after rain is come to an end. Therefore, NBRO is struggling to cope every extreme rain event, which bring about an enormous number of mass movements occurring all over the central highlands of the country.





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