Sri Lanka is a tropical island situated in the Indian Ocean. It has two well defined rainy seasons called North Eastern and South Western monsoons. It encounters several kinds of natural disasters every year. Cyclone, drought and flood take major role and sea erosion and landslides also takes place. Landslides had three places among top 25 natural disasters in Sri Lanka. Fig. 2.1 shows the direct fund allocated by the social services department for the different disaster events in Sri Lanka. There are other different kinds of expenditure for landslides associated with highways such as temporary and permanent remedies, monitoring and instrumentation.
Sri Lanka contains 65,610 km2 areas with inland water bodies and 1340 km coastal line. Nine tenth of Sri Lanka is made up of highly crystalline, non fossiliferous rock of Precambrian age belonging to one of the most ancient and stable parts of the earth crust, the Indian shield. The rest of island is formed of Mesozoic, Tertiary and quaternary sedimentary formations [Cooray, 1984]. The Precambrian rocks are divided into three subgroups called as Vijayan Complex, South Western Group and Highland Complex. The area under central hill country and part of northern plain belongs to the Highland Complex. South west coastal belt of Sri Lanka consists of South Western Group. Low land of north western and south eastern to Highland Complex belongs to the Vijayan Complex series. Entire central hill region is covered by the Highland Complex. Most landslides which occurred in Sri Lanka are based on the Highland Complex. Main rock types found in Highland Group are, metaquartizites, quartz- feldspar schists, marbles and calciphyres, quartzo-feldspahtic granulites, charnockitc gneisses and hormoblende biotite gneisses [Cooray, 1994].
Foliation and jointing are important structural properties of rocks which cause the activation of landslides. Presence of two or more joints was observed in each landslide [Dahanayake 1989]. Weathering is a process acting on rocks and it produce the soil. There are two types of soils as residual and colluviums. Colluvium can be easily found in the highland of Sri Lanka. Most of Sri Lankan mass movement has occurred in colluvium soils [Cooray, 1994].
There are mainly one hundred and three river basins around the island. The river basins which started from highlands of Sri Lanka have large flows. Mahaweli is the longest river and Kalu Ganga contains the largest water flow within a year. Those rivers after coming to the flat terrain cause floods. Kalu Ganga valley, Kalani Ganga valley, Nilwala Ganga valley are some examples for most common flooding of valleys.
Foliation and jointing are important structural properties of rocks which cause the activation of landslides. Presence of two or more joints was observed in each landslide [Dahanayake 1989]. Weathering is a process acting on rocks and it produce the soil. There are two types of soils as residual and colluviums. Colluvium can be easily found in the highland of Sri Lanka. Most of Sri Lankan mass movement has occurred in colluvium soils [Cooray, 1994].
There are mainly one hundred and three river basins around the island. The river basins which started from highlands of Sri Lanka have large flows. Mahaweli is the longest river and Kalu Ganga contains the largest water flow within a year. Those rivers after coming to the flat terrain cause floods. Kalu Ganga valley, Kalani Ganga valley, Nilwala Ganga valley are some examples for most common flooding of valleys.
Sri Lanka is divided into three basic morphological zones. They are the coastal lowland, the upland and the highlands [Cooray, 1984]. The coastal lowlands spread up to 270 m from Mean Sea Level (MSL). The spread is narrower in southern Sri Lanka and wider in north and north western part of the country. All kind of natural hazards discussed in section 2.1 are common in this area, except the landslides which are rare in this region. Hambantota and Kalutara districts are examples for landslide activities in this region. Floods are common in this morphological zone. Floods on May 2003 at Walawe, Nilwala, Gin and Kalu river valleys are best example for this.
The uplands spread from 270 m to 1060 m to MSL. Ridges and valleys are the common features in this zone. Three tenth of the island is covered by the upland zone. Average slope of this area varies from 10 degrees to 35 degrees and steep scarps are also found commonly. Landslides and floods are the main types of natural hazards observed in this region. Landslides are in Matara, Ratnapura, Matale and Kegalle districts. Floods also occur on plateaus in upland region. Gampola- Nawalapitiya area on Mahaweli river valley is one example of flood hazard in this region.
The highland series is spread from 1140 m up to 2524 m from MSL. This is the highest elevation range in the country. Deep valleys and high mountain peaks are common features in this area. All the major rivers begin from this region. Deep escarpment to upland region can be found. Different types of landslides are common in the highland series.
The uplands spread from 270 m to 1060 m to MSL. Ridges and valleys are the common features in this zone. Three tenth of the island is covered by the upland zone. Average slope of this area varies from 10 degrees to 35 degrees and steep scarps are also found commonly. Landslides and floods are the main types of natural hazards observed in this region. Landslides are in Matara, Ratnapura, Matale and Kegalle districts. Floods also occur on plateaus in upland region. Gampola- Nawalapitiya area on Mahaweli river valley is one example of flood hazard in this region.
The highland series is spread from 1140 m up to 2524 m from MSL. This is the highest elevation range in the country. Deep valleys and high mountain peaks are common features in this area. All the major rivers begin from this region. Deep escarpment to upland region can be found. Different types of landslides are common in the highland series.
Most of the natural disasters in Sri Lanka are associated with not only geology and topography but also the rainfall climate of Sri Lanka which is predominantly governed by the seasonally varying monsoons and the associated air masses that are part of the planetary wind regime over South Asia. Therefore, the climate of Sri Lanka can be characterized as a tropical monsoon climate [Malmgren, 2003]. Owing to the annually alternating monsoon systems and their associated winds, two monsoon rainfall seasons, separated by two inter monsoon rainfall seasons can be identified in Sri Lanka [Domroes, 1974]. South western and northeastern monsoons are the two rainy seasons. Fig. 2.2 shows the distribution of average rainfall over the country.
The central highlands, which control the prevailing moisture-laden monsoon winds, act as an important physiographical climatic barrier. Two climatic zones can be distinguished to the west and east of central highlands; the wet zone and dry zone respectively [Malmgren, 2003].
Changing of tropical monsoon climate in Sri Lanka governs all types of natural hazards in country. Change in meteorology together with topography and geology cause activation of landslides. The majority of hazards in Sri Lanka arise out of hydro-climatic variability.
If the geological processes cause loss of life or property then those events become most noticeable. Some geological processes which cause threat to life or property are called natural disasters. This process can be observed throughout the earth’s history. They only become hazardous only if they negatively affect the humans. There would be no natural disasters if it was not for humans and in that case they would only be natural events.
A natural disaster event is commonly defined as the impact of an extreme natural event on an exposed, vulnerable society. If impacts exceed an affected region’s coping capacity thereby necessitating interregional or international help, a large disaster is said to have occurred. The following criteria can be used to define a large disaster [Smith, 1996].
# More than 100 casualties, or
# Economic damage in excess of 1% gross national product (GNP), or
# More than 1% of an impacted country’s population harmed.
Natural disasters can be defined according to the underlying hazard. There are two types as sudden-onset events and slow-onset events. Sudden-onset events can be categorized as extreme geotectonic events and extreme weather events. Earthquakes, volcanic eruptions are examples for extreme geotectonic events. Tropical cyclones, floods are examples for extreme weather events. Fast mass movements are governed by extreme geotectonic events and also extreme weather events. However, these events cannot be modified at all (tropical cyclone) or merely to a lesser degree (floods).
Droughts are the best example of slow onset events. Impact to human behavioral pattern from slow-onset events is significant. For these reasons famines are often treated in a different fashion than other natural disasters, and disaster management options vary from those for sudden-onset events.
