1.1 Hazard, vulnerability and risk

1.1.1 Hazard
Hazard can be defined as follows. Extreme geophysical events which are characterized by concentrated releases of energy or materials, which is pose a largely unexpected threat to human life and can, cause significant damage to goods and the environment. The extent of damage of the event, however, is subjected to societal intervention and will be determined by the exposed elements at risk and inherent vulnerability [Smith, 1996].

1.1.2 Vulnerability
Vulnerability can be defined as the damage or disruption by the impact of hazard to the society in a disaster prone area. The extent to which a community, structure, service or geographic area is likely to be damaged or disrupted by the impact of a particular disaster hazard, on account of their nature, construction, and proximity to hazardous terrain or a disaster prone area. Vulnerability divided into four basic categories. They are physical vulnerability, social vulnerability, institutional vulnerability and economic vulnerability.
· Physical vulnerability: this relates to the susceptibility to damage of engineering structures such as houses, dams or roads.
· Social vulnerability: the ability to cope with impacts on the individual level.
· Institutional vulnerability: referring to the existence and robustness of institutions to deal with and respond to natural disasters.
· Economic vulnerability: the economic or financial capacity to finance losses and return to a previously planned activity path. This may relate to private individuals

1.1.3 Risk
Risk is defined as the probability of impacts or losses occurring. Natural disaster risk can be defined as the following. The exposure or the chance of loss (of lives, persons injured, property damaged and economic activity disrupted) due to a particular hazard for a given area and reference period. It may be expressed mathematically as the probability that a hazard impact will occur multiplied by the consequences of that impact.

Fig. 2.3: Hazard, vulnerability and risk

Fig. 2.3 shows the relationship between hazard, vulnerability and risk with disaster.
Risk = Hazard x Vulnerability [Smith, 1996]
Risk can be minimized by minimizing hazard or vulnerability or both. Disaster risk management helps to minimize the natural disaster risk. It includes risk assessment, disaster prevention and mitigation and disaster preparedness. Mitigation of natural hazards is concerned with reducing risk of disasters.

1.2 Mitigation of natural hazards
Hazard mitigation is defined as sustained actions to reduce or eliminate the long-term impacts and risks associated with natural and human-induced disasters. Mitigating measures are generally taken well in advance of a potential disaster situation to reduce the likelihood or frequency of the event's occurrence, or to avert or diminish the event's impacts. There are two types of disaster mitigation measures named structural and non-structural. As risk is determined by the natural hazard as well as by the existing social and economic vulnerability, there are two areas for addressing risk. One area is to modify the intensity, frequency or location of a given hazard by building dams against floods, reinforcing existing structural engineering measures or using hazard-resistant design. The other area is to modify vulnerability by educating people, devising land-use measures that decrease environmental degradation and installing early-warning systems like radar or by decreasing vulnerability of the population at risk or in economic sectors. A more general vulnerability reduction measure is to improve living standards which will usually improve the capacity to cope with disaster risks. To date, the focus in mitigation approaches has traditionally been on physical and structural measures to modify the hazard [Smith, 1996].

1.3 Landslides in Sri Lanka

1.3.1 Situation of landslides in Sri Lanka
Landslide describes the gravitational movement of body of debris or earth or rock or artificial fill as a unit. The movement may be a slide, flow or fall acting singly or together [Simonett, 1968]. Landslides are the most significant natural hazards in this country, seconds only to flood and cyclones [Dissanayake, 1998]. First six decades in Sri Lanka recorded only six major landslide events, the years since 1981 have registered five major occurrence of landslides [Amaratunga, 1994]. The study of landslides in Sri Lanka indicates that their incident during the past decade (1983-1993) increased rapidly. These landslides have caused damages to about 223 village areas, about 381 deaths and the displacement of 1370 families [Katupota, 1994]. Major part of seven districts comprising the central hill mass namely Badulla, Kalutara, Kandy, Kegalle, Matale, Nuwara-Eliya and Ratnapura are prone to landslides [Amaratunga, 1994]. The present landslide density in the slide prone districts has been estimated to be in order of 1 to 2 landslides per square kilometer area [Dissanayake, 1998]. According to past study [Katupota, 1994] there is a trend of acceleration of landslide hazard in Sri Lanka in coming years.

1.3.2 Features and classification of landslides
Features associated with landslides usually included scarps, tension cracks, shear zones and toes. In many instances only few of those features are present [Baum et al, 1991]. Landslide features are shown in Fig. 2.4. Tension cracks are open fractures in the ground that have rough and irregular surfaces [Fleming et al, 1989]. Transverse cracks are located within the moving mass at the toe of the main slip surface [Burns, 1999]. Scraps are steep exposures of the upper edge of slip surface marked by a sudden increase in slope angle some time nearing vertical [Burns, 1999]. Most prominent feature associated with landslides is the shear zones forming the flank of the landslide [Burns, 1999].

Fig. 2.4: Features of a landslide

As the landslide is beginning to move, usually at first it shows up as en-echelon tension cracks which develop in the shear zone [Fleming et al, 1992]. It is important to have a method of classification to understand the mechanism of land sliding. One of the systems used in world are shown in Table B.1 in Appendix B.

1.4 Mitigation of natural disasters in Sri Lanka
In 1990, the South Asian Association for Regional Corporation (SAARC), based on the collective work of the SAARC countries, published the report titled “Regional study on causes & consequence of natural disasters and protection and prevention of environment”. The most significant development to date has, however, been the release of the draft “Natural Disaster Management Plan” in 1992 (Disanayake, 1998)
The Asian Urban Disaster Mitigation Program (AUDMP) is a ten-year (1995-2004) program designed to respond to the need for safer cities in Asia. The goal of the program is to reduce the disaster vulnerability of urban populations, infrastructure, critical facilities, and shelter in selected cities throughout Asia. The AUDMP projects are being implemented in eight countries, namely Bangladesh, Cambodia, Indonesia, Lao PDR, Nepal, Philippines, Sri Lanka and Thailand. (http://www.adpc.ait.ac.th/audmp/sri.html)
National Building Research Organization (NBRO) proposes for implementing a comprehensive program to identify the vulnerable areas in order to take appropriate measures to reduce the vulnerability of landslides was accepted by the UNDP and UNCHS which agreed to provide the necessary technical and financial assistance. In order to integrate this effort with the regional and the urban planning process, Sri Lanka Urban Multi-hazard Disaster Mitigation Project was implemented commencing 1st of October 1997. The main project activities included hazard and vulnerability mapping, risk analysis, strategic planning, review of policy and procedures, training and professional development and networking.
The project Operational Professional Combine (OPC) functioned in 2003 May with one hundred of academics, professionals, volunteers, from various background with two objectives. Those objectives were,
· To combine all geology and engineering geology expertise in the country to work as one team for a period of one month to carry out field inspection of all landslides/ areas vulnerable to landslides in Ratnapura district and provide essential technical support for immediate implementation work including evacuation, warning and resettlement ( if necessary)
· To study the best mechanism to exploit all national expertise in different disciplines to join hands in similar crisis situation in future to strengthen the national capacity for mitigation and management of a natural hazard.

NATURAL DISASTERS IN SRI LANKA

1.1 Historical background

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.

Fig. 1.1: Fund allocation by the social services department

1.2 Influence of topography, geology and meteorology on landslides and floods

1.2.1 Outline of geology

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.

1.2.2 Topography

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.

1.2.3 Meteorology

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.

Fig. 2.2: Annual average rainfall in Sri Lanka

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.

1.3 Classification of natural disasters

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.