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 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].
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].
1.3.1 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].
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.
