LANDSLIDE RISK ASSESSMENT OF KOTHMALE OYA RIVER CATCHMENT AREA Research Proposal for M.Phil Degree Mrs. H.M. Jayani Rupi Herath Department of Geography University of Sri Jayewardenepura RESEARCH PROBLEM Where (geographical location), When (temporal distribution), why (the reasons) landslides occurred in Kothmale Oya River Catchment Area? Are these landslides occurred due to natural phenomenon or reservoir induces? ( Kothmale Reservoir was constructed including 90 m high rock hill dam across the Kothmale Oya, a six km long reservoir holding 175 million cubic meters of water and underground power station in 1978) INTRODUCTION In Sri Lanka, landslide prone areas and former landslides have been identified using landslide hazard zonation maps. (Sri Lankan landslide hazard zonation maps are of 1: 10000 scale). The Kothmale River Catchment Area is included in to the landslide prone areas which are covered by landslide hazard zonation maps. Significantly in Sri Lanka, many former landslides have been concentrated in to the Central Highlands. OBJECTIVES Assessing spatial and temporal distribution of landslides occurrences in Kothmale Oya River Catchment Area during past fifty years of time. Identification of reasons for these occurrences; Whether there are natural phenomenon (eg: slope degree, rainfall, soil properties, distance from the crest etc) Or man made reasons (eg: reservoir influence, deforestation, insufficient drainage in steep slope areas, ponding of water for paddy cultivation on terraces on unstable slopes etc). Developing a method to assess landslide risk: A Risk Assessment includes; assessing former and potential landslide occurrences. Identification of limitations of landslide risk assessment. What is a landslide? Landslides is a common natural phenomenon in many parts of the world, especially in hilly or mountainous terrains. A landslide event is defined as “the movement of a mass of rock, debris, or earth (soil) down a slope “ (under the influence of gravity) (Cruden 1990). The word “landslide” also refers to the geomorphic feature that results from the event. Other terms used to refer to landslide events include; mass movements, slope failures, slope instability, and terrain instability. The most commonly used landslide classification system is based upon material type and type of movement describes by Cruden and Varnes (1996). Landslides disasters have brought significant economic and social impact causing severe damages to life and property, environment and socio economic life of the society. What is a Risk Assessment ? Risk Assessment is a part of risk management which supports many different issues in the society in order to avoid or minimize damage and costs. Contributes to a sustainable development in both ecological and economical terms. The risk assessment identifies and evaluates the risks (Lundkvist 2001). LITERATURE REVIEW WHAT OTHERS HAVE DONE IN SRI LANKA 1 Johansson, D., 1989. The Kothmale Environment – A Study of the Environmental Impact of the Kothmale Hydropower Project in Sri Lanka. SIDA Evaluation Report, Infrastructure, Sri Lanka. The Kothmale valley is bordered by the cliffs, which have gentle foot slopes, so called talus slopes, formed by colluviums and debris derived from high cliffs and upslope sources by various processes of mass movement. Furthermore paddy fields act as silt traps, which mean that the silt does not reach the reservoir. Some of the tea estates which are neglected or poorly managed may have an effect on the reservoir through the release of silt from erosion due to improper land use practice. 2 Brunsden, D. and Doornkamp, J.C., 1980. Volume: 1 – Landslides, Kothmale Project Geomorphological Survey. During this landslide study, prior to the reservoir construction, identified the whole area as a mecca of mass movement with over 300 discrete failures with variety of adverse geological features such as; • Unstable soil and rock masses in the reservoir area, • Solutioned and cavernous limestone in the reservoir and below the dam site, • Deep and irregular weathering rock associated with strong lineaments representing either master joints or faults 3 Vithanage, P.W., 1986, Landslide damaged houses in the villages around the Kothmale reservoir project area. – Mim. Report to the Mahaweli Authority. States that there is some correlation of the orientation of the cracks observed and the orientation of the fractures (joints and lineaments) previously mapped in the Kothmale reservoir catchment area. Observed that the series of deep well developed fractures along the paddy fields and the village gardens indicate the creep of the land along the lower slopes towards the reservoir bed. This movement is probably the effect of draw down of the reservoir. STUDY AREA Kothmale Oya is one of seven major tributaries of the Mahaweli Ganga. The lowest part of the catchment lies at about 700 meters while the highest point, the Totupolakanda Peak rises to 2,380 meters. The Catchment of the Kothmale Oya can be divided in to three major regions. High level surface region 20% of the area. Altitude of about 2,200 – 2,300 meters Low undulating hills Low level surface region Altitude of about 1,400 meters. Undulating hills 40% of the catchment area. Gorge Region Steep slopes in an area of high relief. 40% of the catchment area. TOPOGRAPHY VEGETATION & LAND USE The environment of the reservoir area is sub divided in to six Ecological units. 1. The aquatic environment 2. The shores of the reservoir 3. Home gardens and paddy fields 4. Humid montane forest 5. Tea cultivations 6. Montane grasslands (Patanas) GEOLOGY Kothmale Oya river catchment area is located in the “Highland Series”. The geological structure in the region is as under: Soil and boulders Pegmatite and vein quartz Quartzite Charnockite with associated rocks and meta-sediments Crystalline limestone and calc granulites Charnockitic Gneiss and Feldspathic Gneiss SOIL The study area solely composed of Red Yellow Podzolic soil type CLIMATE Rainfall Average annual rainfall about 2000 mm to 4000 mm. Temperature The average temperature is about 25C. According to the Modified Köppen’s Classification the study area composed Afi climatic zone. An annual rainfall of more than 2500 mm. 40% - 60% of the rain is acquired through Southwest monsoon. None of the monthly rainfall is less than 50 mm Selection of the study area The Kothmale Oya Catchment Area is one of the major vibrant landscapes in Sri Lanka which experienced significant changes towards terrain, land use, hydrology and social environment during the past fifty years due to the Accelerated Mahaweli Development Program. Kothmale oya river basin has a unusual combination of geological, structural , and topographical features which give rise to a strong tendency to slope failures and underwent significant landscape changes during the past fifty years . MATERIALS - Documentary Evidence MAPS Print format of Topographical Maps of 1:63360 of Hatton (1940) and 1: 50000 of Gampola (1993), Nuwaraeliya(1994) and Badulla (1996) produced by Survey Department. Digital Data of Topographical Maps of 1: 50000 of Gampola, Nuwaraeliya and Badulla produced in 1999 by Survey Department. Geological Maps (1:10000) Forest Cover Maps MATERIALS - Documentary Evidence Remote Sensing Aerial Photographs Printed format ( 1:20000) and Scanned Images of Aerial Photographs (1: 40,000) (TIFF format in 600 dpi) produced in 1970 and 1999 by Survey Department . Others Meteorological Data – Monthly rainfall data from Kothmale sub station of the Meteorological Department Hydrological Data – Monthly incremental flow data, water yield data and stream flow measurements from the Kothmale Project Office. MATERIALS - Field Evidence Field data GPS points of previous landslides Data regarding slope (slope degree etc.) Distance from the crest Forest cover data Data required to geo reference for Aerial Photographs Data required for SINMAP mapping METHODS Produce a landslide risk map of Kothmale Catchment Area based on oral and written evidences of landslides occurred in 1947, 1957, 1958, 1970, and after the construction of the reservoir in 1986. Make an inventory of former occurrences with the reasons. Arrange the time periods according to chronological order. Compare the inventory of landslides with geological maps and forest cover maps Quantitative Analysis Terrain Stability Mapping using SINMAP (Stability Index Mapping) methodology(Tarboton et.al., 2001, Hammond et.al., 1992, Montgomery & Dietrich, 1994) SINMAP model is one of the digital models which describes terrain stability implemented in GIS environment. It will be used to assess the landslide vulnerability of the area and to divide the area in to stability classes. TIME BUDGET Types of Work Time Periods Field Research & Data Collection Dec 2004-Jan 2005 Synthesizing & Analysing Feb –March 2005 Writing the Thesis April –June 2005 END OF THE PRESENTATION THANK YOU ! PUNDALU OYA FALLS – UPPER