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
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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
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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
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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).
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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?
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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 ?
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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
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
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
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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
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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
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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.
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High level surface region
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20% of the area.
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Altitude of about 2,200 – 2,300
meters
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Low undulating hills
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Low level surface region
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Altitude of about 1,400 meters.
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Undulating hills
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40% of the catchment area.
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Gorge Region
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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)
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GEOLOGY
Kothmale Oya river catchment area is located in the “Highland
Series”.
The geological structure in the region is as under:
Soil and boulders
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Pegmatite and vein quartz
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Quartzite
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Charnockite with associated rocks and meta-sediments
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Crystalline limestone and calc granulites
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Charnockitic Gneiss and Feldspathic Gneiss
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SOIL
The study area solely composed of Red Yellow Podzolic soil type
CLIMATE
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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.
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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
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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
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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.
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Geological Maps (1:10000)
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Forest Cover Maps
MATERIALS - Documentary Evidence
Remote Sensing  Aerial Photographs
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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 
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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
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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
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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
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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