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Hazards and
Hazard Maps
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The main hydro-meteorological
hazards affecting Sri Lanka are drought,
flood, landslides and cyclones. Although Sri Lanka has few records of
previous seismic hazards, the Tsunami of 26th December has shown that
there is indeed seismic hazard risk, which warrants further research.
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Drought is
the most significant hazard in Sri Lanka in terms of people affected
and relief provided (excl. The relief disbursements for drought between
1950 and 1985 were SL Rupees 89 million (approximately 1 million US$)
whereas floods accounted for only SL Rupees 7.5 million.
The prevalence of drought maybe surprising given that Sri Lanka
receives on average 1,800 mm of rainfall annually. However, it is
distributed unevenly spatially
and temporally.
A large part of the island is drought-prone from February to April and
on to September if the subsidiary rainy season from May to June is
dry.
There is a
stronger tendency to drought in the South-Eastern district of
Hambantota and the North-Western region of Mannar and Puttalam. The
drought tendency is markedly less in the South-West corner of Sri Lanka
where there is heavy rainfall.
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Heavy
rainfall in the Eastern and South-Western slopes is a principal cause
of the flood risk. In addition, the drainage and topography of certain
districts and land use patterns are also significant factors. For
example, the Districts of Kegalle and Ratnapura have people settled in
flood plains and steep hill-slopes. Flooding has now become an annual
occurence.
The Western slopes receive rainfall in both Maha (September to January)
and Yala (May-August) seasons, and is prone to flooding in these
periods. The Eastern slopes receive most of the rainfall during the
Maha season (September to January). This is also the cyclone and storm
season that can bring heavy rainfall in short time periods. Thus the
two regions show distinct flood
seasonality.
The District of Vavuniya shows a higher flood probability due to
cyclonic storms. Even though the annual rainfall is lower than the
Western highlands, Vavuniya and Mullaitivu in the North have recorded
the highest rainfall intensities in the island.
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Eight
districts in the central highlands are at risk. The highest risk is in
the Kegalle District followed by Ratnapura and Nuwara Eliya Districts.
Even within these Districts, there is spatial
variability at Divisional
Secretariat level. Kalutara, Kandy and Badulla Districts have moderate
risk and Matale and Kurunegala Districts have slight risk.
The frequency of landslides has increased in the recent years. Changes
in land use including cultivation of tobacco on steep slopes, land
clearing in the hills, blocking of drainage ways, and the impact of the
large reservoir construction may because of the increase.
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Cyclones and
storms have made landfall only in the Eastern coast of Sri Lanka except
for a single storm in 1967. The majority of cyclones and storms pass
through the Northern and North-Central parts of the island. There have
been four severe cyclones during the last 100 years as well as a number
of severe and moderate storms. The cyclones that pass through Sri Lanka
originate from the Bay of Bengal during the North-East monsoon which
develops in November and lasts a few months. Cyclone incidence shows
a strong seasonality,
and 80% of all cyclones and storms occur in November and December.
Incidences of cyclones that pass through Sri Lanka in other seasons is
rare due to geography and the regional climatology.
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Sri Lanka is
not located near any of the 12 or 13 main plate boundaries that are
prone to earthquakes. Indeed, it was squarely on a plate that extended
from Australia to India. In the last decades, however this plate is
beginning to rotate on account of accumulation of run off from the
Himalayas in the Bay of Bengal and other reasons leading to a fissure
between the "Australian" and "Indian" plates. Some scientists believer
that this is leading to a new plate boundary across the Southern Indian
Ocean. This boundary is still approximately a 1000 km from the south of
Sri Lanka. Yet, these shifts have to be more carefully monitored and
seismological studies need to be carefully followed. The Geological Survey and
Mining Bureau is the government
agency entrusted with seismological studies and it hosts a seismic
station at Pallekelle in the
Kandy District which is part of the global network of seismographs.
Further research is needed to elucidate the consequences of
compressions set up in the India plate and the impact of the recent
earthquake on the regional hazards and also more precisely estimate the
probabilities of the risk of earthquakes closer to Sri Lanka.
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While Sri
Lanka is far away from the plate boundaries, yet it is close enough to
the highly active seismic zone near Sumatra and other regions to its
South-East that earthquakes generated in these region may lead to a
Tsunami Hazard in Sri Lanka. Tsunamis are rarer in the Indian Ocean as
the seismic activity is much less than in the Pacific. Tsunami's are
extremely infrequent - the last major volcanic explosion in the
Indonesian island of Krakatau led to a Tsunami in Sri Lanka in August
of 1883. The wave heights that resulted however were much smaller than
the 2004 Tsunami. While earthquakes could not be predicted in advance,
once the earthquake is detected it is possible to about an hours notice
of a potential Tsunami for every 500 km distance from the epicenter.
Such a system of warnings is in place across the Pacific Ocean. Once
the large amount of pent-up energy in the compression zones of the
plate boundaries have been released, it takes another buildup of energy
for another event of similar magnitude. Thus another Tsunami at the
same location is unlikely in the short-term from the same epicenter. In
the future, Indian Ocean littoral regions should generate and pay
attention to earthquake and tsunami warnings.
Tsunamis are likely to have a more modest impact on the coastal zone
from Killinochchi to Puttalam as the Tsunami wave shall not build into
a wall due to the presence of neighbouring India.
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