Detection of Flood affected areas of Kalutara District of Sri Lanka by ...

Detection of Flood affected areas of Kalutara District
of Sri Lanka by Using PALSAR Remote Sensing and
GIS Technology
Ajith Rohana Gunawardena
Central Environmental Authority, Sri Lanka
ajith@cea.lk
Abstract
Floods and droughts are more frequently experienced natural hazards in Sri Lanka These
hazards have a strong impact on the country’s socio-economic environment. The government
spends large amounts of money annually to provide relief to families affected by natural
hazards. Flood hazard ranks high among natural hazards in Sri Lanka. Comparing with the other
districts of the country, Kalutara district severely subjects to floods. The floods in May 2003
recorded highest damage since 1992. The heavy rainfall caused severe impact on lives and
economic assets of the people. The floods occurred in June 2008 also was serious after the
floods in May 2003.
Kalutara district is subjected to regular flooding particularly with heavy rains in the upper
catchments areas of the Kaluganga, damaging houses, urban centers schools the roads network,
home gardens, crops, industries and etc. As such inhabitants of this district have been badly
affected by frequent floods. Their livelihood and social life have been disturbed. So far, no
specific adaptation or mitigatory measures have been taken in this regard.
Keywords: floods, droughts

1. Detection of flood affected area and development of
flood map
To having a flood affected and vulnerability area map is very important tool to environmental
management as well as after flood management practices. However, development of such map
by field work is very difficult and time consuming task .To overcome above limitations,
RADAR satellite images and GIS technology can be used to develop flood affected area map.
2. Objective
The main objective of this study are to prepare a flood map by using PALSAR (Phase Array L
band Synthetic Aperture Radar) Remote Sensing and GIS Technology and flood risk map for
Kalutara district overlaying with other map features such as road net work, land uses,
population, transportation, schools, industries and etc. and explore impact on people who live
there .
Once this flood risk map is produced it could be utilized for planning sustainable project,
identification of vulnerable people and resources in the flooding areas and also for preparation
of disaster preparedness plan. The Central Environmental Authority (CEA) as a regulatory
authority concerned with conservation and management of the environment in Sri Lanka
3. Methodology
ALOS PALSAR 1.5 satellite images taken on 03, June.2008 (peak flood period) and on 14
Aprial,2008 ( Non flood day) were used to prepare flood map. According to the situation
report- 2008 of Disaster Management Center, Kalutara was one of the most effected districts in
the Western province due to torrential rain resulting from the South West monsoon and it has
been experiencing flood situation since 30th May 2008.
Acquired images on 03 rd , June were geometrically rectified into UTM (Universal Transverse
Mercator) projection and datum WGS 84 (zone 44) using ground control points obtained
through GPS together with 1: 50,000 topographic maps. Then the image was subjected to
unsupervised classification to obtain 10 different classes According to the color variability flood
affected layer was identify by visual interpolation and developed a flood affected area layer on
the map. Previously collected GPS locations were used to check the accuracy of flooded area in
the map.
Same procedure was followed for the images taken on non-flood day (14 th ,April) to produce
existing water layer (water bodies and ways) and it was overlaid with the flood layer and
extracted actual flood affected area to developed the final flood map.The produced flood map
was overlaid with other relevant map layers such as administrative boundaries, road net work,
and school. Industries, land uses and etc. to explore the impacts on people due to flood.

4. Data and material and software used
ALOS PALSAR 1.5 satellite images, 1:50,000 topographic maps, ArcGIS, ERDAS, Remote 10,
Adobe Photoshop software and GPS data were used to this project analysis.
5. Results and discussion
It is estimated that 7.28 % of the total land area of the Kalutara district has been inundated
during the flood occurred in June, 2008. When contour layer is super imposed on flood map
prepared, it is shown clearly that low lying area of the study area has been inundated and most
of these lands are located in closed proximity to the coastal area. Generally majority of the lands
of the Kalutara district are high elevated ones. Therefore, severity of the flood may be much
higher in these low lying areas and also comparing with the flood damages in the other districts
of Sri Lanka. Highest percentage of flooded area covered by paddy fields (about 52.67 % ) and
followed by Rubber (25.34%) , Home gardens ( 15.8% ) and it can be observed that some
where of all class A and B roads and minor road of the district have been submerged by flood
water. Therefore, the accesses to most of the non-inundated areas have also been affected.
Figure 1: Flood affected area map of Kalutara district developed using PALSAR remote sensing
technology

6. Conclusion
The flooding map made by analyzing the satellite image taken during the real flood time and
the flood risk map prepared by overlaying with other relevant map layers create very
comprehensive and accurate information / data with less effort and less time . The flood risk
map shows actual situation of the area , land uses inundated, affected industries, schools, road
net work, important eco-systems and etc. during the flood.
Finally, Considering the usefulness of these data / information derived under this study , it is
recommended that this type of GIS based studies should be extended to the other part of the
country to identify the real effect of flood as well as other disasters , impacts on people and to
prepare adaptation/mitigation and rescue plans.