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Disasters
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Monitoring and forecast of disasters in bangladesh using remote sensing technology
Protection Against Cyclones:
What can be done to protect ourselves from the cyclones ? A cyclone, is a natural phenomenon like an earthquake or a volcanic eruption. We have to learn to live with it. We have to strengthen the cyclone warning system and adopt protective and relief measure to minimize their onslaught. Strongly built houses have to be constructed high above the sea level to serve as shelter places. people from the low lying areas in the coastal region can be evacuated into these shelters in the event of a cyclonic hit. Coastal embankments have to be made to protect life and property from the onslaught of storm surges. Plantation of trees along the coastal area can also diminish the fury of the storm surges. But above all, cyclone warning system should be improved and warning lead time be increased as much as possible.
Floods
The primary cause of flood in Bangladesh is rainfall in the catchment areas of the rivers of Bangladesh. Situated in the monsoon belt with the Himalayas in the north , Bangladesh falls in the region of very heavy rainfall. About 80 percent of the rainfall occurs during the 5 month period from May to September. The annual rainfall varies from about, 60 inches in the western part of the country to about 200 inches in the north eastern part. At Cherapunjee in Assam very near our Sylhet Border the average annual rainfall is about 500 inches which is highest in the world. But the average rainfall in Bangladesh generates annually only 100 million acre feet of water whereas 1100 million acre feet of water comes from outside Bangladesh. Thus about 90 percent of the water carried by our river system, the Brahmapurtra, the Ganges, The Meghna and other smaller rivers is brought from outside the country. These revers carry water from an areas of about 600, 000 sq miles of which only 7.5 percent lies in Bangladesh. Water enters in Bangladesh through three major channels but the discharge takes place through one major channel. The river system has evolved to carry the normal flow of water generated in the catchment area. Whenever the inflow of water is greater than the carrying capacity of the rivers (and this happens very often ) flood results. The magnitude of the flood depends on the magnitude of excess water that is generated.
Besides the primary cause , namely rainfall in the catchment area, there are other factors which may aggravate the floods.
They are:
- Snow melting in the Himalayas.
- Hydrographic changes in the Brahmaputra basin.
- 2.4 billion tons of sediments carried by the river system of Bangladesh every year reduces the water carrying capacity of the rivers, which worsens the flood.
- Deforestation in the catchment area tends to aggravate the flood.
- Construction of unplanned roads, railways, barrages, embankments etc. also create obstacles to the flow of water and aggravate the flood.
Flood Area Monitoring by Satellites
If the sky is cloud free, NOAA satellite imagery can be processed with computer to delineate the flooded areas . This was done for August 31, September 01, September 10, September 15, September 18 and September 24 for the year 1988. It was estimated that about 30,000 sq. miles of Bangladesh was inundated in 1988.
Flood Forecasting and Warning:
Bangladesh Govt. has taken up both the structural and non-structural measures of flood mitigation. In this respect regional and international cooperation has been sought. A Flood Action Plan with 26 components with the assistance of World Bank has been undertaken. Structural measures of flood control like storage reservoirs, embankments or levees, channel improvements and bypasses or floodways are costly and time consuming. For immediate benefit to public, non structural measures are accomplished at a much smaller cost and time. For flood forecasting a network of hydrological stations connected with telemetering gauses or by telecommunication or teleprinter links with the forecasting centre has been established by the water Development Board (6) . Available hydrological data consist of discharge, water level and rainfall records. Historical records of data have been analysed to prepare forecasting procedure. For major rivers correlating of water levels or discharges between upstream and downstream stations are utilised in preparing forecasting procedure. For revers with smaller catchments rainfall-runoff relation, flood correlation methods are used Extensive modelling of flood in our river system is necessary for effective forecasting. Rainfall estimation can be made using satellite imagery and this can be used as input in these models.
References
- A.M. Choudhury: Cyclone in Bangladesh, Bangladesh Quarterly, Vol. 12, No 1, PP 7-12, September 1991.
- A.M. Choudhury: Flood - 1988 as seen by Satellite , Bangladesh Quest, Vol. 1, No 1, PP., 52-54, Dhaka 1969.
- W.M. Gray: Global view of the origin of tropical disturbances and storms, Mon. Wea. Rev., 96, 669-700, 1968.
- V.F. Dvorak: Tropical Cyclone Intensity Analysis Using Satellite Date, NOAA Technical Report NESDIS 11, Washington D.C. 1984.
- A.M. Choudhury: Cyclone Forecasting, Proc. Of the workshop on Living with Cyclones - Bangladesh land Reclamation Project, Dhaka, 1991.
- M. Alam Miah: Flood Monioring and Forecasting in Bangladesh: Presented at the Seminar on Application of Remote Sensing to Flood Plain Mapping and Flood Monitoring, SPARRSO, Dhaka 1989.
Figure 1 Picture of the deadly cyclone of 29 April, 1991.
Figure 2 Flooded area shown in the NOAA-10 imagery taken at SPARRSO on 15.9.88.
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