ESTIMATING THE EXTENT OF FLOODS IN BANGLADESH USING SPOT DATA

Nine SPOT XS scenes recorded during the dry season (January, February) and after cyclonic storms were analyzed to evaluate both the extent and consequences of floods in Bangladesh, in the deltaic complex of the Ganges. Digital and visual analysis showed the following results: 1) Near the capital, Dahka, within a SPOT scene (3600 km2), the extent of water varies from 324 km2 during the dry season to 1303 km2 after a destructive flood. This means that flood-prone areas in this overpopulated deltaic region are at least four times larger than the hydrographic network observed during the dry season. 2) The Sunderbans are the world's largest mangroves (about 6000 km2 in a single block); two-thirds of this forest (4050 km2) are in Bangladesh, and the rest is in India. These ecosystems seem to resist cyclones. No sign of any kind of mangrove alteration is seen on SPOT images recorded a few weeks after disastrous cyclonic storms (20 August 1987 and 6 September 1988). This is an important fact since the protective role of these coastal swamp forests is tremendously important. 3) The islands located in the very exposed area at the mouth of the Meghna are densely populated (more than 1000 inhabitants/km2). Hathya island has been carefully surveyed. According to the investigations carried out on SPOT images (25 February 1987 and 26 October 1988), the island seems to be at present satisfactorily protected against cyclones and tidal surges, behind newly planted mangroves (Sonneratia apetala Buch.-Ham.) and sheltered by a complex network of embankments which are visible on the images. It was concluded that the spectral separability of classes (water, natural plant communities, land use suitable for coastal monitoring in this part of the Bangladesh.