Assam represents highly flood-prone region characterized by severe hazards of floods. Although occurrence of flood has been an age-old phenomenon in the riverine areas of this region, the extent of damage caused by the flood has increased significantly in recent years. With more than 40 percent of its land surface susceptible to flood damage, the total flood-prone area in the Brahmaputra valley is about 3.2 Mha. (Goswami, 2001). The Brahmaputra valley had experienced major floods in 1954, 1962, 1966, 1972, 1974, 1978, 1983, 1986, 1988, 1996, 1998, 2000 and 2004.
National remote Sensing Centre (NRSC), Hyderabad under Deptt.of Space in association with Assam State Disaster Management Authority (ASDMA) has prepared the district wise flood hazard maps for Assam State using satellite remote sensing data sets. Satellite data due to their synoptivity and frequent coverage of flood affected areas can be directly used for deriving the flood inundation limits.
About 93 satellite datasets acquired from large number of Indian Remote Sensing satellites and foreign satellite datasets, spanning over 10 years from 1998 to 2007, acquired during different flood magnitudes in Assam have been used to create the flood hazard maps for different districts of Assam. All satellite data sets were analyzed and flood layers were extracted. All the flood layers corresponding to a year are combined as one inundation layer so that this layer represents maximum flooded area in one year. All such combined flood layers for 10 years were integrated into flood hazard layer representing the observed flood inundated areas with different frequencies.
The flood inundation represented in different colours indicates varying frequencies as observed during 1998 to 2007. The road and railway lines are shown to indicate the probable frequency of flooding they are subjected to. The normal river course and water bodies are also shown in the map.
Similarly, the layer was also integrated with digital database layers of different districts, these layers include road, rail, village, etc.
The flood hazard zones are categorized into five classes ranging from very low hazard zone to very high hazard zones. Very Low category indicates the areas, which are inundated once or twice during the 10-year period. Similarly, Low indicates three to four times, Moderate indicates five to six times, High indicates seven to eight times and Very High indicates nine to ten times.
Table: Flood Hazard Classification
||Flood Hazard Classification
||Number of times / years the area was subjected to flood inundation during 1998-2007
||9-10 times(almost every year)