1.0 Introduction
Monsoons are formed because of intense low-pressure systems of tropical origin. It causes heavy precipitation and results in flooding of inlands. In 2005, all the southern states of India have received good rainfall from the northeast monsoon (NE).

The rainfall has been as high as 813.09 mm in Tamil Nadu. In Chennai city alone, the actual rainfall was 1869.7 mm, three times the normal rainfall of 701.6 mm during this period.

During the monsoon, nearly all the lowlands and plain areas were flooded. The major reasons for flooding are non-uniform distribution of precipitation both in space and time. Because of the heavy rainfall during the NE monsoon in 2005, many agricultural lands including paddy fields and the low-lying settlements were flooded.

Remote sensing and GIS have a special superiority and potentiality for disaster monitoring and assessment, especially disaster monitoring and assessment. A lot of scientific and practical achievements have been reported in this field. (Ormsby et al. 1985, Liam et al. 1995, Sharma 1996, Dhakal 2002, Sanjay et al. 2004). At present, flood situation and disaster condition can be monitored directly using remote sensing, but rainfall, discharge and water level are mainly acquired by conventional means. Mapping by using remote sensing techniques gained popularity in the present day scientific community (Harun Rasid 1990, Boni et al. 1996). Merging of different sensor data made very precise flood boundary lineation and flood damage assessment and the land cover inundated at the time of monsoon rainfall (Imhoff et al. 1987, McCarthy et al. 2003).

The image from visible band or infrared and near-infrared bands of some high resolution satellites can provide the background data, such as baseline of water body and land use, for flood and water logging disaster assessment. Flood extent maps derived from remotely sensed data are used not only for flood relief management and to develop spatially accurate hazard maps, but also provide distributed validation data for hydraulic models on fluvial flow (Horritt et al. 2001).