Geo-anthropogenic aberrations and Chennai floods: 2015, India

The flood occurred in December 2015 in Chennai, the capital city of the state of Tamil Nadu, India, is a major disaster in the history of the region as it caused the loss of 421 human lives, affected over 2 million people and the loss of properties worth of US$ 80,000 million. There were beeline of observations through media and dailies, all attributing the floods to the indiscriminate urbanization. In the context of lack of any scientific observations on the disaster which occurs often, and at the same time the Chennai region is geologically vibrant with active geodynamics viz. tectonics and tectonically induced riverine and coastal geomorphic processes, a study was undertaken involving all the features related to both geodynamics and anthropogeny to unfurl the mysteries behind the recent flood disaster. In the study, ArcGIS-based spatial databases were generated on the (1) flood inundation interpreted from post-flood MODIS satellite data, (2) geodynamic features related to earth movements, riverine/fluvial and coastal geomorphology interpreted from IRS LISS IV FCC and GeoEye satellite data, (3) anthropogenic features viz. encroached drainages, water bodies and the wetlands extracted from IRS and GeoEye satellite data and (4) integrated geo-anthropogenic data. Using these GIS databases, spatial modelling was carried out between the flood inundation data and the geodynamic, anthropogenic and geo-anthropogenic data sets which revealed that these have significantly controlled the December 2015 floods of Chennai city. The GIS-based spatial correlative study between the flood and the geodynamic features indicated that the land arching, deepening and land subsidence related to post-collision tectonics; and the tectonically tutored phenomenon of river migration and the resultant occurrence of huge system of palaeochannels and the complex drainage patterns, and the coastal geomorphic features viz. palaeobeach ridges and swales, defunct swamps and the wide tidal flats have controlled the floods. The study with anthropogenic features indicated that the unmindful obstructions of drainages, encroached water bodies and encroached segments of the tidal flats and the swamps were the loci for the floods, whereas the spatial analysis between the flood and integrated geo-anthropogenic data gave fine resolution information that the anthropogenic features depending upon the geodynamic features with which these are associated determined the floods; for example, (1) the obstructions of drainages along deflected drainages, compressed meanders and tectonic deepening, (2) encroachment of water bodies in zones of deflected drainages and tectonic subsidence and (3) encroachment of tidal flats and encroachment of Pallikaranai swamp in zones of tectonic deepening. These observations have lead to broader remedial strategies.