Catastrophic flood of August 2018, Kerala, India: Study of partitioning role of lineaments in modulating flood level using remote sensing data

India's Kerala state, sandwiched between the Arabian Sea and the Western Ghats, witnessed a catastrophic flood during the southwest monsoon in mid-August 2018. Unusual precipitation (24% in excess of the normal) coupled with opening of flood gates of dams in the highland and high tide level in the coastal plains were the drivers of the flood. Synthetic Aperture Radar (SAR) data acquired from multiple satellites were used to demarcate area inundated by flood, which covered over 521 km2 in the coastal lowlands. Flood waters receded rather abruptly during the initial days between 18 August and August 21, 2018 where the flooded area dropped from 521 to 395 km2 and water levels dropped from 10 m to 5 m. But subsequent level fall was tremendously slow, taking 40 days to reach ~50% or 260 km2 on September 26, 2018. The chaotic distribution and discharge of flood water, together with structural controls of the coastal lowland, initiated the investigation to examine whether or not geologic lineaments of the basement terrain was involved in modulating the location and discharge of flood waters. The investigation resulted in identifying two scenarios around the lineaments: (1) pooling of flood water over extended periods and (2) rapid discharge of flood water, named as pooling and discharging lineaments, respectively. Flood water recession, for the 40 days, through discharging lineaments was 87.15% when compared to the low rate of recession through pooling lineaments (33.07%). Moreover, morphology-wise discharging lineaments are characterized by high relative relief whereas pooling lineaments were depressions. Results suggest, other than lineament's role in aiding water percolation, lineaments can also act as conduits for rapid discharge.