Analysing Hydro-meteorological Scenario During 2015 Chennai Flood Event And Possible Role of Urbanization

Abstract

Chennai stood still by the extreme rainfall event that occurred in November-December 2015. It received ~300% excessive rainfall from the mean observed amount of 407.4 mm for the month of November. The record-breaking rainfall created history by exceeding the 24-hour rainfall (290 mm) on 1st and 2ndDecember. For analysing this event series, a multi fold observational and modelling approach is adopted in this study. Simulation of the whole event is performed in six experiments using Noah based land surface models with and without multi-physics options and by considering three different types of land use (LU) data to understand the urbanization impact. The land use and land cover (LULC) change analysis from the year 1991 to 2015 is done using satellite images from LANDSAT and IRS collected from BHUVAN data portal of ISRO. The LULC change suggested an urban growth of ~62% and ~74% with respect to the base years of 1991 (LANDSAT) and 2004-05 (BHUVAN) respectively. Study of meteorological parameters from observational as well as re-analysis datasets show that the episodic rainfall event is governed by three synoptic scale systems, which transported significant amount of moisture from the neighbouring Ocean causing heavy rainfall throughout the Chennai city, Tamil Nadu and southern Andhra Pradesh. The modelling results reasonably agree with the GPM observations in a qualitative manner. The role of LU and increased urbanization is quite evident in the model derived spatially distributed rainfall. The flood scenario is governed by the higher surface runoff values; well simulated by the consideration of multi-physics option, which is proved more sensitive to the urban LU. The model simulation with ISRO 2012-13 LU found to be simulating relatively higher rainfall and surface runoff values when considered with Noah multi-physics option and indicate possible role of urbanization in governing local hydrological and hydrometeorological processes.