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Flooding potential zonation using conditional probabilities – A case study on Mumbai’s monsoonal flooding
H. P. Samant
Geology Department, St. Xavier’s College
Mumbai, India
Email: hsamant@gmail.com
ABSTRACT
After the tragic events of large scale flooding due to torrential rains in Mumbai on 26th –28th August 2005, which resulted in the death of more than 1000 persons and an estimated loss to the economy of more than 1000 crores, it was imperative to prepare a ‘flooding potential zonation map ’ for Mumbai.
About 40% of the total land in Mumbai city is low lying marshland that has been reclaimed in the last 200 years. The reclamation has been done without regard to the natural topography and drainage (stream network) of the city. The pattern of infrastructure development in Mumbai is also without regard to the geomorphology. This has resulted in the occurrence of intensive localised monsoonal flooding in more than 30 areas in the metropolitan city. The flooding is further aggravated due to tidal surge in the creeks if rainfall duration and intensity coincides with the rainfall duration. The cause of such localised flooding with regard to the type of human activity and the natural topographic factors has been investigated. The monsoonal flooding was studied by integrating multi-thematic data in a GIS environment. The maps depicting the geology, geomorphology, drainage network, drainage density, slope, and land use were overlaid on the map depicting the flooding locations and the observations were used in a semi-quantitative analysis for generating a flooding potential map for Mumbai and the neighbouring Navi-Mumbai area. The weightages for the different factors were calculated using prior probabilities of flooding, and a flooding potential map was generated. This map can be used for exercising precautions right at the stage of land development and also to plan the flood water disposal systems for the present as well as prospective problematic areas.
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