Flood Inundation Modeling for Malir Watershed of Karachi Considering Future Mean Sea Level Rise

Abstract

Karachi is the one of the largest coastal city of Asia & impacts of sea water level rise are worth studying for betterment of the inhabitants. It is the largest metropolitan of Pakistan with population over 16 million that is expected to reach 27 million by the year 2020. Heavy rainfalls of a high intensity are experienced on occasions with the average annual rainfall varying from 125-250 mm. The urban population along the coast line varies from 10 to more than 7000 persons/km2 which is vulnerable to tropical cyclones as well as resulting floods. There is no fixed cycle for the occurrence of these occasional heavy down-pours. They are dependent upon the local atmospheric disturbances in the sea which occurs from time to time. Rain-storms, cyclones, floods and tidal bores are all natural phenomenon and their recurrence cannot be ruled out. Such natural hazards are responsible for the loss of millions of lives and damages to the properties. Man must therefore, learn to plan and build safely to counteract the devastation which can be caused by these forces of nature. This paper describes the simulation for the extent and depth of flooding in some portion of the Karachi city lying in the catchment area of Malir River due to rainfall event of the year 2007, change in the mean sea level at Karachi coast for the years 2025, 2050, 2075 & 2100 in accordance with the report of Intergovernmental Panel on Climate Change (IPCC) and due to rise of 6m in mean sea level and its backward flow through the estuary of Malir River. A distributed physically based hydrological model named "Institute of Industrial Sciences Distributed Hydrological Model" (IISDHM) was employed for the simulation of flooding. For the flood inundation modeling of a part of Karachi city lying in the catchment area of Malir River, a watershed having 1690 km2 area was delineated. The calibration and validation of the model was carried out for the rainstorms of the year 2007 & 1977 and the maximum simulated flood inundation depths were computed as 2.89 m & 4.41 m on Super Highway Bridge and Haji Shah Ali Goth, respectively. Flood inundation maps for the years 2007, 2025, 2050, 2075 and 2100 were prepared. The results for the future scenario reveal that mean sea level rise in next hundred years will not cause any increase in the flood inundation depths within the simulated watershed boundary of Malir River. However the maximum simulated flood inundation depth for the scenario of 6m rise in mean sea level was computed as 4.41m.