Large Scale Urban Development as A Precursor to Floods in Karachi, An Analysis Based on Historical and Modern Spatial Records

Abstract

Urban floods are a major problem in mega cities, especially in developing countries with rapid and uncontrolled urban development as one of major contributing factors. Flood analysis is necessary to ensure public safety, minimize property damage, protect the environment, and manage financial risks associated with flooding. This study aimed at analyzing large scale urban development as a precursor to floods in Karachi. Temporal analysis of urban development in Karachi City was carried out using historical and modern spatial records. Topographic records of 1944 and 1980 and Landsat imagery of 2000, 2008, 2015 and 2020 were used to map urban extent. Sentinel-1 Synthetic Aperture Radar (SAR) imagery was used to extract flooded areas during 2015, 2017, 2019 and 2020 floods using a global thresholding approach. Results indicate that urban extent is continuously increasing with major increase in urban development observed between 1980-2000 of 286%. Flooded areas extracted by SAR imagery shows that year 2020 experienced maximum flooded area of 64.3 km2 as compared to previous years. In all flood years, core urban areas were most affected by floods. Urban extent was overlaid with streams network of 1944, and it indicated that that out of 3600 km of streams, 870 km of streams have either been blocked or removed. This makes almost 100% in core urban area. Using Google Earth, informal settlements were digitized. The locations of these informal settlements were overlaid with flooded areas which indicated that these informal settlements were at maximum risk in all floods. The study concludes heavy rainfall has led to inundation of core urban areas of Karachi City. This inundation is caused by rapid urbanization, blockage of drainage system of Karachi and existence of informal settlements along major rivers and nullahs of the city.