Flash floods in Malir basin due to urbanization

Karachi is a strategic city of Pakistan since it serves as the gateway for international trade through the country's oldest port. This city has faced rapid population increase and industrial growth which are now causing environmental issues, social problems and economic stresses and challenges. Urban development and industrialization have extensively changed the landuse/landcover (LULC) and drainage pattern of the city. Majority of the old streams have been blocked that caused localized flash flooding in the city after heavy rainfall events. The natural drainage network of Karachi consists of two (2) major river systems-Malir River on the eastern side of the city and Lyari River which passes through the center of the city draining majority of its storm water. These rivers and their tributaries have been affected very badly due to the urbanization and bad urban management. This paper discusses the changes in LULCs over the past 16 years selecting Malir basin as a case study. Hydraulic flood modeling of Malir River is also presented here for 50 and 100 years floods. SRTM (Shuttle Radar Topographic Mission) Digital Elevation Model (1 Arc Second) and topographic sheets of 1975 and 2001 (1:50000) are used to demarcate natural drainage or Karachi District on digital maps and to delineate stream network using Geographical Information System (GIS) software tool Arc Hydro. Historical rainfall data are used to simulate runoff using hydrologic models HEC-HMS and GeoHMS. The flood extent is modeled using hydraulic models HEC-RAS and GeoRAS. GIS overlay analysis combines LULCs and river drainage layers to identify the blocked area and to quantify the specific LULCs that caused the river blockades. Maps of blockages and flood extent can be used for revival of natural drainage network and flood mitigation and preparedness. The beneficiaries of this study may include local disaster risk management, planning and development authorities.