Flood Hazard Mapping Using Integrated Multi-Parametric AHP-GIS and SFFM: Case Study On Adama City

Abstract

Flooding is one of the most common natural disasters, often with disastrous consequences, affecting 170 million people worldwide each year. East Africa has been facing flooding problems due to heavy rainfall. Around 2.5 million people, just in Ethiopia, Sudan, and South Sudan, have been defenseless against the flood water. At least 1.3 million people have been affected by flooding; 481,000 have been relocated; and 360 individuals have lost their lives as a result of devastating floods in the region. Adama City has experienced flooding problems that have led to the displacement of communities in some areas of the sub-city, destroyed different infrastructures, washed away houses, and affected people and the city’s economy. The main focus of the study was mapping the flood hazard and computing flood inundation depth in Adama City using integrated multi-parametric Analytical Hierarchy Process (AHP) with Geographical Information System (GIS) and the Super-Fast Flood Model. To achieve these objectives, the AHP-GIS model was employed with the methodology of collecting hydrological, meteorological, and geophysical data. Furthermore, residents and different experts were interviewed to make judgments among flood causative factors and validate the model's results. Eight flood causative factors such as rainfall, slope, elevation, land use land cover, road network, drainage density, storm drains, and soil type were utilized to generate a flood hazard map of the City. The super-fast flood model simulated flood inundation depth for three cases, such as the past flood event of April 18, 2000, rainfall intensity (35.5 mm/hr) and rainfall intensity of both Indian Meteorological Department (IMD) 55.6 mm/hr and Ethiopian Road Authority (ERA) 78 mm/hr of the 50-year return period for a 0.5-hour storm duration. The generated flood hazard map indicated that Irrecaha, Migira, and some parts of Gara Lugo Kebeles were at high risk of flooding. For past flood event simulations, flood depth was in the range of 0 to 1.8 meters. A 0–1.95 meter and 0–2.3 meter flood inundation depths were obtained from 50-year simulation scenarios of the IMD and ERA methods of Rainfall Intensity (RFI), respectively. The result inferred that the maximum flood depth appeared in Irrecha and Migira kebeles. The obtained model outputs were validated based on experts and local resident’s interviews, as there was no single stream flow gauge station in the city and limited satellite products capturing water extent during rainfall events. The results of the study revealed that poor drainage networks in very low-elevation areas aggravated the flood hazard in the city. This study will be helpful in flood protection, planning and construction of any projects along the city’s flood hazard area, and planning effective emergency responses for flood hazard areas in the city. Furthermore, the developed IDF curve will be helpful in planning and designing the drainage system of the city.