FLOOD MODELING AND RISK MAPPING: (CASE OF KULFO RIVER IN SOUTHERN ETHIOPIA REGION

Abstract

Floods pose significant threats globally, causing immense damage to lives, societies, and economies. This study aimed to assess flood hazards, evaluate vulnerabilities, and determine flood risk along the Kulifo River floodplain. To achieve the objectives of a study, advanced hydrological and hydraulic modeling techniques were analyzed, using data from various sources, including rainfall from the National Meteorological Agency, stream flow data from the Ministry of Water Resources, and land use/land cover data from USGS. The HEC-HMS model accurately calibrated and validated using observed stream flow data, the result of model calibration gives Nash Sutcliffe efficiency (NSE) of 0.81, Percent Bias (PBIAS) of 1.77, coefficient of determination (R2 ) of 0.77, and Relative Mean Square Error (RMSE) of 4.28. During the validation period, the model gives (R2 ) of 0.79, NSE of 0.78, PBIAS of 1.09, and RMSE of 2.13. After model calibration and validation, flood hydrographs for different return periods were generated. These hydrographs served as inputs for the HEC-RAS hydraulic model, integrated with GIS software to map flood inundation areas. The resulting flood inundation maps revealed extensive flood-prone areas along the Kulifo River, with maximum flood depths of 15.2 meters and maximum velocities of 6.9 m/s during a 100-year flood event. Flood hazard maps classified areas into different hazard categories from low to extreme hazard, and 59% of inundated area falling under extreme, very high, and high hazard levels, 41% of inundated area falling under medium, and low hazard levels. Vulnerability analysis considered indicators such as flood depth, velocity, duration, slope, land use, and population density, highlighting 25% of the flooded area as very high and high vulnerability, 20% of the flooded area as moderate vulnerability and 55% of the flooded area as low and very low vulnerability. Combining flood hazard and vulnerability information, a comprehensive flood risk map was developed, identifying 32% of the flooded area as very high and high risk, 15% of the flooded area as moderate risk and 55% of the flooded area as low and very low risk. These high risk zones were concentrated in the Limat area of Arba Minch city, emphasizing the need for mitigation measures and emergency response plans. The flood risk map provided valuable insights for decision-making processes, guiding the implementation of structural and non-structural measures, floodplain zoning, and population relocation. This study's findings contribute to effective flood management, land-use planning, and disaster risk reduction strategies along the Kulifo River