ANALYSIS OF THE EFFECT OF CLIMATE CHANGE ON THE FLOOD HAZARD FOR LOGIA SUB-WATERSHEDE, AWASH BASIN, ETHIOPIA

Abstract

Climate change is likely to increase both the magnitude and frequency of extreme precipitation events, which may lead to more intense and frequent river flooding. Accordingly, this study was to analyze climate change impact on flood hazard in Logia watershed, Awash Basin, Ethiopia. SDSM was used to derive local scale information from global climate scenarios generated by canESM2. Historical weather data collected were used to calibrate and validate down scaling model. After calibration and validation, the downscaling model was used for computing future temperature and precipitation pattern. Future scenario analysis was performed in two-time frames: near-term and midterm under both RCP4.5 and RCP8.5 scenarios and reference for the baseline period of 1988-2017. To estimate the future stream flow of the Logia watershed, the HEC-HMS model was adopted. The historical daily stream flow and precipitation of the Logia watershed were collected to calibrate HEC-HMS model for the period of 1988-2017. For model performance evaluation; Coefficient of determination (R²) and Nash-Sutcliffe Efficiency (NSE) were used during calibration and validation of stream flow. The result of calibration and validation of stream flow R² and NSE were 0.74, 0.72, and 0.77 and 0.74 respectively. HEC-RAS and HEC-GeoRAS models were used to simulate a possible flood event in baseline period, 2020s, and 2050s due to climate change. As a pre-processing stage, a topographic data has been surveyed from the field. Basic geometric data like river cross-section data and the peak discharge calculated in the HEC-HMS model were used as input for the HEC-RAS model, to simulate the water surface profile in the river. The result obtained in HEC-RAS was exported to ArcGIS extension HEC-GeoRAS for post processing. The resulting flood depth of the simulation was used to prepare inundation maps and to analyses flood hazard in the Logia watershed, as well. Based on the result obtained from the future scenarios, down scaled precipitation and temperature were projected to be increased by the 2020s and 2050s for both RCP4.5 and RCP8.5 scenarios. For 50 year return period, the flood hazard map of base period indicates that 336 ha, 482.2 ha, 16.48 ha, and 10.88 ha which correspond to very high, high, moderate and low flood hazard, respectively. The flood hazard map at future showed that approximately 1204.3–1361.9 ha area is expected to be flooded under future climate change scenarios of which significant areas are under agricultural and residential areas. The intensity of annual floods is expected to increase for both RCP 4.5 and 8.5 scenarios. The study investigates that due to the combined effect of projected variation in rainfall and temperature there will be increment of flood hazard in the Logia sub - watershed.