Climate Change Impact on Koka Dam Safety, Ethiopia

Abstract

Climate change is currently having significant global impacts, affecting both living and nonliving entities. One of the prominent consequences of climate change is the increasing occurrence of floods, which pose a threat to various man-made structures including dams, roads, buildings, bridges, and others. This study focuses on assessing the safety of Koka Dam to climate change, employing future projection scenarios of RCP4.5 and RCP8.5. To achieve this, the distributed hydrological SWAT Model was utilized with input from three distinct Models (CanESM2, CNRM, and MPI) operating under RCP4.5 and RCP8.5 scenarios. As a result, the RCM projection suggests that the average annual maximum precipitation is expected to rise by 34.77%, while the mean annual maximum precipitation is predicted to decline by -22.38%. Additionally, the mean annual maximum and minimum precipitation are projected to increase by 7.2??C and 1.5??C, respectively. The accuracy of the model is assessed based on various indicators, including the R2, NSE, and Pbias. The calibration and validation results suggest that the model performed very good and good at both Hombole and Mojo gauging stations, with a good level of correlation and accuracy between the observed and simulated data. In the case of the RCP4.5 scenarios using HEC HMS, the peak inflow and outflow values for the near, mid, and end-future periods are 7752.03 m3/s, 7240.78 m3/s, and 6510.68 m3/s, and 1354.3 m3/s, 1316.2 m3/s, and 1252.5 m3/s, respectively. Similarly, under the RCP8.5 scenario, the peak inflow values are 10456.6 m3/s, 7224.78 m3/s, and 8000.48 m3/s, respectively. The corresponding outflow values are 1505.9 m3/s, 1315 m3/s, and 1372.3 m3/s. According to the routing analysis, the maximum inflow and outflow during the near future, mid-future, and end future under RCP4.5 and RCP8.5 scenarios exceed the probable maximum flood that the spillway at the Koka Dam was designed to handle. This implies that the dam will be at risk of overtopping. In summary, the results suggest that due to the insufficient capacity of the spillway, the Koka Dam would be considered unsafe and vulnerable to overtopping under the projected outflow.