9. Channel Stability Assessment and Stabilization Measure of Mersa River, Ethiopia

The objective of this study is to evaluate channel stability and recommend appropriate mitigation measures for the Mersa River, in Awash River Basin, Ethiopia. The HEC-RAS5.0.7 model was used to evaluate the Mersa riverbed and bank stability, quantify the depth or mass sediment erosion amount, and identify flood-prone areas. To achieve this objective, both field investigations, such as river cross-section data collection and soil sample collection, and experimental tasks, such as sieve analysis and triaxial compression tests, were carried out. The HEC-RAS model simulation with the Yang sediment transport formula is the best fit for the study compared to the Meyer, Peter, and Muller sediment transport methods. For the entire simulation period, the average aggradation was 1.24 m and 0.98 m in the upstream and downstream reaches, respectively, whereas the average degradation in both the upstream and downstream reaches was 1.25 m. The average sediment erosion generated by the Mersa River was 22.47 kt/yr. Both aggradation and degradation were observed in the study reach, but the Mersa River reach was more affected by erosion than deposition. Mersa riverbank stability and toe erosion assessed by BSTEM of HEC-RAS were safe, and the bank toe neither aggraded nor degraded in response to the flow. Additionally, the water surface of the Mersa River was computed using steady flow analysis, showing that floods over the top above the bank and adjacent area (Mersa town) were affected by floods, and the reality was also true. It was shown that there was farmland loss and property damage due to floods. Finally, this investigation showed that the channel bed was unstable while the bank was stable. Different stabilization measures, such as the Gabion bank, check dam, and drop structure, were recommended to prevent flood-prone areas from experiencing floods and to control channel bed instability.