A socio hydrological approach toward riverine flood alleviation for river Manafwa:

Abstract

River Manafwa sub-catchment tends to experience dramatic flooding events, the worst being the most recent one which occurred May 17th, 2019. Its frequent flooding is attributed to the current climatic changes characterized by intense precipitation, unsustainable human activities in the upstream of the catchment sedimentation of the river course resulting into reduction in River capacity. This has led to several socio-economic impacts to the nearby communities, such as loss of lives and destructions of physical infrastructure. The study was conducted to build a hydrodynamic model in combination with Geographical Information System (GIS) for flood inundation mapping in River Manafwa Catchment basin. HEC-HMS and HEC-RAS models were employed for the 50-, 100-, 200-, and 500-year return periods to achieve the overall specified objective. A sensitivity analysis of the hydrological model parameters used in this study showed that the model out puts are more sensitive to curve number compared to other parameters. The HEC HMS model was simulated, calibrated and validated using both Manual and automatic methods and the Nash- Sutcliffe Efficiency (NSE) obtained showing that the model was satisfactory. The simulated peak discharges of 285.5m3/s,362.2m3/s,416.8 m3/s,531.3m/s magnitudes of the respective 50,100,200, 500-year return periods produced maximum channel flood depths of 7.11, 7.17, 7.230 and 7.237m respectively as observed from the gauging station. Flood hazard maps were generated for the return periods in the Ras Mapper environment. From the results of the hydraulic model, it was observed that, the flood plain inundated areas increase with the magnitude of flow within the modelled network indicating a high flood risk level for activities and settlements adjacent to the river banks. The existence of the structural flood mitigation measures, their possible improvements and Community Willingness and ability to pay for adaptation strategies, etc were assessed through field surveys in the majorly affected sub counties of Mazimasa and Himuntu. This study finding are relevant for planning the development of sustainable flood risk adaptation pathways given the established destructions within the sub-catchment due to flooding.