A sociohydrological approach to the mitigation measures used along the flood plains of river Nyamwamba.

Abstract

The study was conducted to determine a sociohydrological approach to the flood mitigation measures used along the flood plains of river Nyamwamba. HEC-HMS and HEC-RAS models were employed to achieve the first objective of developing a hydrodynamic model that estimated the flood risk extent. Results of the rainfall frequency analysis used to obtain the rainfall intensities for different return periods such as 50, 100, 200, 500 and 1000 years were used as an input for the hydrological model. The HEC HMS model was calibrated and validated using automatic methods and the Nash- Sutcliffe Efficiency (NSE) obtained were in the ranges of 0.765- 0.925 for the different return periods showing that the model was satisfactory. 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 simulated peak discharges 93.2m3/s, 138.1m3/s,187.8m3/s, 259.1m3/s and 316.6m3/s of the respective 50, 100, 200, 500, and 1000-year return periods produced maximum channel flood depths of 2.27ft, 2.7ft, 3.10ft, 3.66ft, 3.98ft respectively.