Hydrodynamic modelling of flash floods and its socio-economic impacts in river Nyamwamba Catchment

Abstract

River Nyamwamba in Kasese district, pouring its waters in lake George, has over the years experienced recurrent floods which have threatened the livelihood of communities in the catchment. Destruction of infrastructure such as bridges, houses, and hospitals has been common and to a large extent killing and displacing people. This study assesses flash floods along Nyamwamba River catchment by carrying out hydrological and hydraulic modeling and estimated the socio-economic impacts of these floods on the community using flood depth- damage functions. HEC-HMS is used to conduct hydrological modeling. Flow simulations are forecasted for various return periods producing flow hydrographs for each flood event. HEC- RAS is used to perform hydraulic modeling from runoff data and subsequent flood plain maps developed for each flood event. Model calibration and validation is performed to compare the simulated water surface elevations with the observed values against the model yielding/ appealing results. Several hectares of land were found inundated with various return periods despite the measures that have been implemented. The inundation depths ranged from 0.0012-21.4 m with subsistence farmland having the highest percentage (40%) of land inundated by the flood. The amount of damage as determined using flood damage depth functions yielded subsistence farmland to have the highest amount of damage worthy Euros 1,454,350.54 and 1,589,290.28 at 5yr and 25-year return period. Schools had the highest amount of economic loss worthy Euros 449,799.14, while hospitals had the least amount of damage worthy Euros 149,933.05. In a bid to reduce the devastating impacts of floods, the study recommends, among many already in-place measures, creation of awareness in the communities on flood risk reduction measures, ensure effective and efficient enforcement of riverbank management regulations, adequate protection, and restoration of wetlands in addition to ensuring proper physical planning of settlements. The study generally concludes that the frequency in occurrence and severity of floods in Nyamwamba River catchment has increased posing panics and potential threats to human livelihood in the Kasese region. For instance, a 25-year return period peak discharge of 375 m3/s at the bridge section was found to cause an inundation area of 53.2 hectares. The findings of this study are relevant for the development of Emergency Action Plans encompassing river flooding and flood routing analyzes for the affected downstream areas.