Hydrological and Hydrodynamic Modelling of Flows to Support Establishment of Flood Adaptation Strategies for River Malaba Sub-Catchment in Uganda
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Date
2022-03-22
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Addis Ababa University
Abstract
Many people tend to live in the floodplains along River Malaba due to fertile soils which support agriculture. However, heavy rains in the highlands of Mount Elgon often lead to floods which end up affecting especially the local population in terms of loss of lives and destruction of infrastructure within the low-lying areas of River Malaba sub-catchment in Uganda. This research aimed at presenting a platform for understanding the impacts of flooding on the socio-economy while investigating perceived effectiveness of establishing flood adaptation strategies for predictive risk-based water resources management in the study area. The study had four specific objectives including; (i) to analyse changes in historical rainfall and potential evapotranspiration (PET), (ii) to perform hydrological modelling of extreme peak flows, (iii) to estimate impacts of flooding given the spatial extents of flooding inundations, and (iv) to analyse community willingness-to-pay (WTP) for flooding adaptation strategies. Changes in terms of trends and variability were analysed using nonparametric approach based on the cumulative sum of the difference between exceedance and nonexceedance counts of data points. The second specific objective consisted of determining which hydrological model could best reproduce observed extreme peak flows. Hydrodynamic modelling was performed using a two-dimensional Hydraulic Engineering Center’s River Analysis System model. Double-bound dichotomous choice contingent valuation method was applied to assess the local population’s WTP for flooding adaptation measures. The number of days with rainfall intensity > 5 mm/day and 10 mm/day had insignificant (p>0.05) decreasing trend. The sum of rainfall with intensities > 5 mm/day exhibited a significant (p<0.05) decreasing trend. However, annual maxima rainfall increased (p>0.05), indicating less frequent rains but some events having very high intensity. Variability of rainfall sub-trends was insignificant (p>0.05) and had a common pattern. PET had an insignificant (p>0.05) positive trend. The amplitudes in PET variability were insignificant (p>0.05) and though of generally common pattern. The Australian Water Balance Model exhibited the best performance in reproducing extreme peak flows and it had Nash–Sutcliffe efficiency (NSE) of 0.837. Land-use change had insignificant (p>0.05) influence on determining flood inundation extents. Inundation of rice gardens by the most severe 100-year flood was found to lead to an economic loss of about US$ 39 million. Amongst the infrastructure, churches showed the highest economic losses of US$ 1,623,832 due to flooding of 100-year return period. In general, the local community was aware of the flood citing rainfall variability and longer rainfall durations as main cause of flooding. Post-
flood strategies were more efficient than those practiced before- and during-floods. Among the suggested structural and non-structural strategies, “river training structures” and “flood forecasting and early warning” were highly preferred, respectively. 55% of the households expressed WTP an individual amount between Uganda shillings (UGX) 5,000 (US$ 1.35) to UGX 500,000 (US$ 135.14). Several demographic, social and institutional factors had significant (p<0.01) positive impact on community WTP. This study findings are relevant in supporting policy makers regarding predictive planning and development of flood risk adaptation pathways given the established destructions within the sub-catchment due to flooding.
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Keywords
River Malaba Sub-Catchment, Extreme Peaks Flows, Hydrological and Hydrodynamic Modelling, Flood Adaptation Strategies, Willingness-To-Pay