Assessing Flood Risk And Developing A Framework For A Mitigation Strategy Under Current And Future Climate Scenarios In Nyabarongo Upper Catchment, Rwanda

Abstract

Literally, Rwanda has experienced climate change and variability in terms of frequency, intensity, and persistence of extreme changes, such as floods and droughts as well. The occurrence of rainfall deficits and excess has meaningfully increased in recent years. Yet, these events have had serious impacts on the environment, economy and human lives. Overexploitation of soil and extensive erosion results in soil being washed down the hillsides into valleys causing extensive sedimentation of main rivers, such as Nyabarongo river, and other water-bodies. However, there is lack of knowledge on flood risk potential, flood damage and therefore no flood risk mitigation strategy. The main objective of this study was to assess the flood risk and develop a framework for a mitigation strategy under current and future climate scenarios in the Nyabarongo Upper Catchment using a hydrological model. The hydrological data comprises precipitation and stream flow records for a period of 24 to 46 years, digital elevation model (DEM) dataset of cell 30-meter grid, land use/land cover datasets to create curve number (CN) values of the river catchment were used for hydrological modeling using ArcGIS. The SCS Curve Number method was used to perform Loss model, then SCS Unit Hydrograph for Transform model and Lag method for Routing model to compute the input basin and river reach parameter values for each sub-basin, for instance initial abstraction, curve number, imperviousness and lag time for both basin and river reach, in HEC-HMS to compute a 10-year, 50-year, 100-year, 250-year and 500-year flood for the entire catchment. The catchment was delineated using the Digital Elevation Model and the whole catchment was divided into 4 sub-basins. The results of the analysis show that the upper left sub-basin (W500) has high flood magnitude of 1,128.4 m3/sec compared to the 3 other sub-basins for a 10-year flood, the lower sub-basin (W360) is dominant for both a 50-year (21,054.7 m3/sec) and 100- year flood (54,541.5 m3/sec), and finally the middle sub-basin (W460) is leading at 27,467.7 m3/sec of flood magnitude for a 250-year flood and at 836,578 m3/sec of flood magnitude for a 500-year flood. As result, the integrated river basin approach, public awareness and public participation, land use, zoning and risk assessment along with early warning and forecast system were proposed as the best practices for the mitigation and adaptive management strategy in the catchment.