Flood Modeling of the River Katonga Catchment

Abstract

The Katonga River is located in the southern part of Uganda and it drains into LakeVictoria.The river basin supports a variety of economic activities such as sand mining, fishing, handcraft making, and subsistence farming. The main objective of the research was to use Hydrologic Engineering Centre–Hydrologic Modeling System (HEC-HMS) software to model the continuous flows in River Katonga. This was coupled with hydraulic modeling using the Hydrologic Engineering Centre – River Analysis System (HEC-RAS) to develop flood inundation maps for the catchment. The simulated flows were compared with the observed flows to evaluate whether the model can be used for further prediction. The calibration was performed using data from 2001 to 2003 and validation for the period from 2007 to 2009 on a daily time step. The model performance was evaluated based on computed statistical parameters and visual checking of plotted hydrographs. For the calibration period of the continuous modeling, the Nash–Sutcliffe coefficient was 0.744 and the percent bias (PBIAS) was 3.63 indicating good and very good model performances respectively. For the validation period, the NSE was 0.752 and the PBIAS was 14.14 indicating very good and good model performances respectively. Based on these performance results, the model was found to give a satisfactory prediction of stream flow in the Katonga Catchment. A sensitivity analysis of the model parameters was performed and the different parameters were ranked using their average elasticity ratios considering percentage changes in simulated volume, peak flows,and NSE.The parameters that were more sensitive to change were tension,soil storage and Groundwater 2 storage. Peak discharges for the floods corresponding to the return periods of 2, 5, 10, 25, 50, and100years were input into HEC-RAS as unsteady flows. 2D unsteady flow analysis was carried out to obtain the inundation maps. The inundated area for the 2-, 5-, 10-, 25-, 50- and 100-year return floods were 508, 721, 931, 1215, 1531 and 1785 km2 respectively. The most affected land use in the catchment was subsistence farmland for the 100- and 50-year return floods and wetlands for the 2-, 5-,10- and 25-year return floods. We recommend flood zoning, flood warning systems, and dams as measures to be put in place to mitigate flooding in the catchment.