Application of HEC-HMS and HEC-RAS in the mitigation of floods caused by increased rainfall intensity due to climate change in lower sections of Banda area.

Abstract

Flooding is a major problem that has continued to stress both developing and developed countries due to changes in climate change and land use patterns in most affected areas. Banda area located approximately eleven kilometres East from Kampala Central Business District is one of the areas that has been severely affected by floods due to increased rainfall intensity, changes in land use, continued encroachment on wet lands and its topographic characteristics that have exposed economic activities in lower section settlements at great risks of floods. The main objective for the study was to design and simulated a trapezoidal channel to mitigate impacts caused by floods in lower sections of Banda area. Inorder to provide a solution to the problem, ArcGIS version10.2.1 with HEC-geoHMS and HEC-geo-RAS extensions was used to delineate the watershed, hydrologic modelling was then carried out using HEC-HMS software version4.3 to model the basin hydrologic processes, HEC-RAS was also used to carry out hydraulic modelling of Kawoya river and HEC-geoRAS was finally used in generating flood in undation maps for the area for return periods of 5-,10-,25-, 50-.and 100-years. The trapezoidal channel was then designed using the hydraulic toolbox software to obtain reasonable channel parameters using the 25-year designs tormpeak discharge. The hydrologic model performance was tested using the Nash-Sutcliffe Efficient and the Root Mean Square Error Standard deviation objective function. The model was tested by calibration through carrying out optimization trials for the most sensitive parameters got from the sensitivity analysis with the 5-year designs torm. The obtained optimized parameters were then used to run the model for therestof there turn periods to get the corresponding peak discharges. The hydraulic model was then developed by using obtained peak discharges from HEC-HMS as in put and other parameters such as Manning’sn values and setting boundary conditions for the model. Using results obtained from hydraulic modelling, flood inundation maps were then generated. The obtained Nash-Sutcliff eEfficiency and Root Mean Square Error Standard deviation values were 0.919 and 0.3 respectively, indicating satisfactory model performance. The simulated peak discharges for return periods of 5-,10-,25-,50-and 100-years were 32m3/s,38.3m3/s,45.9m3/s,51.7m3/s,57.6m3/s respectively. And the maximum floods depths for the design storms were 10.6m,10.9m,11.4m,11.7m, and 12.3m respectively. Keywords: ArcGIS, HEC-HMS, HEC-RAS, HEC-geoHMS, HEC-geoRAS, Floods and Inundation mapping