Flood risk and resilience assessment for the Nalukolongo Storm Water Drainage System in Kampala

Abstract

Floods are among the most catastrophic natural disasters around the globe impacting Economic activities, human lives and infrastructure. Changing climate and land use have escalated challenges associated with increased peak runoffs and flood management. Poor solid waste disposal and insignificant maintenance works being done on the existing drainage systems, flooding effects are magnified especially in urban and semi-urban catchments. This study was aimed at assessing the risk of flooding of the Nalukolongo Storm water drainage System and evaluating the effectiveness of various flood resilience measures to mitigate flooding. Flooding, flood Risks, flood assessment and resilience measures are expounded more herein. Software used include Geographical Information System (GIS), Storm Water Management Model (SWMM5)/ Personal Computer Storm Water Management Model (PCSWMM). Hydraulic and hydrological analysis are discussed including peak runoff determination methods. Catchment was modelled using SWMM5/PCSWM and peak runoff quantity of 359.787m3 /s was obtained using the soil conservation services method. Simulating using PCSWMM gave a total peak runoff quantity of 333.23m3 /s. The total flood volume (TFV) of the catchment at baseline was 487234m3 with an average flooding duration (AFD) of 1.26hours. The low impact developments (LIDs) implored reduced the flood volumes as summarized, infiltration trenches reduced TFV by 79% with an AFD of 0.62 hours, rain water barrels reduced TFV by 93% with an AFD of 0.39 hours and detention ponds reduced TFV by 89% with an AFD of 0.46 hours. With land use change consideration, TFV increased to 529710m3 an approximate 9% increment. With incorporation of rain water harvesting barrels as an LID, the total flood volumes reduced by 77% with an AFD of 42minutes. Incorporation of the resilience measures at baseline and after consideration of land use change in the model results in decreased Total Flood Volumes and Average Flood Duration. Rain water harvesting barrels have the most effective impact in reducing the total flood volume and average flood periods. The peak runoff quantity of the drainage system was determined using the soil conservation services (SCS) method. A PCSWMM model was developed and used to evaluate the hydraulic capacity of the existing drainage system and for simulation of flood volumes and durations. ArcGIS enabled subdivision of the study area into number of irregular sub-catchments to best capture the effect of spatial variability in topography, drainage pathways, land cover, and soil characteristics have on runoff generation used to examine the hydraulic capacity of the existing drainage system. Addition of various flood resilience measures to the PCSWMM model indicated great reduction in amount of runoff from the sub-catchments.