Model based evaluation of effectiveness of drainage maintenance on reduction of flood risk in Kampala : case study : Nakivubo Drainage System

Abstract

Nakivubo drainage channel is under intense strain due to rapid urbanization, increased impervious areas, high intense rainfall, poor waste management and siltation, which causes hydraulic overloading resulting into pluvial flooding that damages infrastructure, disrupts traffic movements and economic activity and increases ground water pollution. This study evaluated the effectiveness of maintenance on the hydraulic conveyance capacity of the Nakivubo Drainage system, using the catchment and drainage system characteristics, and evaluated system performance to hydraulic load under varying sediment load (maintenance scenarios) and proposed interventions to the operation and maintenance of the drainage system that enabled achievement of the required service levels. To evaluate the effect of maintenance on drainage system performance, a desired maintenance level equivalent to a corresponding sediment load (0%, 25%, 50%, and 75%) was assumed and modelled. The drainage system performance was evaluated by the number of flooded nodes, flood duration, volume, and depth. The flooded nodes and flood duration increased with increased sediment load which suggested that sediment load significantly contributed to the number of flooded nodes within the drainage system as it reduced the residual functionality of the drainage system. A decrease in average flood volume and flood depth at the flooded nodes from 0% to 75% sediment load was observed. This reduction was attributed to the reduced surface run-off that was routed through the conduits due to increased sediment load and reduced channel cross section area. From the modeling results achieving or implementing 25% maintenance at which ever sediment load scenario, the number of flooded nodes and the flood duration reduce which suggests an increase in the conveyance capacity of the conduits and a general improvement in the residual system functionality of the Nakivubo Drainage system when maintenance was undertaken, as represented by the reduction in sediment load, however, it does not completely alleviate flooding due to within the drainage system as 41 nodes flood at 0% sediment load. The 41 flooded nodes are considered inherent system bottlenecks that required physical inspection and detailed evaluation of the designs of conduits before and after the flooded node and onsite conditions.