Holistic Behaviour of Urban Pond Systems for Flood Risk Mitigation- A Case Study in Metro Colombo Area

In every year Colombo becomes a flood victim causing many negative effects and the current flood control systems are inadequate to address the issue to the fullest. This case study is to enhance the present flood mitigation system in Colombo. Methodology development was targeted for evaluation of the effectiveness of cascade behavior of ponds in flood mitigation. Initially, a pond system with cascade connectivity was identified in the Metro Colombo flood affected area and pond capacities, streamline network and catchment area of each pond were generated using Digital Elevation Model(DEM) and GIS software. Stream inflows were developed using 5-year storm event from IDF curves for Colombo and HEC-HMS software. Simulation of ponds during rainfall event was modelled using HEC-ResSim software and outflows were obtained for both isolated and cascade behavior of ponds. Flood maps of each pond for both cases were modelled using HEC-RAS software. Total flood inundation area of the 5-year storm event when the ponds are acting as isolated flood resisting structures and under holistic behavior is 74.7 ha and 63.9 ha respectively. Thus, the reduction of inundation area when the ponds are connected as a cascade is 10.8 ha which is 14.47% of the total inundation area.