Urban Storm Water Management Modelling: A Case Study of Kalu Oya Basin – Sri Lanka

The Kalu Oya basin is a sub basin of Kelani Ganga and located closure to Colombo (the commercial capital of Sri Lanka). Due to rapid urbanization and other development activities in this basin there is high demand for lands and it has led filling of marsh areas both legally and illegally. Reduction in flood storage area due to this filling and solid waste dumping on marsh areas have led frequent flood inundation and flood impact in Kalu Oya basin particularly in downstream areas. Hence, it has greatly caused economic loss, social disturbances and environment degradations. To make balance between development activities and flood impact there was great need to understand flood mechanism in Kalu Oya basin, develop flood mitigation options and arrive at solution that is acceptable to different stake holders including environment.

A two dimensional model was developed using SOBEK software to replicate the existing condition of the Kalu Oya basin. A 20 m x 20 m DEM generated from LiDAR survey data was used in the 2D model. This model was calibrated based on captured satellite images of flood inundation as there was not observed water level data and then used to simulate mitigation options for different flood return periods. As the mitigation options, canal widening, removing hydraulic bottleneck, trans-basin diversion and installing pump station were individually tested. Further, combination of aforesaid options were tested. Model results showed that 25 year return period flood cannot be effectively managed with any of the individual or combined mitigation options. Therefore, 10 year return period event was selected as the design rainfall event.

As per the model results, it was noted that the flooding could not be completely avoided in Kalu Oya basin. However, the magnitude of the flood impact can significantly be reduced by implementing few mitigations options. From the analysis it was found that from hydrological, social, environmental and financial perspectives the most appropriate mitigation option was combination of canal widening and use the existing wetland to (marshy and paddy lands) to absorb the storm water temporary and release with time.

Key words: Flood, flood mechanism, mitigation options, SOBEK, 2-D model, marsh.