Siam Alam^1,, Afeefa Rahman^{2, 3}, Anika Yunus³

The drainage congestion problem is getting intensified in Dhaka city of Bangladesh over recent years due to rapid urbanization, inadequate drainage channels, and improper operation of the existing channels. The areas of Shantinagar, Rajarbag, Motijheel, Paltan, and T&T Colony serve as vital economic hubs in South Dhaka, yet they are highly prone to flooding during the rainy season. This study aims to address this problem by employing mathematical modeling to design a robust drainage network capable of effectively managing stormwater for existing and projected rainfall scenarios over 50 and 100 years. To achieve this, we utilize the frequency analysis method to estimate the design storm corresponding to 50 and 100 years return period for the study area. EPA SWMM 5.1 is employed to calculate the parameters of each sub-catchment, conduit network, junction node, and outlet for the drainage model setup. Through model simulation, we analyze the water level of each of the junctions to generate an inundation and hazard map of the study area. The accuracy of our SWMM model is validated in two ways: firstly, by comparing the inundation map produced from the model's water level outputs with the observed flood extent derived from Google Earth imagery, and secondly, by cross-checking the model's simulated runoff coefficient with manually estimated coefficients based on the land use map created from Google Earth. The total runoff volume from the model simulation is used to design the minimum cross-section of each of the conduits. By incrementally adjusting the conduit areas we analyzed the response of the flood hazard scenario of the study area. The findings of this analysis provide insights into the percentage increase in cross-section area required for drainage channels to effectively manage the runoff volume for a 50- and 100-year return period. With the developed model, we are equipped to design and size new components of the drainage system necessary for flood control during extreme scenarios. We anticipate that the outcomes of this study will prove valuable to relevant organizations and experts involved in addressing the drainage congestion issue in Dhaka City. The findings offer insights and recommendations for effectively managing and mitigating flooding through the design and implementation of improved drainage infrastructure.

urban flood, drainage network, runoff volume, SWMM, Dhaka City, Bangladesh