Mathematical modelling of stormwater drainage of the catchment of boalia khal including low impact development measures

Abstract

Dhaka city is one of the fastest growing megacities in the world and is facing a massive challenge to deal with rainfall-induced flooding. Severe storm events due to climate change and landuse changes are playing a key role in the acceleration of urban flooding. The main drivers of rainfall-induced flooding are fast & unplanned urbanization and incapacity of stormwater drainage systems to manage the peak runoff volume. The catchment of Boalia khal in the northeastern part of Dhaka city is one of those areas which face frequent flooding induced by intense rainfall during the pre-monsoon and monsoon periods. This study aims to obtain an optimum combination of Low Impact Development (LID) measures for stormwater management in the Boalia khal catchment area. This research focuses on investigating the influence of different individual LID (Rainwater Harvesting (RWH), Soak-Away Pits (SAPs), Porous Pavement (PP), Wetland Basin (WB)) and a combination of LID measures for the proposed drainage system to reduce the runoff volume, flood extent and pumping duration in the Boalia khal catchment area. A 1D-2D coupled drainage model was developed using the DHI MIKE URBAN interface. The design rainfalls were distributed using the Alternating Block Method. The drainage system of Dhaka city has been designed to handle a 1-day, 5-year rainfall event, while the pumps have been designed to cope with a 2-day, 5-year rainfall event. Consequently, the current systems are ill-equipped to deal with rainfall events that lead to urban floods, such as 1-day 10-year and 25-year occurrences. To assess the effectiveness of LID (Low Impact Development), this study conducted an analysis of various scenarios considering these different rainfall events. The peak water level is reduced by 8.4 cm at upstream of Boalia Khal and by 19.2 cm at downstream for a 25-year rainfall event after the application of low impact development measures. The combined (RWH-SAPs, PP and WB together) application of LID measures decreased the flood-affected area by 14% for a 10-year storm event and 15% for a 25-year storm event compared to without LID. Integrating RWH-SAPs and PP in the future drainage system may reduce pumping duration by up to 14%, equivalent to 3.0 hours. The addition of Wetland Basins alongside these LID measures can lead to a more substantial reduction of pumping duration up to 53%, i.e. 10 hours for a 10-year storm event.Thus, the application of porous pavement is effective for the reduction of flood extent and WBs are more effective for the reduction of pumping duration along with flood extent. To get optimum performance of wetland basins the invert level of the inlet control structure was set below the pump start level. It was found that climate change could lead to increased flood extents and longer pumping durations. However, the application of LID strategies mitigated these impacts, reducing flood extents and pumping durations under climate change scenarios.LID measures reduced the flooded area by 20% and average pumping duration by 43% for a 100-year storm event.