Abstract:
Dhaka, the capital of Bangladesh, frequently experiences water logging as the storm water drainage systems are often unable to manage peak runoff volume. Begunbari khal is one of the major drainage khals of Dhaka that carries storm runoff to the Balu river from catchments in eastern and central Dhaka. Three tributary khals, Shutibhola and Gojaria khals from the north and Nasirabad-Nandipara khal from the south, discharge runoff into this khal. Hatirjheel, which is now the largest surface water body within Dhaka, also plays an important role in the inflow of Begunbari Khal. It serves very important hydrologic functions of draining and detaining storm water from a large area of Dhaka city. Although designed to carry storm water, the storm sewers discharging into Hatirjheel carry both storm water and dry weather flow. This study aims to assess the drainage capacity of Hatirjheel- Begunbari Khal drainage system considering predicted precipitation variability due to climate change as precipitation intensity and pattern are expected to be altered in future due to climate change and such changes are likely to lead to severe flooding in urban areas of Bangladesh. The study involves DEM based catchment delineation using Arc Hydro tool based on GIS. The DEM was reconditioned prior to catchment delineation to reflect the existing topography and storm sewer network. A future land use scenario was developed based on the analysis of land use maps to estimate future runoff coefficient. Regional Intensity-Duration-Frequency (IDF) relationship for Dhaka city was developed to determine peak rainfall intensity considering historical observed as well as predicted future precipitation data and then validated using measured rainfall data of 2014. Once the necessary parameters were determined, the rational fromula was used to assess the flow. Since Hatirjheel is a regulated system, different operational scenarios were considered while assessing the flow. Finally HEC-RAS was used to model the reach of Begunbari khal downstream of Hatirjheel and adequacy of different cross-sections of the khal was checked considering gravity flow drainage and at the same time ignoring backwater water flow effects.The proposed future land use scenario facilitated to predict future flow by providing future runoff coefficient of the study area. The extreme rainfall events of Dhaka showed that rainfall characteristics over time do follow a simple scaling process. While validating the developed IDF curves, it was seen that scaled estimates are relatively close to measured data. While checking adequacy using HECRAS, all the sections of Begunbari Khal appeared to be inadequate except some upstream cross sections for historical extreme rainfall events when both Hatirjheel and Rampura regulator gates are closed. It implies that Hatirjheel plays a very important role in the conveyance capacity of Begunbari Khal. The high inflow at the downstream of Begunbari Khal even with closure of the Hatirjheel and Begunbari regulators depicts that the eastern catchment have marked contribution to the flow of the khal. It was also found that under future climatic scenario there is a possibility of significant overflows which is alarming.
