Evaluation of different adaptive measures to develop a sustainable stormwater drainage system at Kallayanpur catchment of Dhaka city

Abstract

Dhaka, one of the densely populated cities of Bangladesh is experiencing the negative consequence of rapid urbanization. The Kallayanpur drainage catchment is located in the western part of Dhaka city where about 95 ha retention pond area with 20 m3/s pumping capacity was sufficient. But the retention pond area had reduced to 42 ha in 2020.The catchment experiences waterlogging in every monsoon because of increasing population density, reduction of wetlands, and illegal encroachment of water bodies. Moreover, the trend line analysis of annual maximum daily rainfall over the last 30 years (1991 -2020)showsan increasing rate of 1.28 mm/year.It is also expected that, in future extreme rainfall event will be more frequent. The drainage situation is likely to be deteriorate for Kallayanpurif adequate measures are not taken.To develop a sustainable drainage system, stormwater drainage modelling plays an important role. This study tried to improve the understanding of the Kallyanpur drainage system and helped to predict the hydrologic and hydrodynamic response by using MIKE URBAN. Relevant past studies and literatureshave been reviewed, and key information have been gathered. Based on them a methodology has been devised. Data were collected from various sources and have been analyzed to develop hydrological model, 1D hydrodynamic model, and 1D-2D coupled overland flow model of the study area. The model calibration and validation were carried out using water level data from the Kallayanpur pumping station. Following that, several model scenarios were simulated based on specified adaptive measures, such as increased pumping capacity, and LIDs like rainwater harvesting, permeable pavement, and green roof. However, the model scenarios have been developed by considering design hyetographs generated from Alternating Block Method (ABM) and design hyetograph for Dhaka proposed by JICA study (1987). Also, it was considered that design rainfall will be increased by 14.4% in the future due to climate change effect. An acceptable level of model accuracy has been achieved during model calibration where the value of coefficient of determination (R2) and Nash-Sutcliffe efficiency (NSE) were found as 0.883 and 0.60 respectively.Also, a satisfactory level of model accuracy has been achieved during model validation.Following that, the results of the subsequent simulations show that the drainage system is pump-dependent and will require additional pumping.For intense rainfall events, requirement of additional pumping capacity is very high whereas for moderately intense rainfall events additional pumping requirement is significantly less though a much longer pumping duration is required.For design rainfall with climate change and with existing operating rule (i.e., pump start level 4.0mPWD, pump stop level 3.5mPWD), additional requirement of pumping capacity varies from 22m3/s – 55m3/s.But, requirement of additional pumping capacity has reduced to 17m3/s – 45m3/s with new operating rule to start pumping at 3.8mPWD.However, total storage amount by LIDs is about 0.74Mm3 which is 23% of total accumulated flow in the study area for design rainfall event with climate change effect. Thus, by combining LIDswith new operation rule, requirement of additional pumping capacity has stand to 9m3/s – 37m3/s. Incorporation of LIDs help to prevent shallow depth inundations and also providing additional advantages.Considering this, an adaptation strategy has been developed for the study area combining the grey and green structures (G&G). This approach is likely to be the most effective for stormwater management in the study area. The adaptation strategy proposed that, an additional pumping capacity of 37m3/s will be required in the Kallayanpur pump station. Besides, the pump operation rules need to be adjusted so that the system CAPEX and OPEX are optimized. Among the LIDs, rainwater harvesting has been found as the most effective solution. This solution can be adopted in the first phase. Following that other LIDs alternative likepermeable pavement and green roof can be adopted in later stage. For this proper design practices and regulations need to be formulated.Right now, the area of the Kallayanpur retention pond should be preserved, and if possible, further land should be acquired to improve the retention pond capacity.As the drainage systemisfound pump dependent during intense rainfall events, regular maintenance of drainage network and retention pond is required.