Study on water quality parameters due to withdrawal and flow augmentation in the Dhaka peripheral river system

Abstract

This study focused on the water quality of Dhaka peripheral river network of 195 km consisted of Turag, Buriganga, Dhaleshwari, Shitalakhya, Balu and Tongi Khal. 1D hydrodynamic and water quality model of Dhaka peripheral river network is developed in HEC-RAS. The hydrodynamic model was calibrated and validated for the year 2014 and 2016 respectively for Manning’s n=0.025 in all cross sections along the river network. The water quality model was calibrated and validated for January-June of the year 2014 and July-December for the year 2016 respectively for dispersion coefficient, D = 0.07 m2/s in all cross sections. The result of performance evaluation for the calibrated-validated hydrodynamic and water quality model has shown good match between the observed data and the simulated data. To develop the flow scenario, available source of water has been identified by calculating monthly mean flow (MMF) and environmental flow. While calculating the environmental flow, two approaches have been considered, Tennant method and Flow Duration Curve method. MMF has been compared to environmental flow to determine the amount of available water to withdrawal for each month of Turag, Buriganga, Dhaleshwari, Shitalakhya, Balu and Tongi Khal. The analysis of water quality parameters showed that the water quality is satisfactory in high flow season, but unsatisfactory in low flow season, except for Dhaleshwari and Shitalakhya. Based on the output, three withdrawal scenarios and four augmentation scenarios have been developed. The response of the most important water quality parameters DO and BOD to the withdrawal and augmentation scenario have been assessed in this study. Results show that low flow season in all rivers mostly exhibit the conditions for flow augmentation scenario due to less amount of MMF compared to environmental flow. Only few months of Dhaleshwari and Shitalakhya exhibit the conditions of withdrawal scenario as the MMF was more than environmental flow. In few months of low flow season, Turag, Buriganga, Balu and Tongi Khal exhibit the situation when MMF was more than environmental flow but no available water to be withdrawal due to poor condition of water quality. Results indicate that the withdrawal scenarios decrease the amount of DO and increase the amount of BOD. Conversely, augmentation scenarios increase DO up to 16.43% and decrease BOD up to 8.58% for 100% of available flow augmentation. Highest value of DO and lowest value of BOD for a specific month of a location were observed when 100% of available flow was augmented among the four augmentation scenarios. DO increased in maximum amount in May at Dhaleshwari and BOD decreased in maximum amount in April at Shitalakhya for 100% of available flow augmentation as the amount of available flow is quite high in Dhaleshwari and Shitalakhya compared to other Dhaka peripheral rivers. The effect of flow augmentation is not that much remarkable at Turag, Buriganga, Balu and Tongi Khal due to severely polluted water of Turag, Buriganga and less amount of available flow compared to Dhaleshwari and Shitalakhya. Though increases of DO and decreases of BOD occur in response to the augmentation scenario, the amount of change in DO and BOD is not that much significant even for 100% augmentation of available flow because it does not satisfy the inland river water standard of DO and BOD. It is tough to improve the water quality by implementing only the withdrawal and augmentation scenario without controlling the external source of pollution as the poor condition of water quality and flow availability. This research will greatly contribute and introduce new method for cleansing the heavily polluted river in future.