Study on the hydrodynamics of Dhaleswari-Buriganga river system for increase of lean flow in Buriganga

Abstract

This study work has been conducted to assess the simultaneous impact of reduced dissolved oxygen due to the extreme pollution induced on the Buriganga River and to observe the hydrodynamic parameters of the Dhaleswari-Buriganga River system and to increase the lean flow of the Buriganga River by diverting water from Jamuna River to Buriganga River through Dhaleswari River. There are three scenarios considered in this study, one is hydrodynamic scenario, other is water quality scenario and other one is augmentation of the lean flow of the Buriganga River and necessary analysis done to increase the flow. The Buriganga River is choked with industrial effluent and untreated sewage through numerous outfalls. Thousands of industrial units and sewerage lines dumping huge volumes of toxic wastes into Buriganga River increasingly polluting the water. One of the most important parameters frequently considered in river pollution studies is Dissolved Oxygen. From January to June and in December, the value of DO was also very low because of absence of water flow in the river. Because of gradual sedimentation in the Turag- Buriganga and Tongi khal-Balu-Lakhya river systems, the conveyance capacities have decreased, causing no flow conditions during the dry season, and consequently the navigational drafts have been reduced, although DO increased with the increase of river flow during the other periods of the year and it remained below the standard value of 6 mg/l for surface water according to DoE (2000). This parameter has been analyzed to find out the trend of degradation of DO around the year from 1988 to 2011. A thorough sensitivity analysis has been done for the dissolved oxygen (DO) value of 2 mg/l, 4 mg/l and 6 mg/l and finally dissolved oxygen (DO) value of 6 mg/l has been taken as critical value, which must be maintained for healthy aquatic lives in the water. The low DO content could also be linked to high turbidity and thus low photosynthesis that adds oxygen to the water. It is obvious that in such low DO state, no aquatic life can survive and thus the river reaches to a dying stage. In this situation, without augmenting the flow it will be impossible to recover the river water from its dying stage. This study is based on the assessment of hydrodynamic and flow augmentation of Buriganga River by using a mathematical model namely HEC-RAS. The mathematical model supported has, therefore, been taken up to develop stable river maintain augmenting the dry season flows of the Buriganga River from Jamuna through South Dhaleswari River. Considering these vulnerable situations, a hydrodynamic and a water quality model is set up then the models were calibrated and validated at both the Dhaleswari and Buriganga Rivers. Moreover, the desired discharge for which the channel is redesigned is determined by sensitivity and conveyance analysis. Finally, from sensitivity and conveyance analysis the Dhaleswari River has been redesigned to carry the discharge of 700 m3/s to maintain a healthy dissolved oxygen value of 6 mg/l for which the Buriganga River must carry 400 m3/s diverted from the Jamuna River. Which involves a huge earthwork about 80.6 Mm3.