Author(s): Ronak Prakash Das; Manas Kumar Das; Bibhas Chandra Barman; Dhrubajyoti Sen

Linked Author(s): Dhrubajyoti Sen

Keywords: Sewer flow; Water quality; Dissolved Oxygen; Biological Oxygen Demand; Simulation

Abstract: Urban drainage canals in many deltaic cities discharge partially treated wastewater into tidal rivers, where the hydrodynamic conditions strongly influence the transport and transformation of pollutants. The city of Kolkata, located in eastern India on river Hooghly, disposes a large fraction of its wastewater through three major drainage canals – Bagjola, Kestopur, and the S torm Weather Flow (SWF) channels – which convey combined wastewater and storm runoff to a smaller tidal creek Bidyadhari. Although observational studies have reported poor water quality in these canals and in the receiving river, the influence of tidal hydrodynamics and gate operations on the downstream transport and partial self-purification of pollutants has not been systematically investigated. The present study examines the dynamic variation of two key water-quality indicators, dissolved oxygen (DO) and biochemical oxygen demand (BOD₅), in the drainage canals and the receiving river using a coupled hydrodynamic–water-quality modelling framework. The canals are represented using a one-dimensional model, while the tidal Bidyadhari River is simulated using a depth-averaged two-dimensional model implemented in the MIKE+ hydrology-hydraulics computation platform. The numerical simulations show that tidal reversals and the opening and closing of flap gates at the canal outfalls strongly influence the spatial distribution of DO and BOD₅ in the receiving waters. During rising tides, enhanced mixing and natural reaeration contribute to partial oxygen recovery in the river, whereas during falling tides the release of accumulated polluted canal water produces localized deterioration of water quality near the outfalls. The results highlight the important role of tidal flushing and gate operation in governing the self-purification capacity of tidal receiving waters and provide insights that may assist in the management of urban drainage discharges in deltaic environments.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.279

Year: 2026