Optimization of salinity intrusion control through freshwater and tidal inlet modifications for the Indus River Estuary

Abstract

Increasing salinity levels in the tidal rivers are becoming a significant challenge to inland water quality. Salinization of the adjoining water resources and agricultural lands have prompted the water managers to optimize the solutions for mitigation of adverse effects. Keeping this in view, a hydrodynamic model coupled with a water quality model was used to analyze the propagation of tides from the Arabian Sea into the Indus River and their subsequent effect over longitudinal salinity distribution. Field observations of hydraulic parameters and salinity were used for validation of the model predictions in dry (no river flow) and wet (specific river flow) hydrological conditions. Results suggest that the tidal amplitude dissipates landward; salinity intrudes about 58 km (near Dandari) into the Indus River Estuary (IRE) at spring tide. Salinity in the proximity of Dandari point is sensitive to variations in fresh water supplies. A complete neap-spring tidal cycle was chosen for scenario-based evaluation of the impact of fresh water over the longitudinal salinity distribution. A daily river discharge 250 m3/s was found optimum for limiting the salinity interface (≤1) up to 31 km from the estuary mouth. In addition, tidal inlet modifications based on the theoretical framework of stable channel design combined with the optimum river discharge were tested. The cross-sectional modifications based on the regime/Lacey theory proven better and salinity interface was further pushed back up to 27 km. Thus, it is maintained that synergic impact of the optimized freshwater quantity and the estuary mouth modifications are considered as one of the pragmatic solutions for control of salinity intrusion and achieving the specific environmental conditions along the alluvial estuary.