Salinity Distribution in River Network of a Partially Mixed Estuary

The salinity distribution in the estuarine channel network of a partially mixed estuary was investigated by quantifying salt intrusion length and stratification with response to tidal circulation, seasonal variation, topography, and river discharge. The studied Karnafuli River estuary responds to a power-law relationship between discharges and salt intrusion lengths with exponent −0.64. In the river network, two salinity sinks were identified, that is, a river confluence and a channel bend. At the estuary, the surface salinity level during the dry period increased by 1.2–1.9 times that of the wet period due to residual salinity. On the other hand, the surface salinity level during the wet period decreased due to high river discharge. Again, a high salinity pulse was observed during flood tide and lower concentration was found in surface salinity during ebb tide. The spring–neap transition also causes salinity variation, reaching the lowest concentration at the end of the neap tide-wet period. Applying a subgrid scale Horizontal Large Eddy Simulation model in Delft3D, the spatial distribution of salinity showed that during flood tide the centrifugal force acted stronger than the opposing barotropic pressure gradient, and the centrifugal force moves the low-density water toward the river bend, which resists salinity intrusion. High salinity stratification was observed with high freshwater discharge and low stratification was found at a shallow water depth of the estuary. The surface salinity responds faster than the channel bottom salinity flux with changing forcing conditions (tide and discharge), while the bottom salinity circulation follows an unsteady process.