On the dynamics of density currents in submerged estuarine channels

ABSTRACT

Classical river hydraulics has been developed primarily for unidirectional, homogeneous flows and has been successfully applied to rivers, canals, and hydraulic structures. Its direct application to estuarine systems, however, becomes inadequate where freshwater and seawater coexist and interact with tidal forcing and engineered infrastructure. In estuaries, flow acceleration, a, becomes more significant than in river flows; yet, it has largely been neglected in the literature due to the widespread use of tidally averaged approaches. Density stratification fundamentally alters flow resistance, energy gradients, and sediment-transport mechanisms, while navigation channels, ports, and coastal reservoirs further complicate the hydrodynamics. This study develops a unified theoretical framework for stratified estuarine hydraulics by extending the classical concepts of uniform flow, gradually varied flow, and rapidly varied flow to density-driven two-layer systems. Governing equations are reformulated to explicitly incorporate reduced gravity, interfacial shear stress, and baroclinic pressure effects. Analytical solutions are derived for uniform stratified flow, interfacial water-surface profiles, critical conditions, and rapidly varied flows, including hydraulic jumps and sudden channel expansions. The framework establishes new theoretical foundations for estuary hydraulics and supports the design and operation of coastal reservoirs and other engineered estuarine systems in densely populated coastal regions.