Tidal hydrodynamics

Abstract

Although it is astronomical forcing (the gravitational effects of the moon and sun on the oceans) that makes the tides so predictable, it is the hydrodynamics (i.e., physics of the water movement) that determines the size and timing of the tides. When we speak of the tidal hydrodynamics of estuaries, we are speaking of shallow-water tides, which have much larger tidal ranges and faster tidal currents than open-ocean tides. In shallow water, the hydrodynamics also transfers tidal energy, through various nonlinear processes, to new frequencies. These so-called shallow-water tidal constituents include higher harmonics of particular astronomical constituents (called overtides), which distort the simple sinusoidal shape of the tide and tidal current curves that are found in the open ocean, often causing asymmetry in the tidal cycle in an estuary. These same nonlinear processes also lead to interactions between the tide and nontidal phenomena such as river discharge and wind-induced changes (e.g., storm surges). In this entry, we have explained how each of these mechanisms work and the importance of tidal hydrodynamics in moving the water and affecting the processes in estuaries.