Positive hydrology in wave-dominated estuaries

Positive hydrology processes in wave-dominated estuaries

Freshwater input

Freshwater enters from the catchment. Although the volume of freshwater input varies regionally and temporally (depending on local catchment and climate conditions), it is often relatively high in positive estuaries.

Freshwater flow

Water circulation in wave-dominated estuaries generally ranges from well mixed to salinity-stratified, depending on the degree of wave mixing, volume of freshwater input, and climate (Nichols et al., 1985). ‘Positive’ wave-dominated estuaries have lower salinity water towards their head, with the salinity of the water in the central basin and next to the inlet approaching that of the adjacent ocean water. The volume of freshwater causes stratification (or layering) in the water column, which varies with seasonal flow. Buoyant low-salinity fresh water floats above the denser, high-salinity ocean water.

Salt wedge

A ‘salt-wedge’, or intrusion of denser saline marine water penetrates through the entrance along the bed of the estuary. Some mixing occurs at the interface between the fresh and marine water. The distance that the salt-wedge penetrates is dependant on tidal range and the amount of fluvial flow received by the estuary. During high fluvial flow events (which may be seasonal), fresh floodwater may push the salt water beyond the mouth (Eyre, 1998, Kurup et al., 1998). However, the large volume of central basins typical of wave-dominated estuaries tends to reduce this effect (Hossain et al., 2001).

Outflow of brackish water

Exchange of ocean water and estuarine water occurs through the entrance of the estuary, although the amount of exchange depends on the size and length of the entrance channel. In positive wave-dominated estuaries, the outflow of freshwater exceeds the inflow of marine water. During dry conditions, the entrance of the estuary may be intermittently closed.

Internal currents

Wind-induced currents drive the internal circulation of wave-dominated estuaries. Secondary circulations can be generated by tides, and can be influenced by coriolis effects in estuaries with very large basins. However, inside wave-dominated estuaries, tidal ranges are often small (~0.1 m) compared to tidal ranges in the ocean. Internal circulation patterns are disrupted during extreme high-flow events.

Evaporation

While significant evaporation can occur in wave-dominated estuaries characterised by positive circulation, evaporation (by definition) does not exceed the amount of freshwater input (Heggie et al., 1999b).