Negative hydrology in wave-dominated estuaries

Negative hydrology processes in wave-dominated estuaries

Catchment of 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 typically relatively low in negative estuaries.

Periodic minor stratification

The volume of freshwater entering the estuary is typically too low to cause significant stratification. When freshwater input does occur (during seasonal rains or extreme runoff events), minor short-lived freshwater stratification may occur.

Mixing

High evaporation rates result in elevated salinity in the central basin. This produces hyper-saline water that sinks beneath the intruding sea water, and may flow out of the entrance (Lennon et al., 1987, de Silva Samarasinghe et al., 1987b). A small amount of mixing occurs between the stratified layers.

Marine exchange

Exchange of sea 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 ‘negative’ wave-dominated estuaries, the inflow of marine water exceeds the outflow of freshwater. In such cases, the hyper-saline water is usually exported to the ocean (Heggie et al., 1999b). The entrance of the estuary may be intermittently closed.

Internal circulation

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

Evaporation is the dominant process in ‘negative’ wave-dominated estuaries, due to arid climatic conditions. Aridity and evaporation may vary seasonally, however by definition evaporation in ‘negative’ estuaries is larger than freshwater input (Veeh et al., 1995). Consequently, negative estuaries tend to have longer residence times than positive estuaries (Smith et al., 1989).