Peter Ridd

Tidal mixing and trapping in mangrove swamps

Lateral trapping in mangrove swamps is a dominant process controlling longitudinal mixing in mangrove-fringed tidal rivers. In the dry season, the effective value of the longitudinal diffusivity is increased by the trapping effect by typically two orders of magnitude, from its value in the absence of swamps. In the wet season, freshwater is trapped within the swamp and the adjoining boundary in the tidal river, for long periods following the passage of a short-lived flood. This may have important ecological implications. A fluid-mud layer appears to be created by strong tidal currents over colloidal mangrove mud.

Outwelling from tropical tidal salt flats

Abstract The Norman River flows into the Gulf of Carpentaria, north-east Australia. The estuary is surrounded by 250 km2 of vegetation-free ‘saline supratidal mud flats’ which are only inundated by the tides at spring high tide in summer when the seasonal sea level is 0.5–1 m higher than in winter. The first summer tidal inundation of the salt flats in November 1985 resulted in a readily measurable export from the salt flats of salt and nutrients. Outwelling values of 90 g salt m−2 per tidal cycle, 1·0 mmol silicate m−2, and 3×10−2 mmol orthophosphate m−2 were measured. About a quarter of the 4000 km2 of salt flats along the southern shore of the Gulf of Carpentaria outwell into the Gulf, the rest outwell into the estuaries where salt outwelling increases the strength of the salinity maximum zone of the estuaries, further isolating the rivers from the Gulf during the dry season.

Mixing and Trapping in Australian Tropical Coastal Waters

Evidence is presented indicating the existence in tropical Australia of a coastal boundary layer, often visible as a nearshore band of turbid water. The coastal boundary layer inhibits mixing between the nearshore and offshore water by two effects, firstly by trapping in mangrove swamps, and secondly due to frictional effects which inhibit inertial jets. Contrast was found between coastal boundary layer dynamics in the Gulf of Carpentaria, which has shallow inshore water and a smooth coastline, and the continental shelf of the Great Barrier Reef which has deeper inshore water and a more rugged coastline. In the Gulf, the coastal boundary layer maintains its integrity for much longer time scales than in the Great Barrier Reef continental shelf.

Settling of Ocean-dumped Dredged Material, Townsville, Australia

Mud is dredged from the Port of Townsville, Australia, and its navigation channel and is dumped offshore in typically 12 m depth. There are two stages to the settling: initial, rapid descent as a negatively buoyant jet, forming a high concentration suspension near the bottom, followed by subsequent slower settling of mud flocs. In calm weather, mud flocs settled out of this layer in about 15 min and the suspension did not move out of the dump site. In rough weather, the settling of mud flocs was inhibited by wave-induced turbulence, and the suspension was mobile and was transported away from the dump site. In quiescent waters, this suspension compacted to about 8°0 of its original volume in approximately 4 days. Even after compaction the mud was resuspended, presumably by pumping by long waves since no strong currents were observed, forming a mobile, 1 m thick, high concentration suspension at the bottom.