Circulation through the mouth of Langebaan Lagoon and implications

In March 1997 a two-weeks field survey was conducted in Langebaan Lagoon and Saldanha Bay. The aim of this survey was to farther our understanding of the processes driving the mixing and the exchange at the Langebaan Lagoon-Saldanha Bay interface. The parameters measured included currents, water-levels, temperature, salinity, density and wind. The nature of the flow at the Langebaan Lagoon inlets was ascertained by combining statistical analysis of the measurements to a theoretical understanding of the system hydrodynamics. The flow in the vicinity of the straight was predominantly driven by the tide. It was found that during high tidal range periods, there existed an asymmetry between the ebb and the flood flows at both of the lagoon's inlets. When tidal forcing was strong, water particles released at the lagoon inlets during the ebb were subject to long drifts. The outflow from the east inlet appeared to take the form of a turbulent jet. At the west inlet strong frictional interactions between the flow and land boundaries occurred, causing the flow to rapidly expand and lose momentum and therefore impeding the formation of a jet. It was established that, generally, buoyancy forcing on the Langebaan Lagoon outflow would be small and that water issuing from the lagoon during the ebb would remained attached to the sea-bed as it propagated into Saldanha Bay. However, when Saldanha Bay was strongly stratified, the east inlet ebb jet would lift off from the bottom as it reached the 8m depth contour. The large drifts resulting from the sink-like nature of the inflow and the jet-like nature of the outflow induced a very rapid and strong exchange between Langebaan Lagoon and Saldanha Bay. The propagation of the lagoon effluent also contributed extensively to vertically stir the water-column in Big Bay. As the tidal range weakened, the regions of influence of the ebb and the flood overlapped to a greater extent and the exchange between the lagoon and the bay decreased significantly. The asymmetry between the ebb and the flood flows at the Langebaan Lagoon inlets generated a Lagrangian residual circulation, with the east inlet constituting the entrance for Saldanha Bay water, while the west inlet would be the exit route for Langebaan Lagoon water. Southerly winds, contributed to the overall residual circulation by driving water out of the Lagoon. Bibliography: 126-133 pages.

Grantor dc:publisher.institution

Department of Oceanography

Year dc:date.issued

1999

Author dc:creator

• Krug, Marjolaine

Advisor dc:contributor.advisor

• Largier, John

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/19383

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/19383