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New South Wales Government
Currents occur within the deep ocean, over the continental shelf and within the offshore and nearshore zones. They may be quasi-steady and persist for several hours to several weeks (ocean and shelf currents), or they may be oscillatory with periods of seconds (currents under waves). Currents may be limited to the surface or to the seabed, or they may extend over the full depth of water. Surface currents can have a different direction to those at the seabed.
The largest currents are those of the open ocean, which are driven by global scale interactions between the atmosphere and the sea.
Along the NSW coast, the most significant ocean current is the East Australia Current (EAC), which consists of a series of warm water eddies that originate in the Coral Sea and slowly move southward (see Figure B6.1).
Along the NSW coast, the continental shelf is some 20 to 30km wide. Continental shelf currents consist of the EAC, the counter currents associated with its eddies, internal waves, coastal trapped waves, tides and local wind induced currents. At any one time, the shelf current is a complex mix of these components.
Shelf and ocean currents are generally of little significance within the shallower waters of the nearshore zone. This area is the preserve of wave induced currents which include:
The passage of waves through shallow water causes a back and forth oscillation of water particles above the seabed (see Appendix B5). The motion of this oscillatory current can bring sediment into suspension to be transported elsewhere.
Shoreward Mass Transport
Wave action generates a quasi-steady shoreward current at the seabed. Outside the surf zone, this current is normally small and its effects are negligible. Within the nearshore zone, this onshore current can be of considerable importance. As long period ocean swell moves towards and through the surf zone, the onshore current transports sediment shorewards. This onshore movement is responsible for the rebuilding of beaches after storm erosion.
Rip cells are a mechanism whereby the water pushed shoreward by wave action and, to a lesser extent, by onshore winds can escape seawards. Rip cells can be identified by a strong longshore current in the nearshore gutter (feeder current) and a return jet of water seawards through the offshore bar (rip current). This is shown in Figure B6.2. Depending on wave conditions, nearshore bar bathymetry and state of the tide, rip cells may establish, dissipate and re-establish. Some rip cells have a more permanent nature, being associated with a controlling feature such as a headland, breakwater, offshore reef, creek, or stormwater outlet. Different rip types have been classified by Short (1985) and are typically associated with the various beach types as defined by Wright and Short (1983).
Rip cells can move significant volumes of sand offshore during storm events. Water is deeper in a rip channel, allowing larger waves to penetrate and attack the beach. Rip currents readily move offshore any sand eroded from the beach. At Wamberal Beach during storms in 1978, the maximum beach erosion occurred opposite a rip cell (PWD, 1985).
Rip cells are important to recreational amenity as they can present dangers to unsuspecting swimmers.
Longshore currents are generated by waves breaking at an angle to the beach, by feeder currents to rip cells, and from longshore variations in water level resulting from nearshore wave conditions and wind stress. Longshore currents are an important mechanism for transporting sand along a shoreline and into and out of the active beach zone (see Appendix B7).
Discharges from rivers, creeks, lagoons and stormwater outfalls can cause currents within and through the surf zone. These currents are usually controlled by the tide, with ebb tide effects being more noticeable. However, following heavy rain, freshwater outflows may become the dominant process.
Discharges from small lagoons and creeks often flow alongshore within a nearshore channel before crossing the offshore bar at a rip location. Larger rivers tend to penetrate the surf zone as a jet normal to the shore.
Gordon, A.D. and Hoffman, J.G., (1984). "Sediment Transport on the South East Australian Continental Shelf". Proc. 19th International Conference on Coastal Engineering. Houston, Texas, 1984.
Komar, P.D. (1976). "Beach Processes and Sedimentation". (Prentice-Hall, New York: 1976). ISBN 0-13-072595-1.
Nielsen, A.F., (1985). "Wamberal Beach and Avoca Beach Coastal Engineering Advice". Civil Engineering Division Report No. PWD 85040, Public Works Department of New South Wales, May, 1985.
Short, A.D. (1985) "Rip Current Type, Spacing and Persistence, Narrabeen Beach, Australia".Mar. Geol., V65, pp 47-71.
Wright, L.D. and Short A.D. (1983). "Morphodynamics of Beaches and Surf Zones in Australia".In P.D. Komar (Ed.), CRC Handbook of Coastal Processes and Erosion. CRC Press, London, pp 35-64.