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New South Wales Government
A coastal entrance is an important feature that affects the hydraulics and amenity of the estuary and nearby coastal areas. Entrances are subject to a variety of processes that affect their behaviour: tides, waves, currents, sediment movement, floods, etc. The interaction and ever changing nature of these factors may cause entrances to migrate along the coastline, to shoal, to close up, re-open, etc. At best, an ever restless dynamic equilibrium exists that may be punctuated by irregular and sudden changes of behaviour.
Estuaries are of importance to human use and recreation. Their entrances provide access to and from coastal waters for commercial fishing boats and pleasure craft. Changes to entrance behaviour can be detrimental to boat movements and the use of the estuary.
Estuaries along the New South Wales coast can be classified into the three types listed below and shown in Figure B11.1.
Drowned River Valleys
Drowned river valley estuaries generally consist of a semi-enclosed bay with a deep and wide entrance, such as Broken Bay, Sydney Harbour, Port Hacking and Batemans Bay. Typically, such estuaries experience the full ocean tidal range. Entrances are characterised by submerged tidal deltas formed by the deposition of marine sands over the muddy sediments of the estuary floor. The seaward face of the delta is subject to wave and current attack.
Barrier estuaries such as Wallis Lake and Lake Macquarie, are characterised by long, narrow entrance channels meandering through broad tidal sand flats. Strong tidal flows occur in the entrance channel. Frictional effects progressively reduce the tidal range, which in the estuary basin may be as small as 5% of the ocean tide. Wind waves and wind-induced currents are of more importance to water circulation than tidal currents. The entrances of Barrier estuaries normally remain open to the sea.
Mature "river estuaries" such as the major north coast rivers Tweed, Richmond, Clarence and Macleay, are examples of infilled barrier estuaries.
Saline Coastal Lakes
Most saline coastal lakes are small enclosed waterbodies located behind beach foredunes, e.g. Dee Why Lagoon and Coila Lake. Typically, entrances are closed or "choked" and undergo irregular episodic opening by flooding. This results in the lake waters being non-tidal or weakly tidal at best. Generally, lake waters are saline to brackish, although heavy rains may significantly lower salinity levels. Water circulation and mixing are caused mainly by wind. When the entrance is closed, water inflows are lost by evaporation and percolation through the porous sand barrier. After heavy rains, the entrance berm may be breached by superelevated lake waters (Gordon 1981). In these circumstances, the lake becomes saline and tidal for a period of weeks to months until surf zone processes reform the beach berm and close the entrance.
The entrance of any tidal inlet on a littoral drift coast is subject to the competing influences of scouring caused by tidal and flood flows and shoaling caused by sedimentation from the adjacent littoral zone. The interaction of flow, waves and littoral drift is always varying and leads towards what has been termed a "dynamic equilibrium" (Bruun, 1978).
Features of a typical tidal inlet on a littoral drift coast are shown in Figure B11.2. These include an offshore entrance bar, swash bar and shoals, ebb tide channels, marginal flood tide channels and inner shoals. These features are intimately related and together reflect and affect the hydrodynamic response of the inlet.
Conditions of estuary entrances are controlled by the tidal range and discharge, river flow, wave exposure, entrance channel geometry and the rate of longshore sediment transport.
Sand entrained by waves is transported into the coastal entrances by flood tide currents to be deposited on inner shoals. Depending on inlet and shoal configuration, part of this material is returned in a seaward direction by the ebb tide currents. However, some of this material remains inside the inlet. Sediment trapped in this way tends to be deposited at the landward end of the entrance channel, building up the inner shoals.
If sediment inflow from coastal waters becomes too great the entrance may close. A flood event will be required to break the sand plug and re-establish an entrance channel, at which time scoured sand is returned to nearshore waters.
Bar and shoal formations will be largest at those inlets with the greatest tidal volumes and those where longshore transport is greatest.
Other significant factors affecting the behaviour of coastal entrances are man-made and include breakwaters and training walls. These are constructed to stabilise the location of entrances and maintain their depth. As shown in Figure B11.3, these works can significantly modify the hydraulic behaviour and sedimentation processes both within the estuaries and along their adjacent coastlines (Druery and Nielsen, 1980; Nielsen and Gordon, 1980; Nielsen, 1981; Williams, 1981).
Entrances controlled by a single breakwater often have an elongated and unstable offshore bar. With double breakwaters, the offshore bar becomes smaller and more compact.
Bruun, P., (1978). "Stability of Tidal Inlets". Developments in Geotechnical Engineering 23. Elsevier, Amsterdam, 1978. ISBN 0-444-4-41728-1.
Druery, B.M. and Nielsen, A.F., (1980). "Mechanisms Operating at a Jettied River Entrance". Proc. 17th International Conference on Coastal Engineering, Sydney. ASCE, March, 1980.
Gordon, A.D., (1981). "The Behaviour of Lagoon Inlets". Fifth Australian Conference on Coastal and Ocean Engineering, Perth. I.E.Aust., November, 1981.
Nielsen, A.F., (1981). "The Application of a 1-D Numerical Tide Model to a Small Estuary". Proc. First Conference on Hydraulics in Civil Engineering, Sydney. I.E.Aust., October, 1981.
Nielsen, A.F. and Gordon, A.D., (1980). "Tidal Inlet Behavioural Analysis". Proc. 17th International Conference on Coastal Engineering, Sydney. ASCE, March, 1980.
Roy, P.S., (1984). "New South Wales Estuaries: Their Origin and Evolution". Coastal Geomorphology in Australia. ISBN 0-12-687880-3.
Williams, G., (1981). "Investigation and Management of Estuarine Lake Inlets". Proc. First Australian Conference on Hydraulics in Civil Engineering, Sydney. I.E.Aust., October, 1981.