Coastal Formations
Tasmania has a large variety of coastal formations - sea-cliffs, bays, beaches, and estuaries are just a few examples. The following sections describe the State’s coastal geomorphology.
- Hard-rock Coasts
- Beach/bay Formations
- Coastal Inlets
- Open Drainage Systems
- Closed or Semi-closed Drainage Systems
- Straits and Channels
- Further Information
Hard-rock Coasts
Example of rocky coastline
at the Tasman Peninsula
(Land Information Services)
Where the coast is exposed to the forces of the ocean, soft sediments can be removed and the bedrock directly eroded by the sea. In such areas, sea cliffs and sea-stacks are common.
Much of the south-western coast from Cape Sorell to South East Cape is like this, as are parts of the south-eastern coasts of south Bruny Island, the Tasman Peninsula, Maria Island and Freycinet Peninsula.
Given the appropriate conditions, other formations may include sea-caves, rock arches, or wave-cut platforms, examples of which are all seen on the Tasman Peninsula.
More resistant zones of bedrock gradually wear away to form rocky reefs offshore.
Beach/bay Formations
Example of beach and bay formation at
Safety Cove, Tasman Peninsula
(Land Information Services)
Beach/bay formations are found along hard-rock coasts where a bay area forms between rocky headlands and soft sediments accumulate. Even without protecting headlands, very large beaches can develop on exposed coasts; for example, Ocean Beach on the west coast.
Beach/bay systems can be classified according to their degree of exposure to wave energy, although each site is unique and the range of conditions is large. Their exposure will depend on the strength and direction of the prevailing wind, the distance the wind blows over open water (the fetch—this helps determine swell height); local currents; and, influencing all these, the shape and structure of the local landforms (the geomorphology) and the presence and nature of any offshore protection such as reefs.
Exposed beaches/bays: these may experience direct continuous ocean swells, as on the west coast (Ocean Beach, Prion Bay), or less frequent swells, as on the east coast.
Semi-exposed beaches/bays: these areas receive some protection from headlands or reefs, but are exposed to moderate wave action, at least sporadically. They may receive swells by wave refraction or reflection; for example, Shoal Bay on Maria Island.
Sheltered beaches/bays: these are enclosed in bays or protected by islands or reefs. They experience low wave-energy and fetches are short. There are often extensive intertidal flats and shallow subtidal areas that may have large stands of seagrass.
The coast of the south-east of the State has many such habitats; for example, the beaches of the north coast of the Tasman Peninsula from Lime Bay to Cascades Beach.
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Coastal Inlets
St Helens
(Photograph courtesy of Wonderer Photographics,
Garry Richardson)
The forces that shape a coast also shape estuaries and other coastal inlets. Their geomorphology may be primarily controlled by the type of bedrock in the area or by sedimentary processes that are, in turn, shaped by factors such as weather and biota.
A distinctive feature of estuaries, lagoons and similar systems is that they contain modified salt water and are connected to the sea either continuously or intermittently by waves, sea spray or tides. They also receive material from their terrestrial catchments.
The formation of some of the large marine bays (such as Macquarie and Bathurst harbours) was controlled by faults and other geological structures. Similarly, the estuaries of the Derwent, Huon and Tamar rivers also show geological origins.
Open Drainage Systems
These permanently open estuarine systems vary considerably in width, depth and flow characteristics.
Tidal rivers, creeks and tributaries: these do not have extensive widened sections or bays, and have small to very small tidal flats. Their flow rates of fresh water are often very high, especially on the west coast, and as a result their surface salinities are generally much lower than the ocean’s, although deeper systems may have salt-water wedges (a tongue of marine water which penetrates up the estuary under the less dense river water). Examples include the Arthur and Pieman rivers on the west coast, and the Forth and Emu rivers on the north coast.
Estuaries: these generally have wide bays into which major rivers flow. They often have several tidal channels and extensive flats exposed at low tide. Most of the major coastal inlets in Tasmania fall within this category, including the Tamar and Mersey rivers and Port Sorell on the north coast, the Derwent and Huon rivers on the south, and Macquarie Harbour and Port Davey on the west coast. The northern rivers, especially, have extensive shallows.
See also Fisheries Habitats 
An open drainage system
Cremorne
(Land Information Services)
Tidal arms: these are often large shallow basins with little drainage from the surrounding catchment, and are therefore essentially marine. They are connected to inlets or bays and are inundated with coastal or estuarine water at each tide. Examples include East Inlet and West Inlet near Circular Head, Kelly Basin in Port Davey, Pipeclay Lagoon near Hobart, and Blackman Bay on the southern east coast.
Open lagoons: these inlets are often very large and have extensive shallows and channels less than 4 m deep that connect directly to the sea. While they may be fed by rivers, the inflow is small compared with the size of the inlet. Examples are Pitt Water in the south, Great Swanport and Musselroe Bay on the east coast, and North East River on Flinders Island.
Closed or Semi-closed Drainage Systems
Although occasionally open to the sea by channels or storm surges, these brackish to freshwater basins are usually closed by sedimentary barriers. They can go from being intermittently open to being closed for decades or centuries, and back again. They include examples such as Hibbs Lagoon on the west coast.
Bar or beach dammed rivers: these may drain through seepage channels over the bar or beach, and be penetrated by sea water over the same barrier during storms. Their salinity is generally low. Examples are the Scamander River on the east coast and the Nelson and Thornton rivers on the west and south coasts.
Bar or beach dammed lagoons: these estuarine systems are brackish, with salinity increasing over the time since their last closure (which can be many years ago). They are often deep and highly productive. Examples include Big Lagoon and Dianas Basin on the east coast.
Coastal basins: these are not connected to other drainage systems. Dune lakes often develop in coastal areas where the build-up of sand traps water behind newly formed dunes. Others occur in topographic low spots where the groundwater percolates up to the surface and may produce saltpans.
Straits and Channels
These pass between landmasses such as islands to connect two larger bodies of water. They vary considerably in depth and width, but may have large relatively shallow areas. Examples include Mercury, Robbins and Waterhouse passages, and Franklin Sound.
Further Information
Contact: Coastal and Marine Branch
Coastal and Marine Branch
6th Floor, Lands Building, 134 Macquarie Street
GPO Box 1751
Hobart TAS 7001
Phone: 03 6233 3963
Fax: 03 6233 6800
Email: Coastal.Enquiries@environment.tas.gov.au
