The coastal zone is not a stable and constant environment, but a dynamic place that can change rapidly in response to natural processes such as seasonal weather patterns.
Waves, winds, currents, tides and storms are the major forces on the coast. The results of actions and interactions of these natural forces on the shoreline and near-shore seabed are called coastal processes. These include erosion and deposition, movement of dunes, longshore drift, and the effects of storms on the coastline.
There are complex issues at the land and sea interface and an understanding of the connectivity between the catchment and the coast is essential in managing coastal issues and landscapes. Activities in the catchment can affect the water quality of river systems and wetland areas, which in turn impacts on the health of coastal and marine environments. This can have implications for natural values such as seagrass beds, fish stocks and biodiversity.
Many coastal foreshores are still responding to the last sea level rise around 6 000 to 7 000 years ago and will continue to respond to increases in sea level associated with climate change.
The geology, alignment of coastal landforms and local geomorphology (orientation and shape of coastal landforms) can all influence how coastal processes operate. The alignment of the coast influences the way waves are bent (refracted) as they approach the shore and the way they are aligned to the coast, and therefore how sand is moved along the shoreline.
Waves are formed by winds blowing over the sea. Waves are the most important force on sandy coasts because they transport sand on- and offshore and can also move sand along the shore. In winter, beach profiles often become steeper due to stronger wave action, which removes sand from the beach and deposits it on offshore sandbars. In summer, calmer waves bring sand back to the beach (see figure below).
Click on the image to view a larger sizeFigure: Beaches and foredunes undergo major cycles of change, as storm waves drag sand out to the surf zone and calmer swells bring the sand back onshore. The beach, foredune and offshore sandbar are all part of the one dynamic system. Source: Community Coastcare handbook (Thorp 2005)
The relative amounts of summer deposition and winter erosion can mean that sand dunes gradually move either seaward or landward. Beaches backed by parallel ridges (eg. Seven and Nine Mile, Rheban and many in the north-west) demonstrate a history of accretion (sand build-up) and seaward expansion. Others, like Roches Beach in the east and Ocean Beach in the west, appear to have entered a new phase of sustained erosion.
CurrentsNearshore currents are created by winds, waves, tides and river flows. These currents may transport large amounts of sand and alter the shape of offshore sandbars.
Longshore currents (longshore drift) occur when waves come in at a slight angle to the shoreline. Each wave washes sand grains diagonally up the beach, the sand moves down the beach with the backwash and is then edged a little further along the beach by the next wave. As a result, the sand is carried in a zigzag pattern along the beach and can cause an overall movement of sand and currents in one direction, called longshore drift. The effects of longshore drift can be observed by noting where sand builds up against a solid object (e.g. a groyne) or by the way a spit forms across a river mouth.
Rip currents can also carry large amounts of sand to offshore bars during wilder weather.
TidesTides play a more indirect role in transporting sand and altering the shape of sandy coastal areas. The ebb and flow of tidal waters can create tidal currents that transport sand and are mostly seen in areas where there is a narrow passage between landforms.
Tides play an important role in coastal estuaries where the regular inundation of low-lying areas often supports saltmarsh and wetland habitats which are important water filters and fish nursery areas.
In the lunar month, the highest tides occur roughly every 14 days, at the new and full moons, when the gravitational pull of the moon and the sun are in line. These are called spring tides (Tarbuck & Lutgens 1987).
On Australia's east coast, the highest tides occur during the winter months of June, July and August, and the summer months of December, January and February. The highest of each of these periods (i.e. one in winter and one in summer, totalling two per year) are known as the king tides.
The winter king tide usually occurs at night and may go unnoticed. Consequently the summer king tide, usually during the daytime, catches the most attention (Tarbuck & Lutgens 1987).
WindWind takes over as the main dune-forming process above the high tide mark. Wind picks up sand grains deposited by waves and moves them landwards until vegetation (or other obstructions) slows the wind, and the sand grains fall. Sand grains become trapped in vegetation and form dunes. Wind is also a cause of dune erosion and blowouts, often when vegetation has been disturbed and sand becomes mobile.
Storms can dramatically change beach, dune and bar systems in a very short period. Storms produce high energy and steep waves that can erode beaches, destroy foredunes and even erode the dunes behind. It often takes years after a storm before the original dune is restored; intervention may be required if public safety is at risk.
In some locations, 90% of the amount of sand moved by the annual longshore drift can occur in just two weeks during very stormy periods. A major storm or succession of storms is likely to occur every 20 years or so.
Shoreline Erosion and RecessionErosion is the wearing away of land or the removal of beach or dune sediments by wave action, tidal currents, wave currents, wind or drainage. Erosion may result in long-term losses of sediment and rocks, or merely the temporary redistribution of coastal sediments.
Recession is the result of ongoing erosion with no sediment replenishment, resulting in the landward retreat of the shoreline.
The impacts of sea level rise and climate change will result in more frequent and severe storm events and higher sea levels will increase the rate of coastal erosion and lead to coastal recession in many coastal areas.
Sediment budget refers to the balance between sediment added to and removed from a sandy coastal system by coastal processes. When more material is added than is removed, there is a surplus of sediment and the shore builds seaward. Coastal erosion occurs when coastal processes remove more material from the shore than is added.
The movement of sand and sediment in coastal areas is very important to the way humans use the coast. Sand can build up in unwanted areas (river channels) and be eroded from valued sandy landforms such as beaches. Understanding coastal processes is essential to ensuring that coastal works do not cause unwanted changes to sand movement such as increasing erosion (Page and Thorp 2010).
For more information see
, Tasmanian Coastal Works Manual.