Coastal beaches and dunes begin with the accumulation of marine sand that is transported to the coast by waves and currents. In the case of dunes, sand is subsequently reworked by strong onshore winds (greater than 5 m/sec ) and then deposited behind the beach, principally being trapped in vegetation, but also accumulating at the base of cliffs and hills.
In Australia there is a variety of coastal dune landforms. They range from relatively small shore-parallel foredunes that sit immediately behind the beach, tens of metres wide and a few metres thick (Photo 1); to a series of relict foredunes that may extend hundreds of metres to a few kilometres inland; to large dune fields comprising transgressive or parabolic dunes that can extend hundreds of metres to several kilometres inland (Photo 2). Dune fields comprise a variety of surface dune types (e.g. transverse dunes), as well as hills and ridges that can rise a few hundred metres above the shoreline and represent long-term accumulation of large volumes of sand.
Photo 1. An incipient foredune and established foredune (Mahia Peninsula, NZ). Photo by Patrick Hesp.
Photo 2. An active parabolic dune (300 m wide) at Dongara, WA. Photo by S. Chape used with the permission of Patrick Hesp.
Communities of plants that grow on beaches and dunes are known as dune vegetation. There are usually three main zones of dune vegetation that are arranged roughly parallel to the coastline. The zones reflect changes in the nutrient status and moisture content of dune soils, which increase in a landward direction, and changes in the degree of exposure to strong winds, salt spray and sandblast, which decrease in a landward direction . The zones consist of:
The loss of dune vegetation is a major trigger for dune erosion. Dune vegetation traps windblown sand and holds it on the foredunes . The exposed, dry sand is easily mobilised by high-velocity winds and large volumes of sand can be rapidly transported, sometimes forming large depressions in the dunes termed blowouts (Photo 3). Downwind from blowouts, drifting sand can smother the surrounding vegetation and cover roads and properties (Photo 4) . Dune vegetation also contains many native species and is valued as habitat and for its own intrinsic biodiversity. Beaches and dunes are important feeding, breeding and roosting grounds for sea turtles and shorebirds .
Photo 3. A blowout in the crest of a foredune at Hawks Nest, NSW. Photo by Patrick Hesp.
Photo 4. The downwind margin of a transgressive dunefield in NZ. Photo by Patrick Hesp.
Dunes serve as natural buffers, protecting the land from storm tides and waves of short duration, and wind. Beaches are also an important aspect of the Australian lifestyle, and have cultural significance for Indigenous people . Some coastal dune deposits also have scientific values because they contain relatively detailed records of present, Holocene and earlier interglacial sea level high stands that can be related to global climatic change [6,7,8]. Economically, beach and dune systems are important for the coastal tourism industry and represent an important sand resource for the construction industry and beach replenishment works. They may also contain valuable heavy-mineral sand deposits.
The erosion of beaches and dunes, especially foredunes, may be a natural process and is often balanced by the supply of sand from the nearshore continental shelf to beaches by currents and waves [3,4]. In some cases, sand from adjacent dunes may replenish beach systems during erosional periods . Listed below are some human activities that have modified dunes and that can induce dune erosion, which may accelerate beach erosion.
Photo 5. Recreational use of beaches and dunes at Dark Point in Myall Lakes National Park: (a) 4WD vehicles on beach; and (b) tobogganing down a dune (photos by Caroline Wenger).
Photo 6. Bitou bush humps at Dark Point in Myall Lakes National Park (Photo by Caroline Wenger). Bitou bush (Chrysanthemoides monilifera) is a weed of national significance. The South African native was planted along the coast in NSW between 1946 and 1968 to reduce dune erosion. The weed has since spread inland by up to 10 km and is now found along 80% of the coastline. The species can out-compete and replace native flora, destroying the food source for native animals and creating an environment that is favourable for colonisation by other highly invasive weeds. More information
Coastal areas that comprise unconsolidated sediments are typically dynamic systems, and their responses to disturbance may be difficult to predict. However, some beach dune systems may be considered more at risk to erosion after human disturbance. These include prograded or stationary barriers on coastlines with either a low shelf-sediment supply, little 'protection' offered by rocky headlands, or direct exposure to strong wave energy or longshore drift currents .
Reducing the disturbance of dunes is an obvious method of controlling erosion problems. Land-use planning needs to ensure buildings and infrastructure are positioned behind the dune system. Land conservation groups recommend the careful retention of vegetation in sensitive areas and revegetation activities in cleared areas. This needs to be combined with the restriction of vehicular and human traffic .
Beach and unvegetated dune areas can be estimated from aerial photography or satellite imagery at 1:25 000 scale .
The cost of beach rehabilitation can be estimated from the number and dollar costs of beach nourishment programs and the number and dollar costs of walls, groynes and ramparts .
Special-purpose studies have been undertaken in most states and have created a baseline of historical data. However, different methods may create problems for inter-comparisons between studies.
The the Department of Environment, Water, Heritage and the Arts website provides guidelines for monitoring and State of Environment Reporting of beach and dune indicators.
1. Short, A.D., Trembanis, A.C. and Turner, I.L. 2000. Beach oscillation, rotation and the Southern Oscillation, Narrabeen Beach, Australia. Coastal Engineering, 3, 2439-2452.
Brendan Brooke, Geoscience Australia