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New South Wales Government
The major causes of coastal erosion are the waves and elevated water levels associated with storms. Wind also promotes coastal erosion in the form of windborne sediment transport. Although such erosion is slower and less dramatic than the effects of storm waves, the inexorable inland march of migrating or "transgressive" sand dunes can smother coastal developments. "Blowouts" on coastal dunes can result in vegetation loss, potential dune migration, reduced amenity and the loss of sand from the beach system. (see Figure B9.1)
There are three distinct modes whereby the wind can transport non-cohesive sediments or "sand" in a downwind direction. These are "suspension", "saltation" and "traction", and are depicted in Figure B9.2.
Suspension refers to the incorporation of fine grains into the atmosphere itself (silt sized particles or smaller). Saltation is the process whereby larger grains (sand sizes) are briefly brought into suspension before falling back to the surface, thereby moving downwind in a series of "hops". Traction refers to the rolling, sliding and pushing of larger particles along the surface.
Saltation is the most significant form of transport for beach sand. The particles most readily moved by wind have diameters of 0.1 to 0.2mm (fine sand). A threshold wind velocity is necessary to initiate sand movement. Typically this is of the order of 20 km/h. Above this threshold velocity, transport varies as the cube of wind velocity.
Sand is transported in a variety of directions in response to changes in wind direction. Where adequate wind data are available, it is possible to estimate potential sand drift. This information can be depicted in the form of a "sand creep" diagram which shows the compass sector from whence the sand originates. Over the course of a year, a number of sand creep movements cancel out. The "sand drift" vector shows the net result of all creep movements and represents the long term direction and amount of potential sand drift.
Figure B9.3 shows potential sand creep diagrams and potential sand drift vectors for selected locations along the New South Wales coast. These figures reflect the effects of average annual wind speed and direction, and represent potential sand movement from a bare sand surface (i.e. not vegetated).
The actual sand drift at a specific location, as opposed to potential drift, depends upon the degree of surface protection. Dense ground cover suppresses sand movement. Shrubs spaced at intervals of three to five times their height significantly inhibit sand drift. Thus, actual sand movement will be less than or equal to the potential values shown in Figure B9.3.
With regard to potential sand creep, it is apparent that Gabo Island, Newcastle, Cape Byron and Cape Moreton are the most active locations. Along the South and Central Coast Sectors, potential sand drift is offshore (easterly) or along shore (north-easterly). In contrast, in the Mid-North and North Coast sectors, potential sand drift is onshore (north-westerly).
A natural beach will often have a dune on the landward side of the beach berm. Dunes are formed by the action of onshore winds. Sand deposited on the beach by surf zone processes is carried inland by the wind. Topographic features and vegetation interfere with wind transport and cause deposition in the lee of obstacles. In this way an "incipient dune" is initiated. The shielding effects of this dune promote further deposition and so the incipient dune grows in size. As the dune grows, a "slip face" on the leeside develops and the ability of the wind to transport sand up and over the dune is progressively reduced. In time, a "dune system" will be created, consisting of an incipient dune, a foredune and hind dunes (see Figure B9.4). The growth of sand dunes is described in several Government publications (PWD, 1986; SCS, 1990).
The size and character of sand dunes are governed by the shape and size of the beach embayment, its orientation to the prevailing wind and wave climates, the grain size and amount of sand available or supplied to the beach, and the type and state of dune vegetation.
Dune vegetation is the primary factor determining the stability of a sand dune. A graduation of primary to secondary to tertiary species occurs from the incipient dune to hind dune areas (see appendix B9 and Figure B9.4). The vegetation canopy provides "aerodynamic" protection to underlying species from salt laden winds.
In many instances, natural dune vegetation is quite fragile. Significant damage can lead to total degradation and loss of the protective vegetation cover. This in turn leads to dune "blowout" and migrating dunes (see Appendix B8). The importance of dune vegetation and the various types of plants relevant to dune management are discussed in government publications (PWD, 1986; SCS, 1990), and in Appendix B8.
A stable incipient dune and foredune provide a natural buffer against storm wave attack. On the New South Wales coast, it is quite normal for the incipient dune to be eroded away by wave attack every five years or so. Further, the foredune may suffer significant erosion every 10 years or so.
During a storm, sand stored in the dunes is mobilised by surf zone processes and becomes part of the sediment budget of the beach (see Appendix B7). Given that the sediment budget is balanced, restoration of the prestorm dune occurs by natural processes in the months, or perhaps years, following the storm event.
Wave erosion of the dune produces a pronounced face ("dune escarpment") which effectively reflects wave energy back to sea. Initially, the dune scarp is near vertical. As the sand dries out, the scarp slumps back to the natural angle of repose (a slope of about 1V:1.5H).
Bagnold, R.A., (1941). "The Physics of Blown Sand and Desert Dunes". (Methuen, London, 1941).
Belly, P.Y., (1964). "Sand Movement by Wind". US Army Coastal Engineering Research Centre, TM No.1, 80pp.
Berg, N.H., (1983). "Field Evaluation of Some Sand Transport Models". Earth Surface Processes and Landforms, Vol. 8, pp. 101-114.
Chepil, W.S., (1945a). "Influence of Moisture on Erodibility of Soil by Wind". Proc. Soil Science Society of America, Vol. 20, pp. 288-292.
Chepil, W.S., (1945b). "Dynamics of Wind Erosion I - Nature of Movement of Soil by Wind". Soil Science, Vol. 60, pp. 305-320.
Chepil, W.S., (1945c). "Dynamics of Wind Erosion II - Initiation of Soil Movement". Soil Science, Vol. 60 (No. 5), pp. 397-411.
Chepil, W.S., (1945d). "Dynamics of Wind Erosion III - The Transport Capacity of the Wind". Soil Science, Vol. 60, pp. 475-480.
Chepil, W.S., (1959). "Equilibrium of Soil Grains at the Threshold of Movement by Wind". Proc. Soil Science Society of America, 1959, pp. 422-428.
Greeley, R. and Iversen, J.D., (1985). "Wind as a Geological Process". (Cambridge Univ. Press, New York, 1985).
Horikawa, D. and Shen, H.W., (1960). "Sand Movement by Wind Action - on the Characteristics of Sand Traps". Beach Erosion Board Technical Memorandum, 119.
Horikawa, K., Hotta, S., Kubota, S. and Katori, S., (1984). "Field Measurement of Blown Sand Transport Rate by Trench Trap". Civ. Eng. in Japan, Vol. 27, pp. 213-232.
Hsu, S.A., (1971). "Wind Stress Criteria in Aeolian Sand Transport". J. Geophysical Research, Vol. 76, pp. 8684-8686.
Hsu, S.A., (1973). "Computing Aeolian Sand Transport from Shear Velocity Measurements." J. Geology, Vol. 81, pp. 739-743.
Kadib, A., (1964). "Calculation Procedure for Sand Transport by Wind on Natural Beaches". Misc. paper 2-64, U.S. Army Coastal Engineering Research Center.
Lee, J.A., (1987). "A Field Experiment on the Role of Small Scale Wind Gustiness in Aeolian Sand Transport". Earth Surface Processes and Landforms, Vol. 12, pp. 331-335.
Lettau, K. and Lettau, H.H., (1978). "Experimental and Micro-meteorological Field Studies of Dune Migration". In H.H. Lettau and K. Lettau, editors, Exploring the Worlds Driest Climate, University of Wisconsin-Madison, Institute for Environmental Studies, IES Report 101, pp. 110-147.
PWD & SCS (1986). "Beach Dunes - Their Use and Management". Prepared by the Department of Public Works and the Soil Conservation Service of New South Wales, 1986.
SCS, (1990). "Coastal Dune Management: A Manual of Coastal Dune Management and Rehabilitation Techniques", ed. P.A. Conacher et. al. Prepared by the Soil Conservation Service of New South Wales, 1990.
Svasek, J.N. and Terwindt, J.H.J., (1974). "Measurements of Sand Transport by Wind on a Natural Beach". Sedimentology, Vol. 21, pp. 311-322.