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In recent years, many coastal dunes in New Zealand have been planted with grasses and sedges in an effort to stabilise dunes and control coastal erosion. Describe the relationship between vegetation, wind flow and sand transport on a coastal dune, and discuss the effectiveness of dune planting as a technique for controlling coastal erosion.
Vegetation, wind-flow and sand transportation are all inter-dependant in the coastal dune environment. Air movement and not water movement form the coastal dune, unlike most coastal features, and is therefore quite unique (Pethick 1984). According to Pethick, the coastal dune is characterised by an “interaction between sand transport by the wind and vegetation cover.”(1984) Under both natural and human induced circumstances the dune can become unstable, and this can lead to coastal erosion. The plantation of vegetation such as Marram Grass can stabilise the dunes and control coastal erosion (Viles and Spencer 1995). Throughout the world this approach to dune stabilisation has been met with variable success, usually depending upon the condition of the dune, and how widespread the coastal erosion is.
Dune formation is a complex process, and is a good example of the interaction between wind, sand and vegetation. Wind-flow over a sandy surface such as a beach will experience friction with the sand, and therefore the wind speed will decrease closer to the surface. On a flat beach, the height of zero wind-speed is one-thirtieth of the grain diameter, with a typical diameter of one millimetre; this distance is very small indeed (Pethick 1984). The wind applies a force to sand grains lying above the zero wind-speed threshold, which can be increased either through an increase in the velocity of the wind, or through an increase in the roughness of the surface of the sand. A few loose grains of sand will be moved in this method, and will travel by a method of saltation.
Sand is transported in three different ways: saltation, suspension and surface-creep. Saltation is by far the most important of these methods, as 75% of sand is moved by saltation. Saltation in the process of sand grains being flicked up by the wind and carried forward a certain distance, before the grains return to the surface. A wind-speed of at least 5 metres per second is required to life the sand grains off the surface, while a wind-speed of at least 4.5 metres per second is required to keep the sand grains in the air (Pethick 1984). As the sand grains return to the ground they impact into the surface, this results in many more grains being flicked up into the air. Wind-flow and sand combine to produce sand transportation, a vital prerequisite for dune formation.
If an object is placed in the way of sand that is moving, then there is a chance that the sand will be deposited. Sand is dispersed when there are wind variations; this is because the object may create a lower wind-speed. If the wind is sufficiently low (lower than 4.5 metres per second) then the sand will not be lifted any more by the wind, and transportation will cease. Sand will pile up on each side of the object, resulting in a ridge parallel to the wind direction, and streamlined by the wind. This type of dune is called the shadow dune, and will cease to exist if the obstacle cannot keep up with the rate of deposition. If it does not, then the sand will bury the object, and a smooth surface will occur again.
Vegetation is such an object that can keep up with the rate of deposition; because it grows, vegetation can rise above the sand, and therefore continue to exert its influence upon the sand grains. But there are few plants that can survive the harsh environment on the upper slopes of the beach; Disphyma australe, the New Zealand ice plant, is an example of such a plant (Fisher et al. 1975). Therefore these embryo dunes resemble an unconnected series of low mounds, sparsely covered in vegetation, and about 1-2 metres in height. As the embryo dunes grow, they extent laterally and catch more sand as well as growing in height. Once the dunes have grown in height, the more traditional plants that are associated with coastal dunes, such as Marram Grass and Spinifex are found (Pethick 1984). The coverage of the dunes increases from intermitted plants, to a more complete cover that is far more effective in catching the sand grains.
The increased vegetation cover introduces a very marked roughness to the beach surface, without vegetation, the zone of zero wind-speed is from 0.002-1 centimetres in height. However, dense vegetation can increase this zone up to 18 centimetres in height (Pethick 1984). Vegetation interacts with the process of sand transportation to create this foredune; it does this by intercepting descending grains partway through their process of saltation. The leaves of the vegetation absorb much of the energy of the grains, as well as providing an obstruction to the grains moving forwards as wind transport is a thousand times less effective over a vegetated foredune than it is over the beach (Van de Var et al. 1989). Some grains can still rise above the zero wind-speed zone, but most will fail to do so and the dune will grow in size. Deposition in the foredune system can vary from 0.3-1.0 metres per year growth in their height (Pethick 1984). The foredune can therefor grow from the 2.0 metres that it was as an embryo dune, up to a 10 metre high ridge of dunes parallel to the coastline. Vegetation can also prevent the loss of sand during storm conditions by preventing wind erosion of the dunes; once again vegetation is interacting with the wind and sand to create and maintain the coastal dune environment (Weedon 1999).
In New Zealand, coastal erosion is a problem that has been well publicised, especially in the Omaha region north of Auckland. This is due to natural erosion being encouraged by a high-energy wave climate that is caused by prevailing westerly winds. Human impact on the coastline in New Zealand is quite high, with a heavy concentration of New Zealanders living within thirty kilometres of the coast due to the expansion of holiday communities in the last forty years. A lack of dune protection has led to coastal erosion in many of these cases because the dune provides a necessary buffer zone between the coast and the sea. If the balance of the dune is upset then the security of the entire coast is put into question as “fixed coastal dunes can be seen as a form of coastal defense” (Viles and Spencer 1995).
The beach and the coastal dune are interdependent; therefor the management of the beach must consider the foredune system. Damage to the foredunes through trampling or direct removal of the sand can threaten the balance of the coastal environment. In response to the problem various plants, especially Marram Grass, have been used to “facilitate a stabilisation of the dunes” (Viles and Spencer 1995). The plantation of dunes is meant to help the dune grow larger, and be more effective in its role of protecting the coast and is often used along with other methods of dune stabilisation such as fences, walkways to prevent trampling, and direct addition of sand.
The success or failure of dune plantation is difficult to assess, and can be variable according to the situation as well as other methods that are used to stabilise the dunes, or the coastline as a whole. The placement of groynes or boulders can also help replenish the sand on a beach to stabilise the coast, but they can be ugly and decrease the aesthetic value of an area such as a tourist resort. Plantation can aid this process by creating a dune ridge, although the ridge may not behave naturally upsetting the sand budget of the entire area.
Along areas of the Spanish coast such as Devasa del Saler, Valencia, dune plantation has been very successful in preventing coastal erosion. The dunes had been destroyed for a tourist resort between 1970 and 1973. Through the regeneration of the dunes over 16 years as a result of dune plantation, the coastline was stabilised. However this was in an area with a low energy dissipative beach, with a tidal range of less than fifty centimetres and was therefore the ideal situation for such a plantation; unfortunately the task is less clear and a lot more difficult in other areas experiencing coastal erosion. (Viles and Spencer 1995)
Dunes that have been damaged by localised, specific human impacts can be restored if they are limited to a certain spatial scale. But if the area becomes too large, or the wave energy is too high, there is generally little that dune plantation can do to protect the coastline. This is especially the case in areas where the coastal erosion is a natural process, and is only affecting people because they happen to live near the eroding coastline; because there is no imbalance in the system, it can be very difficult to address the problem. Coasts that are characterised by long term instability are also very difficult to manage (Viles and Spencer 1995).
Vegetation, wind flow and sand transport interact in a complex way to create, and then maintain, the coastal dune system. At all stages of dune building the interdependency of the three is clearly evident, and without any of them coastal dunes would not occur. Sand dune plantation to protect the coast from erosion has its merits in certain situations, however the variability of the coastal environment invariably means that there is no simple solution to the problem of coastal erosion.
BIBLIOGRAPHY:
Fisher, Muriel E., E. Satchell, Janet M. Watkins, Gardening with New Zealand Plants, Trees and Shrubs, Auckland, 1975.
Pethick, J., An Introduction to Coastal Geomorphology, London, 1984.
Strahler, A. and Strahler A., Physical Geography – Science and System of the Human Environment, New York, 1997.
Van de Ven, T.A.M., D.W. Fryrear and W.N. Spaan, Vegetation characteristics and soil loss by wind: Journal of Soil and water Conservation. vol. 44, 1989.
Viles, Heather and Tom Spence, Coastal Problems: Geomorphology, Ecology and Society at the Coast, London, 1995.
Weedon, Anita Marie, The Interaction of Wind and Complex Dune Formation at Muriwai Beach, Auckland, 1999.
