Dynamics, threats and management of dunes
Processes and mechanisms driving natural dynamics & ecosystem development
Coastal sand dunes are aeolian landforms, found along the majority of the world’s coasts. This ecosystem located at the spatial transition between terrestrial and marine environments can be found in coastal areas where a supply of sand‐sized material (within the size range 0.1-1.0 mm) is available to be transported by winds. The coastal dune system is composed of three compartments: the shoreface (submerged beach), the beach (intertidal and supratidal) and the dune. These three compartments, being under permanent exchanges, must be considered as a whole (Figure 1). Coastal morphodynamic variability is caused by a variety of factors ranging from wind and wave climate, tide, relative sea level, sediment supply, vegetation, and coastal dynamics at regional and local scales. Due to these factors, the coastal zone is a highly dynamic environment at temporal scales ranging from wind bursts and wave breaking (seconds to minutes), to storms and variability in wave and wind climate (days to seasons), to interannual climate variability and sea level fluctuations (years to centuries, etc). The genesis of an aeolian dune is divided in three phases (Clemmensen et al., 2001): (1) dune field formation; (2) accumulation of sediment deposits; and (3) preservation of the deposited sediments. In order for coastal dune formation to start, there must be both adequate sediment availability and sufficient wind energy capable of transporting this sand landward (Aagard et al., 2007). After the initial formation of the dune field deposits, accumulation of sand occurs when the influx of sediment is greater than the losses, creating a positive sediment budget. The third phase of preservation of sediments occurs when the dune system is stabilized either by rise of the ground‐water table, the growth of vegetation (Clemmensen et al., 2001), or reduction of wind stress (Tsoar, 2005). Vegetation is necessary to trap the sand for allowing dune growth, but also to stabilize the soil. In this way, pioneer species facilitate the establishment of other species (less tolerant of salinity, wind…), increasing biodiversity richness (flora and fauna), see Shore protection vegetation. A variety of factors affect the availability of sediment for dune formation, including changes in sea level, changes in sediment transport from continental and oceanic sources, and the presence of vegetation, as well as the impacts of human activities. In addition, the variability of the wind, both in the direction and magnitude, can play an important role in the mobilization and landward transport of sediment. The interactions between all of these forcing factors produce a variety of different environments in which dune formation may occur, as described in the article Dune development.
Vulnerability & threats
During the last thirty years, almost 75% of Mediterranean coastal dunes have been damaged or destroyed, principally by tourism (Géhu, 1985; Salman & Strating, 1992; in Van Der Meulen & Salman, 1993). Dune destruction can have different causes. For example, the presence of natural vents, which are eroded by storms, overwash and sea flooding events. The vulnerability of coastal dunes to flooding depends on the characteristics of the dune system itself: height, width, conservation status etc. It also depends on the characteristics of the event (e.g. sea level rise, duration and intensity of storms), see Dune erosion. Higher dunes are more resistant to flooding but possibly more susceptible to erosion while lower dunes might be more vulnerable to flooding. Climate change will lead to a rise of mean sea‐level, a likely increase of storms intensity and frequency and a more contrasted distribution of drought and precipitation between winter and summer. These changes will modify coastal erosion and sea‐flooding hazards (IPCC, 2014). Dune dynamics is driven by naturally occurring disturbances, which can be both common and incidental. However, when these disturbances increase in intensity or frequency or when they are reduced, substantial alterations in community dynamics can occur (Martinez and Psuty, 2004). Dunes are thought to be fragile because only a slight disruption (either natural or human induced) may lead to change and long‐term progressive alteration (Carter, 1988) and their natural diversity can easily be compromised.
For thousands of years, human activities have been impacting the coastal environment of the Mediterranean Basin through agriculture, husbandry and the deliberate use of fire. In recent decades, tourism has caused important damage to coastal landscapes with the urbanization of the coast, the increase of summer visitors, and the introduction of invasive or exotic species. The most heavily affected habitats are the sandy coastal systems, and coastal dunes in particular (Tzatzanis et al., 2003). The pedestrian and motorized pathways all over dunes lead to vegetation destruction and therefore enhanced weathering and erosion (Moulis and Barbel, 1999). Waste deposits and invasive species are also destruction factors. The potential for dune recovery is dependent on the sediment supply in each area and on the intensity of human impact. Dune plants are especially sensitive to disturbance and are heavily affected by humans. Without dune plants, the integrity and preservation of a stable dune complex cannot exist. Anthropogenic impacts combined with natural coastline retreat accelerate the destruction of the dune vegetation (Araujo et al., 2002), ultimately leading to dune destruction.
Key processes to focus on for maintaining ecosystems integrity
Damaged coastlines are not attractive locations for tourism or leisure. Dune system vulnerability is related to conditions that accelerate erosion and ecosystem degradation. The entire dune system with beach and shoreface must be taken into consideration, see Active coastal zone. Blocking natural processes eventually leads to destruction of the dune system (Bodéré et al., 1991). The natural dune-rebuilding process can take several years. In order to withstand rapidly succeeding storms, it may be desirable to restore a storm-eroded dune rather than relying on natural recovery processes (O’Connell, 2008). Damage of the dune system accelerates sand transit inland; this sand cannot nourish the beach anymore.
There are different ways to protect or restore dunes. The protection of wildlife is important because fauna and flora are an integral part of the dune system: vegetation stabilizes sand, whereas fauna controls plant growth and interactions. Sand dunes provide a unique wildlife habitat. The trampling of visitors must be limited by installing fences. People walk through the dune because walking in the sand is difficult. Since they are looking for a hard surface to walk on, hard paths must be built. A marked path (through fences and with educational panels) is already a discouragement.
For restoration of a totally or partially destroyed dune landscape, it can be necessary to stimulate natural vegetation regeneration by planting indigenous species, and preferably plants characteristic for the first stage of dune colonisation: sand-trapping plants like Ammophila arenaria, or Elymus fractus because they permit sand retention by their stems and sand stabilization by their roots. Moreover, it is crucial to avoid invasive species and to limit them when they are already here. For further details, see Shore protection vegetation.
Another action of dune rehabilitation could be artificial sand supply to ensure dune system dynamics. The nourishment of the lowest part of the white dune can restore the natural topography of the dune belt in order to make it less sensitive to natural aggressions (waves and marine wind), and to limit the risk of marine submersion, see Shore nourishment. This must be adapted to the morphology of the dune with consideration for the sensitivity of the natural environment (Heurtefeux et al., 2007).
Current management practises
Four coastal management approaches are generally distinguished (Heurtefeux et al., 2011, Figure 2), see also Shoreline management. Most usual is the so-called 'holding the line' approach. The goal is to protect developed areas, traditionally by using hard structures (Klein et al., 2001). The “do the minimum” approach corresponds to the use of natural processes to reduce risks but permitting natural coastal evolution. Some of the techniques used with this approach attempt to limit rather than to stop coastal erosion and cliff retreat. The “do nothing“ approach is seldom put in practice. One of the well-known examples of the “do nothing” approach is the municipality of Happisburg, in the county of North Norfolk (UK). Storm waves attacking the cliff base caused the cliff to collapse; houses on the cliff were totally destroyed. Do nothing is one of the policies adopted when it is too late to intervene or when a cost-benefit analysis shows that coastal defense works exceed the value of the properties. Finally, the Managed Realignment (M.R.) approach, shortly introduced below, is practiced more recently.
The purpose of the M.R. is to avoid heavy structures in order to respect the natural dynamics of the dune-beach system and to preserve ecosystem functionalities. The natural ability to restore an original state after a disturbance requires that the dune-beach system be considered in its entirety.
Managed Realignment involves different types of measures. One of these measures is to move the economic assets on the coast to the hinterland. This makes it possible to build a new line of defense behind the beach, restore natural areas by the sea and create a buffer between the sea and the economic assets. Another measure is to ban building in vulnerable coastal areas.
- Dune development
- Dune erosion
- Dune stabilisation
- Shore protection vegetation
- Sand Dunes in Europe
- Shoreline management
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