Difference between revisions of "Dynamics, threats and management of dunes"
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[[Category: Coastal defence]]
[[Category: Coastal defence]]
Revision as of 12:12, 12 July 2012
Processes and mechanisms driving natural dynamics & ecosystem development
Coastal sand dunes are aeolian landforms, found along the majority of the world’s coast. 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 0.2-2.0 mm) is available to be transported by winds. The coastal dune system is composed of the 3 compartments: the submerged beach, the emerged beach and the dune. These 3 compartments under permanent exchanges must be considered as a whole. Coastal morphodynamic variability is caused by a variety of factors ranging from climate and climate variability, relative sea level, sediment supply, vegetation, and coastal dynamics at global, 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 storm and growing season variability (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 begin, 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. Finally, the third phase of preservation of sediments occurs when the dune system is stabilized either by the ground‐water table rising or the growth of vegetation (Clemmensen et al., 2001), or a change in other climatic factors (Tsoar, 2005). Vegetation is necessary to trap the sand in order to make for dune growing, but also to stabilize the ground. The pioneer species, by this action, will facilitate the establishment of other species (less tolerant of salinity, wind…), increasing biodiversity richness (flora and fauna). 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.
Vulnerability & threats
During the last thirty years, almost 75% of Mediterranean coastal dunes have been damaged or destroyed, principally by tourism consequences (Géhu, 1985; Salman & Strating, 1992; in Van Der Meulen & Salman, 1993). There are different kinds of destruction causes. First, the natural vents which are eroded by storms or/and sea level rise, 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 intensity and impact of the event (e.g. sea level rise, storm intensity). The taller dunes are more resistant to flooding but possibly more susceptible to erosion while the shorter dunes might be more vulnerable to flooding. In the next century, 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 wetness between winter and summer (GIECC 2001, 2007, in Vinchon et al., 2008). These changes will modify the coastal erosion and sea‐flooding hazards. Dune dynamics are driven by naturally occurring disturbances, which can be both common and recurrent. However when these disturbances increase in intensity or frequency or are removed there can be substantial alterations in community dynamics (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 might be compromised easily.
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 damages on 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 introduction 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 the natural regression process of the coastline induce the acceleration of the destruction of the dune vegetation (Araujo et al., 2002) ultimately leading to 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 defined as a set of conditions producing an acceleration of the erosion rhythm and system degradation. It is really important to not block natural processes which could destroy the system (Bodéré et al., 1991) and to take into consideration all the dune system with beach and foreshore. The natural dune- rebuilding process can take several years, and it may be desirable to rebuilding a storm-eroded dune quicker than natural processes O’Connell, 2008). Dune damaging accelerates sand transit towards inland and then this sand cannot nourish the beach anymore (Pinot, 1998). There are different ways to protect or restore dunes. Firstly, the protection of wildlife is important because fauna and flora are an integral part of dune system: vegetation stabilizes sand whereas fauna control plants growing and interactions. Sand dunes provide unique wildlife habitat. We must limit the trampling of visitors by paths and beach access setting up with fences. People walk through the dune because it is difficult to go on the sand. As they are looking for a hard surface to walk, we must provide them one. A marked pathway (by fences and why not with educative panels) is already dissuasion.
In case of dune landscape restoration, totally or partially destroyed, it could be necessary to stimulate natural vegetation regeneration by planting indigenous species, and preferably plants characteristic of the first step of dune colonisation: builder sand plants like Ammophila arenaria, or Elymus fractus because they permit sand stabilization and input. Moreover, it is fundamental to avoid invasive species and to limit them when there are already here.
Another action of dune rehabilitation could be sand input to ensure dune system dynamics. The nourishment of the lowest part of the white dune will restore a homogeneous altimetry of the dune barriers in order to make it less sensitive to natural aggressions (waves and marine wind), and to limit the risk of marine submersion. 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
According to Heurtefeux et al. (in press), there are different coastal management modes which have been used and are yet to be employed. In the last twenty years, four global approaches to manage coast have been developed (Figure 1). First, there is the approach by holding the line. It has persisted to be used. Traditionally, the goal is to protect developed area using hard structures (Klein et al., 2001). The “do the minimum” approach corresponds to the use of natural processes to reduce risks but permitting coast natural changes. Some of the techniques used with this approach attempt to limit rather than to stop coastal erosion and cliff’s retreat. The “do nothing“ approach, which is rare but can be found is one of the more famous cases of “do nothing” approach is the municipality of Happisburg, in the county of North Norfolk (UK). The storm waves reached the coast with important damages on the bottom of the cliff, the cliff fell with major impact on the houses which were totally destroyed. Do nothing is one of policy adopted when it is too late, when any decision has been thought before, when the cost benefit analysis shows than the defense front to the sea exceeds the value of the properties. Finally, the Managed Realignment (M.R.) approach is quite recent. Its definitions and its particularities will be presented below.
The M.R.’s aim is to avoid heavy structures to respect dune‐beach system and its intrinsic transfers in order to not damage ecosystem functionalities. It’s necessary to consider dune-beach system in its totality, and thus its natural capacity to return at an initial state after a perturbation.
There are many special features of Managed Realignment. One of these features is to move back the economic assets on the coast to the hinterland. It is also to create a new defense line behind the beach and facing the sea to restore natural areas and to create a buffer between the sea and the economic assets. Another feature is to avoid the construction of new economic assets in areas where they would be vulnerable (Heurtefeux et al., in press).
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