Difference between revisions of "Dynamics, threats and management of dunes"

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UNDER CONSTRUCTION
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==Processes and mechanisms driving natural dynamics & ecosystem development==
 
==Processes and mechanisms driving natural dynamics & ecosystem development==
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[[Image: sand dunes.JPG|thumb|center|350px| Figure 1: Dune section.]]
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Coastal sand [[dunes]] are aeolian landforms, found along the majority of the world’s [[coast]]s. 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, [[Shore protection vegetation|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<ref name= "Clem" >CLEMMENSEN L. B., PYE K., MURRAY A., and HEINEMEIER J., 2001. Sedimentology, stratigraphy, and landscape evolution of a Holocene coastal dune system, Lodbjerg, NW Jutland, Denmark. ''Sedimentology'', '''48''': 3-27.</ref>): (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<ref>AAGARD T., ORFORD J., and MURRAY A.S., 2007. Environmental controls on coastal dune formation; Skallingen Spit, Denmark. ''Geomorphology''. '''83''', 29-47.</ref>). 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<ref name= "Clem"/>), or reduction of wind stress (Tsoar, 2005<ref>TSOAR H., 2005. Sand dunes mobility and stability in relation to climate. ''Physica A'', '''357''': 50‐56. </ref>). 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]].
  
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 seasonal 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<ref name= "Clem" >CLEMMENSEN L. B., PYE K., MURRAY A., and HEINEMEIER J., 2001. Sedimentology, stratigraphy, and landscape evolution of a Holocene coastal dune system, Lodbjerg, NW Jutland, Denmark. ''Sedimentology'', '''48''': 3-27.</ref>): (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<ref>AAGARD T., ORFORD J., and MURRAY A.S., 2007. Environmental controls on coastal dune formation; Skallingen Spit, Denmark. Geomorphology, ''83'': '''29-47'''.</ref>). 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<ref name= "Clem"/>), or a change in other climatic factors (Tsoar, 2005<ref>TSOAR H., 2005. Sand dunes mobility and stability in relation to climate. ''Physica A'', '''357''': 50‐56. </ref>). 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==
 
==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<ref>Salman, Strating, 1992. '''In:''' VAN DER MEULEN F. And SALMAN A.H.P.M., 1993. Gestion des dunes côtières de Méditerranée. ''The first International Conference on the Mediterranean Coastal Environment''. 167‐183.</ref>). 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<ref>GIECC 2001, 2007. '''In:''' VINCHON C., BALOUIN Y., IDIER D., GARCIN M., MALLET C., 2008. La réponse du trait de côte au changement climatique: Evolution des risqué côtiers en Aquitaine et en Languedoc‐Roussillon dans le siècle à venir. The littoral : challenge, dialogue, action. Lille-France. 11pp.</ref>). 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 when they are removed, there can be substantial alterations in community dynamics (Martinez and Psuty, 2004<ref>MARTINEZ M.L. and PSUTY N.P., 2004. Coastal Dunes.  Ecology and Conservation. ''Ecological Studies''. '''171''', 386pp. Available from: [http://www.vliz.be/imis/imis.php?module=ref&refid=70677]</ref>). 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<ref>CARTER R.W.G., 1988. Coastal environments. An introduction to the physical, ecological and cultural systems of the coastlines. Academic Press, New York.617pp. Available from:[http://www.vliz.be/imis/imis.php?module=ref&refid=11046]</ref>) and their natural diversity might be compromised easily.</br>
 
  
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<ref>TZATZANIS M., WRBKA T., SAUBERER N., 2003. Landscape and vegetation responses to human impact in sandy coasts of Western Crete, Greece. J. Nat. Conserv. '''11''', 187‐195.</ref>). The pedestrian and motorized pathways all over dunes lead to vegetation destruction and therefore enhanced weathering and erosion (Moulis and Barbel, 1999<ref>MOULIS D., and BARBEL P., 1999. Restauration des dunes. Réhabilitation et gestion des dunes littorals Méditerranéennes Françaises. Collection: Manuels et Méthodes. BRGM Ed., 75-91.</ref>). 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<ref>ARAUJO R., HONORADO J., GRANJA H. M., NEVES DE PINTO S., BARRETO CALDA F., 2002. Vegetation complexes of coastal sand dunes as an evaluation instrument of geomorphologic changes in the coastline. Littoral 2002, The changing Coast: 337-339. EUROCOAST/EUCC, Porto-Portugal : 337‐339.</ref>) ultimately leading to to dune destruction.
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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<ref>Salman, Strating, 1992. '''In:''' VAN DER MEULEN F. and SALMAN A.H.P.M., 1993. Gestion des dunes côtières de Méditerranée. ''The first International Conference on the Mediterranean Coastal Environment''. 167‐183.</ref>). 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 [[Sea level rise|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<ref>WONG P.P., LOSADA I.J., GATTUSO J.-P,. HINKEL J. , KHATTABI A. , MCINNES K.L., SAITO Y., and SALLENGER A., 2014. Coastal systems and low-lying areas. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 361-409.</ref>). 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<ref>MARTINEZ M.L. and PSUTY N.P., 2004. Coastal Dunes.  Ecology and Conservation. ''Ecological Studies''. '''171''', 386pp.</ref>). 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<ref>CARTER R.W.G., 1988. Coastal environments. An introduction to the physical, ecological and cultural systems of the coastlines. Academic Press, New York.617pp.</ref>) and their natural diversity can easily be compromised.</br>
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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<ref>TZATZANIS M., WRBKA T., SAUBERER N., 2003. Landscape and vegetation responses to human impact in sandy coasts of Western Crete, Greece. J. Nat. Conserv. '''11''', 187‐195.</ref>). The pedestrian and motorized pathways all over dunes lead to vegetation destruction and therefore enhanced weathering and erosion (Moulis and Barbel, 1999<ref>MOULIS D., and BARBEL P., 1999. Restauration des dunes. Réhabilitation et gestion des dunes littorales Méditerranéennes Françaises. Collection: Manuels et Méthodes. BRGM Ed., 75-91.</ref>). 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<ref>ARAUJO R., HONORADO J., GRANJA H. M., NEVES DE PINTO S., BARRETO CALDA F., 2002. Vegetation complexes of coastal sand dunes as an evaluation instrument of geomorphologic changes in the coastline. Littoral 2002, The changing Coast: 337-339. EUROCOAST/EUCC, Porto-Portugal : 337‐339.</ref>), ultimately leading to dune destruction.
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==Key processes to focus on for maintaining ecosystems integrity==
 
==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<ref>BODERE J.C., CRIBB R., CURR R., DAVIES P., HALLEGOUET B., MEUR C., PIRIOU N., WILLIAMS A., YONI C., 1991. La gestion des milieux dunaires littoraux. Evaluation de leur vulnérabilité à partir d’une liste de contrôle. Etude de cas dans le sud du Pays de Galles et en Bretagne occidentale. Norois n°151. Poitiers-France. 279‐298.</ref>) and to take into consideration the entire 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 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 plant growth and interactions. Sand dunes provide a 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 a dissuasion.</br>
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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<ref>BODERE J.C., CRIBB R., CURR R., DAVIES P., HALLEGOUET B., MEUR C., PIRIOU N., WILLIAMS A., YONI C., 1991. La gestion des milieux dunaires littoraux. Evaluation de leur vulnérabilité à partir d’une liste de contrôle. Etude de cas dans le sud du Pays de Galles et en Bretagne occidentale. Norois n°151. Poitiers-France. 279‐298.</ref>). 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<ref>O'CONNELL J., 2008. Coastal Dune Protection & Restoration. Marine Extension Bulletin Woods Hole Sea Grant & Cape Cod Cooperative Extension.</ref>). Damage of the dune system accelerates sand transit inland; this sand cannot nourish the beach anymore.  
  
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 for the first step of dune colonisation: builder sand plants like ''[http://www.marinespecies.org/aphia.php?p=taxdetails&id=403902 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.</br>
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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.</br>
  
The protection of implanted root native vegetation against wind erosion with weed permeable fences (“ganivelles”) and biodegradable geotextiles (Heurtefeux ''et al.'', 2007)<ref name= "Heurt">HEURTEFEUX H., GROSSET S., RICHARD P., SIRE E., 2007. Restoring a highly damaged site: Canet-en-Roussillon (Western French Mediterranean coast). ''ICCD'', 2007.  Montpellier-France. 7pp.</ref><ref name= "Heurt2">HEURTEFEUX H., GROSSET S., VALANTIN P.‐Y., 2007. Une approche alternative de la gestion des risqué côtiers, l’exemple de la Petite Camargue. Territoires en movement 2007‐1. Les risqué côtiers. 11pp.</ref> is also a priority.</br>
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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 ''[http://www.marinespecies.org/aphia.php?p=taxdetails&id=403902 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]].<br>
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The protection of planted native vegetation against wind erosion with weed permeable fences (“ganivelles”) and biodegradable geotextiles (Heurtefeux ''et al.'', 2007)<ref name= "Heurt">HEURTEFEUX H., GROSSET S., RICHARD P., SIRE E., 2007. Restoring a highly damaged site: Canet-en-Roussillon (Western French Mediterranean coast). ''ICCD'', 2007.  Montpellier-France. 7pp.</ref><ref name= "Heurt2">HEURTEFEUX H., GROSSET S., VALANTIN P.‐Y., 2007. Une approche alternative de la gestion des risqué côtiers, l’exemple de la Petite Camargue. Territoires en movement 2007‐1. Les risqué côtiers. 11pp.</ref> is also a priority.</br>
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Another action of dune [[Ecosystem rehabilitation|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<ref name= "Heurt"/><ref name= "Heurt2"/>).</br>  
  
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<ref name= "Heurt"/><ref name= "Heurt2"/>).</br>
 
  
[[Image: Policy options.JPG|thumb|right|250px| Figure 1: Policy options for coastal management (European Commission, 2004 ; in Heurtefeux ''et al.'', in press<ref name= "in press">HEURTEFEUX H., SAUBOUAP., LANZELLOTTIP., BICHOT A., 2011, in press. Coastal risk management modes: The managed realignment as a risks conception more integrated. Montpellier-France.</ref>]]
 
 
==Current management practises==
 
==Current management practises==
According to Heurtefeux ''et al.'' (in press)<ref name= "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 coasts 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 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 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 the policies adopted when it is too late, when any decision has been thought of before, when the cost benefit analysis shows than the defence 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.</br>
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[[Image: Policy options.JPG|thumb|right|350px| Figure 2: Policy options for coastal management (European Commission, 2004)]]
  
The M.R.’s aim is to avoid heavy structures to respect dune‐beach system and its intrinsic transfers in order not to 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.</br>
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Four coastal management approaches are generally distinguished (Heurtefeux ''et al.'', 2011<ref name=H11>Heurtefeux, H., Sauboua, P., Lanzellotti, P., and Bichot, A. 2011. Coastal Risk Management Modes: The Managed Realignment as a Risk Conception More Integrated. 10.5772/16804.</ref>,  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<ref>KLEIN R.J.T., NICHOLLS R.J., RAGOONADEN S., CAPOBIANCO M., ASTON J., and BUCKLEY E.N., 2001. Technological options for adaptation to climate change in coastal zones. Journal of Coastal Research ''17 (3)'', 531–543.</ref>). 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. </br>
  
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 defence 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<ref name= "in press"/>).</br>
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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.</br>
  
 +
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<ref name=H11/>.
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{{THESEUS}}
 
==See also==
 
==See also==
[http://www.theseusproject.eu/index.php?option=com_remository&Itemid=2&func=select&id=41 Theseus Official Deliverable 3.3-Natural habitats for coastal protection and relevant multi-stressor coastal risks. Report and European Scale overview.]
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* [[Dune development]]
 
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* [[Dune erosion]]
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* [[Dune stabilisation]]
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* [[Shore protection vegetation]]
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* [[Sand Dunes in Europe]]
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* [[Shoreline management]]
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==External link==
 +
[https://www.environment.nsw.gov.au/resources/coasts/coastal-dune-mngt-manual.pdf COASTAL DUNE MANAGEMENT A Manual of Coastal Dune Management and Rehabilitation Techniques]  
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==References==
 
==References==
 
<references/></br>
 
<references/></br>
  
[[Category: Coastal defence]]
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[[Category: Biodiversity and habitat loss]]
 
  
 
{{ 4Authors
 
{{ 4Authors
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|AuthorFullName4= Grosset, Stéphanie
 
|AuthorFullName4= Grosset, Stéphanie
 
}}
 
}}
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[[Category:Coastal protection]]
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[[Category:Soft coastal interventions]]
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[[Category:Physical coastal and marine processes]]
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[[Category:Beaches]]

Latest revision as of 11:23, 8 October 2021

Processes and mechanisms driving natural dynamics & ecosystem development

Figure 1: Dune section.

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]): (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[2]). 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[1]), or reduction of wind stress (Tsoar, 2005[3]). 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[4]). 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[5]). 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[6]). 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[7]) 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[8]). The pedestrian and motorized pathways all over dunes lead to vegetation destruction and therefore enhanced weathering and erosion (Moulis and Barbel, 1999[9]). 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[10]), 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[11]). 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[12]). 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.

The protection of planted native vegetation against wind erosion with weed permeable fences (“ganivelles”) and biodegradable geotextiles (Heurtefeux et al., 2007)[13][14] is also a priority.

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[13][14]).


Current management practises

Figure 2: Policy options for coastal management (European Commission, 2004)

Four coastal management approaches are generally distinguished (Heurtefeux et al., 2011[15], 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[16]). 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[15].


Theseus.jpg

See also


External link

COASTAL DUNE MANAGEMENT A Manual of Coastal Dune Management and Rehabilitation Techniques

References

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  2. AAGARD T., ORFORD J., and MURRAY A.S., 2007. Environmental controls on coastal dune formation; Skallingen Spit, Denmark. Geomorphology. 83, 29-47.
  3. TSOAR H., 2005. Sand dunes mobility and stability in relation to climate. Physica A, 357: 50‐56.
  4. Salman, Strating, 1992. In: VAN DER MEULEN F. and SALMAN A.H.P.M., 1993. Gestion des dunes côtières de Méditerranée. The first International Conference on the Mediterranean Coastal Environment. 167‐183.
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  7. CARTER R.W.G., 1988. Coastal environments. An introduction to the physical, ecological and cultural systems of the coastlines. Academic Press, New York.617pp.
  8. TZATZANIS M., WRBKA T., SAUBERER N., 2003. Landscape and vegetation responses to human impact in sandy coasts of Western Crete, Greece. J. Nat. Conserv. 11, 187‐195.
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  10. ARAUJO R., HONORADO J., GRANJA H. M., NEVES DE PINTO S., BARRETO CALDA F., 2002. Vegetation complexes of coastal sand dunes as an evaluation instrument of geomorphologic changes in the coastline. Littoral 2002, The changing Coast: 337-339. EUROCOAST/EUCC, Porto-Portugal : 337‐339.
  11. BODERE J.C., CRIBB R., CURR R., DAVIES P., HALLEGOUET B., MEUR C., PIRIOU N., WILLIAMS A., YONI C., 1991. La gestion des milieux dunaires littoraux. Evaluation de leur vulnérabilité à partir d’une liste de contrôle. Etude de cas dans le sud du Pays de Galles et en Bretagne occidentale. Norois n°151. Poitiers-France. 279‐298.
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  13. 13.0 13.1 HEURTEFEUX H., GROSSET S., RICHARD P., SIRE E., 2007. Restoring a highly damaged site: Canet-en-Roussillon (Western French Mediterranean coast). ICCD, 2007. Montpellier-France. 7pp.
  14. 14.0 14.1 HEURTEFEUX H., GROSSET S., VALANTIN P.‐Y., 2007. Une approche alternative de la gestion des risqué côtiers, l’exemple de la Petite Camargue. Territoires en movement 2007‐1. Les risqué côtiers. 11pp.
  15. 15.0 15.1 Heurtefeux, H., Sauboua, P., Lanzellotti, P., and Bichot, A. 2011. Coastal Risk Management Modes: The Managed Realignment as a Risk Conception More Integrated. 10.5772/16804.
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The main authors of this article are Heurtefeux, Hugues, Milor, Mercedes, Bichot, Amandine and Grosset, Stéphanie
Please note that others may also have edited the contents of this article.