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The Driving Factors of Coastal Evolution Definitions and Context

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The Driving Factors of Coastal Evolution Definitions and Context 12/07/2013 Definitions and context Bourail, New Caledonia, M.Garcin 2012 o The coast is the interface between ocean and lands in consequence the The driving factors of coastal coast is under the influences of • marine processes Woodroffe 2003 • continental processes evolution • human communities Toward a systemic approach o The coast evolves at different nested timescales : from geological time scale M. Garcin With contribution from (pluri‐millennia), historical time scale G. Le Cozannet New Caledonia, M.Garcin 2012 (centuries, decadal), event time scale (few hours to few days) to physical process (hours, minutes, seconds…) e.g. Woodroffe 2003, Coco & Murray 2007 Coastal mobility a response to forcing factors • Climate change does not only Raiatéa, French Polynesia, M.garcin 2006 affect sea level but others parameters like : • If, at least a marine or a continental variable – air and sea temperatures, evolves, the coast tempts to adapts to this new – atmospheric and oceanic circulations etc. Coastal retreat condition – all these parameters affect the CL0 CL1 ocean, as well as lands and SL0 : Sea level at t0 • Characterization of the pluri‐decadal coastal SL1 : Sea level at T1 SL1 societies…and thus the coasts Sea Level Rise CL0 : Coastline at T0 mobility is relatively easy, but identifying the SL0 CL1 : Coastline at T1 cause(s) of this mobility is not trivial • Impact of CC to coastline change can’t be reduced to a simple • The present question is: what is (and will be) the mechanistic projection of sea level impact of climatic sea level rise on coasts ? landward The question is: Why it’s impossible to directly assign a new coastline to a given variation of the sea level? Other External geodynamics processes Biological processes Anthropogenic Coastal Climate change actions and changes impacts Forcing Factor: The Climate The 5 main families of forcing factors Internal geodynamics processes Erosion, accretion, • Coastal changes is transport... driven by 5 main Other External • Evolution of climatic parameters geodynamics categories of factors : processes are spatially highly variable – Climate Temperature, – Sea level Sea Level, Biological – Others external winds, processes – Temperature storms, geodynamics processes pluviometry... – Salinity and acidity of ocean etc. – Internal geodynamics Coastal Anthropogenic processes Climate change actions and • Others climatic parameters and changes impacts – Biological processes Tectonic, vertical Sea defences, processes acting on the coastal movements, gravels – Anthropogenic actions isostasy... extraction, evolution and impacts mining, Internal daming.... – Dominant winds (trade winds…) geodynamics processes – Storms and Cyclones Intensity and frequency – => even if there remains high uncertainties > => the coast, the forcing factors and related parameters constitute the “Coastal system” on the effects of CC on cyclogenesis > Numerous interactions and feedbacks are existing between factors & attribution of causes – Atmospheric Pressure of coastal changes to one or another factor is complex and difficult Cazenave and Llovel 2010, Stammer, B.Meyssignac, Becker et al. 2012, 1 12/07/2013 Other External geodynamics processes Erosion of lateritic soils Biological processes Coastal Anthropogenic Climate change actions and changes impacts External geodynamics External geodynamics Internal geodynamics processes processes processes New Caledonia , M.Garcin 2012 BRGM • CC can potentially modify winds climate (trade winds, storms and cyclones) inducing wave • The CC affects also precipitations (pattern, frequency, intensity) climates change (wave height, orientations, on lands which indirectly modify: frequency…) which modify: – At the watersheds scale – the wave energy during “normal” and extremes • Runoff and surficial erosion of soils Waves generated during the events at coast TC04B cyclone in Sri Lanka (Dec. 2000) • Triggering of landslides (impact on frequency and intensity) – The cross‐shore currents • => inducing indirectly modification the supply of sediment to the – The long‐shore drift currents rivers High coastal erosion during – => can modify the coastal behaviour (erosion the Xynthia storm stability accretion) and the level of coastal – Water and sediment discharges of rivers at outlets erosion and flooding hazards at which each • => modification of the sediment supply at the coast and thus the coastal segment is exposed coastal sedimentary budget which can lead to a modification of – The wave climate changes have an the coastal behaviour and mobility (accretionstabilityerosion) impact on the coastline position – likely indirect incidence of rainfall changes on the La Tranche/Mer, France , M.Garcin 2010 coastline position Other External geodynamics The subsidence effect processes Biological processes Coastal Anthropogenic Climate change actions and changes impacts Internal geodynamics Internal geodynamics processes processes Morton et al. (2004) Fault Regional • At coast the relative sea level is climatic sea level rise • Vertical movements are not necessarily Local sea level regression composed by: homogeneous at the regional scale or Uplift / Local sea level at T0 – the regional climatic sea level Subsidence Local sea level at T1 even at island scales – vertical movements of ground Regional and local ground movements Uplift – Vertical movements of each site must be • The vertical movements can counteract, compensate or characterized • amplify the climatic sea level rise giving the local relative sea Local sea level transgression Studying the geological and Local sea level at T1 geomorphological context, neotectonic and level Local sea level at T0 seismicity – Vertical movements can be of different origins : • Monitoring the ground movements using Subsidence • tectonic (Tuvalu, Alexandrie, Loyalty islands …), GPS, InSAR etc.. • isostatic (isostatic rebound linked to ice sheets melting : Fennoscandia or Laurentides, hot spot volcanoes…) – Vertical ground movements have consequences for the coastline position • Thermic and linked to the cooling of volcanoes… and coastal hazards (erosion and flooding) e.g. Mörner 1976, Peltier 2004, Ballu et al. 2011, Wöppelmann et al. 2013 e.g. Wöppelmann et al. 2013 Other External Other External geodynamics geodynamics processes processes Biological Biological processes processes Example of biological Coastal Anthropogenic Coastal Anthropogenic Climate change actions and Climate change actions and changes changes impacts Example of biological factors: impacts Internal Internal geodynamics geodynamics processes the reefs processes factors : the reefs • Reefs play a crucial role in the evolution of • CC induced also an increase of SST and of the acidity of ocean tropical islands as a biological resource, a illustration of the response of reefs to variations in • Reefs are very sensitive to these parameters; exceeding a threshold for sediment suppliers, and a protective barrier Holocene sea level (Woodroffe 2003, 2012) one of these parameters leads to a coral bleaching ...and decreases their against swells and waves during storms and adaptation capacity to accommodate to the sea level rise. cyclones • => What will be the impact on the reefs (barrier and fringing) of SST, salinity and acidity changes? • Climatic parameters like sea level, ocean • What will be the impact of reefs eoltionevolution on the coastal mobility, on temperature, salinity and acidity are crucial coastal hazards ? for coral reef life – The reefs have been able to accommodate Coral bleaching from 1993 to 2012 (Reefbase Org) the sea level changes during the Quaternary as demonstrated by numerous researchers… – But the remaining question is: what will be the capacity of coral reefs to accommodate in the context of higher rate of sea level rise as predicted in the future decades ? e.g. Pirazolli et al. 1986, 1988; Bard et al.1990, 1996, 210, Woodroffe 2008, Cabioch 2003, Camoin et al. 2013 2 12/07/2013 Other External geodynamics processes Biological processes Coastal Anthropogenic Climate change actions and Example of biological factors : the changes impacts Internal Biological processes geodynamics mangoves processes Shells ridge (Mt St Michel France) Six patterns of mangrove development in relation • In more temperate seas, biologic Dumbéa, New Caledonia, M.Garcin 2009 to changes in sea‐level Woodroffe 2002 productivity of benthos (bivalves, gastropods…) can be affected by CC • Important role of the mangroves in the coastline evolution: role in the sedimentation rates and protection against erosion and waves during extreme events with incidences on the production of bioclasts • Mangroves are influenced by rises of sea level, CO2, air and water temperatures, Société Géologique et Minéralogique de Bretagne precipitation: pattern, frequency and intensity – This modification of bioclasts production can modify the sedimentary • All these parameters influence the mangroves evolution, what impact on the mobility Shells ridge detail (Mt St Michel budget of bioclastic sandy beach and of coastline ? France) shells ridges Identification of least and most vulnerable world’s mangrove forests – Modifications of the beach resilience to (Alongi 2007) e.g. Plaziat 1995, erosion and in consequence to coastline Woodroffe 2002, Gilman et al. 2006, Done et al. changes. 2006, Alongi 2007 Société Géologique et Minéralogique de Bretagne e.g. Alongi 2007 Other External geodynamics processes Biological processes Coastal Anthropogenic Climate change actions and changes Groynes impacts Direct anthropogenic
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