FORM OSE post Graduate Training School

Transcription

1 FORM OSE post Graduate Training School Coastal Hazard Assessment and Risk Management CERG, June 2011 Coastal processes in space and time: analysis and risk management implications Edward ANTHONY Aix Marseille Université & Institut Universitaire de France, CEREGE, UMR CNRS 6635, Europôle Méditerranéen de l Arbois, B.P. 80, Aix en Provence Cedex, France

2 Complexity regulated by various types of organic links and paradigms Forced behaviour involving morphodynamic interaction Self organization or free behaviour Equilibrium disequilibrium Negative positive feedback Relaxation Resilience Scale Sediment heterogeneity (variable mixing, segregation) Anisotropy/Patchiness etc

3 Forcing parameters Waves Currents Wind flow River flow Freshwater saltwater interactions Exceptional events storms, tsunamis, earthquakes, landslides, volcanism, etc Impacts of direct and indirect human intervention + Climate Change

4 The complexity of the transition zone from fluvial to marine environment (Dalrymple and Choi, 2007)

5 Are these rhythmic forms generated by: (1) self organized behaviour? (2) infragravity waves? (3) high angle waves? (4) or some other mechanism yet to be documented?

6 Background conditions Sea level Tectonics Inherited coastal morphology Sediment supply (rivers, eroding coasts, shoreface) Society (social, cultural, economic, physical) Hazards & Risk Management Climate Change

7 Wide range of methodological approaches and techniques Field experiments and sampling Remote sensing Laboratory analyses Modelling (conceptual, statistical, physical ) (increasing recourse to geochemistry and geophysics)

8 Coastal sediment budgets Terrestrial inputs (ca. 90%, mainly river catchments x 10 9 tons) Cross shoreface exchanges (%?) Coastal sediment sources (cliffs ca. 5%, and coastal erosion %?) Biogenic production (corals, mangroves, salt marshes %?)

9 1. Coastal sediment budgets: from inland source to the coast Source to sink (Carter et al., 2010)

10 Humans have increased the sediment transport by global rivers through soil erosion by 2.3 ± 0.6 billion metric tons per year, but yet reduced the flux of sediment reaching the world's coasts by 1.4 ± 0.3 billion metric tons per year because of retention within reservoirs. Over 100 billion metric tons of sediment are now sequestered in reservoirs constructed largely within the past 50 years, especially in Africa and Asia (Syvitski et al., 2009).

11 1. Coastal sediment budgets: from inland source to the coast Major challenge: datasets Rhône delta sediment input reduction ( ): 62 Mm 3 Breakdown around 1960: before dam building (Provansal et al., submitted) (Sabatier et al., 2009)

12 Evolution from river dominated to wave dominated deltaic shores = Propensity for retreat (Vella et al., 2009)

13 1.Coastal sediment budgets: from inland source to the coast Major challenge: sediment redistribution processes (Van Maren, 2007)

14 Fine grained systems Tidal pumping, density circulation, floc buildup and breakdown, estuarine turbidity maximum, sedimentation/erosion Wolanski, 2007

15 1. Estuarine muddy sedimentation 2. Longshore supply of mud to lowenergy (high wave energy dissipation) coasts Mudflat coasts + mud bank coasts

16 20% of the 800 million tons of mud per annum from the Amazon forms individual mud banks that migrate alongshore to the Orinoco strong morphodynamic and socio economic impacts. Challenge: better grasp of mud bank formation processes (estuarine front activity)

17 The mud banks are each up to 5 m thick, 10 to 60 km long, and 20 to 30 km wide, and each may contain the equivalent mass of the annual mud supply of the Amazon (Anthony et al., 2010). They number up to 15 or more, separated by inter bank areas. Challenge: mechanisms and rates of mud bank migration

18 Bedload: from river mouth bars to the longshore drift system? + sporadically high energy systems (wadis) Moa river delta, Sierra Leone

19 2. Sediment exchange between shoreface and coast Embedded spatial and timescales

20 2. Sediment exchange between shoreface and coast Challenge #1 datasets A cyclone impacted wave dominated system: Moruya beach, New South Wales: monthly profile surveys since 1972 (Thom and Hall, 1991)

21 The eastern English Channel and southern North Sea coast of France: a storm tide dominated system North Sea Urban area Cliffs Beaches Dunes Artificial shoreline English Channel Estuaries

22 2. Sediment exchange between shoreface and coast Challenge #1: datasets (Aernouts, 2005)

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24 2. Sediment exchange between shoreface and coast: Challenge #1 datasets A shoreface gradient dominated system: Danish North Sea coast, 2 profiles a year since 1976 Aagaard, submitted

25 2. Sediment exchange between shoreface and coast: Challenge #2 cross shore and longshore processes poor knowledge of shoreface processes strong imbalance in favour of beach studies Black et al., 2008

26 Sediment losses to the shoreface Storms Tsunamis High energy forcing Exacerbated on coasts: (1) with a lowered energy buffering capacity (eroding coasts) (1) with a steepened shoreface (equilibrium following progradation notably involving beach ridges) Likely to be strongly impacted by sea level rise

27 Nice beach, French Riviera, an example of chronic offshore loss of recharged gravel

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29 600,000 m 3 since to 25% of the total volume of Holocene gravel (2.25 x 10 6 m 3 ) locked up in the Nice barrier Anthony et al., 2011

30 3. Longshore sediment transport (fundamental to coastal stability) Major challenges: 1.Sediment cell definition and dynamics 2. Longshore transport processes 3. Longshore drift rates (May and Tanner, 1973)

31 8 From two (?) initial natural cells to eight? (directly or indirectly due to coastal engineering structures)

32 Cayeux gravel barrier, Picardy, proximal section

33 Recent cell division in the distal zone of the barrier (effect of sediment heterogeneity sand and gravel (Anthony and Dolique, 2001)

34 If cell defintion is hinged on waves in the breaker, surf and swash zones, how do we define sediment cells on mixed storm tide dominated macrotidal coasts? (Anthony and Orford, 2002)

35 Somme

36 4. Medium term (1 10 yrs) planform evolution of embayed beaches Challenges: mechanisms and datasets Type 1: ENS0 induced beach rotation in Australia (Ranasinghe et al., 2004)

37 Bank phase Inter bank phase Type 2: mud bank induced beach rotation in French Giuana (Anthony et al., 2011)

38 Beach rotation accretion (2004) Beach rotation erosion (2001)

39 5. Beach morphodynamics Challenge: characterization of modal states and change range (Short, 1999)

40 (Short, 1999)

41 (Short, 1999)

42 Observational datasets: remote sensing (aerial videography), ARGUS monitoring Palm beach, Australia, (Ranasinghe et al. 2004) Truc Vert beach, Aquitaine (Castelle et al., 2007)

43 6. Bars Major challenges: 1. Mechanisms of formation (competing hypotheses: Wave breaking, infragravity waves, self organization, swash) 2. Migration patterns cross shore and longshore (critical to beach and dune accretion + erosion) 3. Cycles (temporal frame of bar life cycles)

44 (Dronen and Deigaard, 2007) (Castelle et al., 2007)

45 How do beach systems with complex intertidal bar topography work? (Anthony et al., 2005)

46 (Sedrati & Anthony, 2007)

47 (Sedrati and Anthony, 2007)

48 (Reichmüth and Anthony, 2008)

49 7. Bars also exist on muddy, wave dominated coasts! (Anthony et al., 2008)

50 (Anthony et al., 2008)

51 a Swell under-consolidated bottom > 750g.l. -1 b <10 g.l. -1 diluted suspension Wave damping zone Solitary waves g.l.-1 liquefaction transport fluid mud Tidal channels 0 m -5 m > 750g.l g.l.-1 liquefaction transport Mud bars <650 g.l.-1 MHWL MLWL c Tidal channels > 750g.l. -1 liquefaction transport g.l.-1 <650 g.l mud.-1 bars Mud bars > 750g.l. -1 >2.45 m d Tidal channels g.l.-1 > 650g.l. -1 > 650g.l. -1 > 750g.l g.l.-1 <650 g.l.-1 >2.45 m 1.5 m > 750g.l. -1 (Gardel et al., 2011)

52 8. The swash zone: strong asymmetric flows high sediment transport Major challenge: understanding beachface equilibrium 1. Acceleration 2. Turbulence 3. Infiltration/ exfiltration (Masselink and Puleo, 2006)

53 9. Aeolian streamers on a North Sea beach: Challenge: understanding processes at the beach dune interface

54 Baas, 2008 Hypotheses on the complex problem of aeolian streamer formation

55 EXTRAIT P4 (Anthony et al., 2009)

56 (Anthony et al., 2009)

57 (Donnelly et al., 2006) Volta delta, W. Africa 10. Surge and overwash: Challenge: processes and modelling parameters Mana, F. Guiana

58 11. Tidal flats Challenges: bed shear stress, wave attenuation and sedimentation processes

59 Floc dynamics, biota, muddy sedimentation (including sequestering of pollutants)

60 (Christiansen et al., 2006)

61 Wave modulation by depth (tides and storms) (Le Hir et al., 2000) (Kim, 2003)

62 The erosion equation over tidal flats (Le Hir et al., 2007)

63 Flow canopy interactions (honami) (Leonard and Croft, 2006)

64 Mangroves as waveenergy buffers (Quartel et al., 2009)

65 Modelling channel formation in salt marshes (D Alpaos et al., 2006)

66 (Temmerman et al., 2010)

67 Marsh response to environmental change (sea level rise) (Morris, 2006)

68 Thank you!