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N. Tambroni , G. Seminara

Trieste, OGS, 22 Luglio 2014. A one-dimensional eco-geomorphic model of marsh response to sea level rise: Wind effects, dynamics of the marsh border and equilibrium*. N. Tambroni , G. Seminara. DICCA, Dipartimento di Ingegneria Civile, Chimica ed Ambientale, Università di Genova.

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N. Tambroni , G. Seminara

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  1. Trieste, OGS, 22 Luglio 2014 A one-dimensional eco-geomorphic model of marsh response to sea level rise: Wind effects, dynamics of the marsh border and equilibrium* N. Tambroni, G. Seminara DICCA, Dipartimento di Ingegneria Civile, Chimica ed Ambientale, Università di Genova *Tambroni, N., and G. Seminara (2012),J. Geophys. Res., 117, F03026, doi:10.1029/2012JF002363

  2. WHAT ‘S WETLAND FATE IN A CENTURY OF GLOBAL WARMING? CAN THEY SURVIVE SEA LEVEL RISE?

  3. wetlands (velme e barene) surface: 435.68 km2 (≈ 80% of the lagoon territory) • BARENE (SALT MARSHES) - colonized by halophytic vegetation; - submerged only at high tide • VELME e BASSIFONDI (TIDAL FLATS) - not vegetated; - submerged, emerging only for exceptionally low tides Venice lagoon wetlands

  4. Morphological degradation of Venice lagoon: main evidences Comparison between the first bathymetry (1810) and the current bathymetry. Salt marsh border collapse End XIX century Nowadays A view of the lagoon during an extreme event of low tide occurred in January 2002 (-0,7 m). (courtesy of G. Cecconi- CVN) A typicalview of the lagoonatlowtide. (archivio Alinari). Progressive loss of salt marshes areas from about 110 Km2 in 1790 to 30 Km2 at the end of the XX secolo Progressive deepening of the tidal flats: The average depth of the tidal flats has increased for the last century by 60 cm, 40 cm e 30 cm respectively in the basins of Malamocco, Lido and Chioggia. Salt marshes have undergone siltation for the last years

  5. Day et al., 1999

  6. Wetlands Tidal Flats Canals Sea MECHANISM GOVERNING WETLANDS LONG TERM EVOLUTION Eustatism and subsidence Sediment availability Mineralogenic Organic

  7. THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS TIDAL CHANNEL TIDAL CHANNEL + TIDAL FLATS TIDAL CHANNEL + TIDAL FLATS + SALTMARSHES 1D numerical model

  8. THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS TIDAL CHANNEL + TIDAL FLATS TIDAL CHANNEL TIDAL CHANNEL + TIDALFLATS+ SALTMARSHES 1D numerical model

  9. M.S.L. initial bottom 80 cycles 200 cycles 2000 cycles 500000 cycles Morphodynamics of tidal channels, Lanzoni and Seminara’s model, JGR 2002 Main features: • 1D numerical model: De S.Venant + Exner. • Sediment transport equal to local transport capacity • M2 tidal forcing at the inlet and channel closed at the other end. Main results: Bottom Evolution

  10. Summarizing… …on the long term morphodynamic evolution of straight tidal channels • 1D Numerical model (Lanzoni & Seminara, JGR 2002 ) • Laboratory observations • (Tambroni et al., 2005) • It exists a bottom equilibrium configuration

  11. i) VEGETATION ii) SEA LEVEL RISE iii) WIND Developments Novel Ingredients:

  12. 1. Modelling vegetation • GROWTH OF VEGETATION • As soon as the channel bed emerges, allow growth of vegetation (using the depth dependent productivity of biomass measured for Spartina by Morris et al., 2002)

  13. Morris, 2000 Observed productivity of the salt marsh macrophyte Spartina alterniflora, measured annually since 1984, Depends on depth below mean high tide (MHT) of sites in high (o) or low (●) marsh

  14. 1.2 Modelling the effects of vegetation • GROWTH OF VEGETATION • As soon as the channel bed emerges, allow growth of vegetation (using the depth dependent productivity of biomass measured for Spartina by Morris et al., 2002) • EFFECTS OF VEGETATION OPPOSING RESUSPENSION SEDIMENT PRODUCTION Once vegetation is present, assume sediments entering the marsh to be intercepted by vegetation and settle in the marsh, while no sediments leave the marsh • Organic sediments are produced in proportion to aboveground biomass B(kg/m2) (Randerson, 1979, Day et al., 1999)

  15. Morphology, vegetation andsea level rise:the fate of tide dominated salt marshes Sea level rise 0, 3.5, 20 mm/yr NO WIND

  16. Bmax=1kg/m2; u sea rise =0 mm/y Marshaggrades and slowlyprogradesseaward

  17. Bmax=1kg/m2; u sea rise =3.5 mm/y Marshkeeps up with sealevel rise butslowlyretreats

  18. Bmax=1kg/m2; u sea rise =20 mm/y Marsh can notkeep up with sealevel rise

  19. Bed profilesafter 1000 yrs : • sealevel rise 3.5 mm/y • in the presence of vegetation with Bmax= 1 Kg/m2 • in the presence of vegetation with Bmax= 3 Kg/m2 Strongly productive vegetation allows the marsh to keep up with sea level rise

  20. wind z Wind stress driven Wind setup driven 2. Modeling the effect of wind acting on the shoals Two distinct effects: • The first: generation of wind waves, whose amplitude is strongly dependent on the shoal depth and on the wind fetch. • (YOUNG&VERHAGEN,1996) ii) The second: generation of currents driven by the surface setup induced by the shear stress acting on the free surface (ENGELUND, 1986) Set-up ĉ D Uwind

  21. Sediment Flux Two distinct contributions: the flow field induced by wind setup may be as significant as tidal currents in determining the direction and the intensity of the advected sediment flux! i) The first: advection by tidal currents ii) The second: advection by wind currents (driven by wind stress and wind setup) wind z Set-up Utidal ĉ D Uwind qs tidal qs wind

  22. tw Hs Sh(kD) tw=0.5fwrw(pHs)2/(TSh(kD))2 tw=0.5fwrw(pHs)2/(TSh(kD))2 tw=0.5fwrw(pHs)2/(TSh(kD))2 Morphological implications of wind resuspension in shoals. What can we envisage on purely physical ground ? m.h.w.l. Wind direction qs wind m.s.l. deposition erosion ηlocal and instantaneous laterally averaged bed elevation psediment porosity qs total sediment flux per unit width tw=0.5fwrw(pHs)2/(TSh(kD))2

  23. Bed profiles after 100 yrs : • no sea level rise • in the presence of vegetation with Bmax= = 1 Kg/m2 DEPOSIT EROSION

  24. …what about the long term evolution? Wind resuspension over tidal flats is not able to compensate the effects of sea level rise! Timescale of the natural evolution process is very large. In the absence of strong anthropogenic (or climatic) effects, variation undergone by these systems are so slow to be hardly perceived. Morphodynamic equilibrium is a rather exceptional and unstable state!

  25. An example of competition among different species: Sea level rise3.5 mm/year NO WIND

  26. Future Developments Waves and currents interactions The role of wave breaking Biofilm role on salt marsh stability Thank you

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