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.
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
CAN THEY SURVIVE SEA LEVEL RISE?
surface: 435.68 km2 (≈ 80% of the lagoon territory)
- colonized by halophytic vegetation;
- submerged only at high tide
- not vegetated;
- submerged, emerging only for exceptionally low tides
Venice lagoon wetlands
Comparison between the first bathymetry (1810) and the current bathymetry.
Salt marsh border collapse
End XIX century
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
MECHANISM GOVERNING WETLANDS LONG TERM EVOLUTION
Eustatism and subsidence
+ TIDAL FLATS
+ TIDAL FLATS +
1D numerical model
+ TIDAL FLATS
1D numerical model
Morphodynamics of tidal channels, Lanzoni and Seminara’s model, JGR 2002
…on the long term morphodynamic evolution of straight tidal channels
(Lanzoni & Seminara, JGR 2002 )
ii) SEA LEVEL RISE
of the salt marsh
macrophyte Spartina alterniflora,
Depends on depth below
mean high tide (MHT)
of sites in high (o) or low (●) marsh
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
Sea level rise 0, 3.5, 20 mm/yr
Bmax=1kg/m2; u sea rise =0 mm/y
Marshaggrades and slowlyprogradesseaward
Bmax=1kg/m2; u sea rise =3.5 mm/y
Marshkeeps up with sealevel rise butslowlyretreats
Bmax=1kg/m2; u sea rise =20 mm/y
Marsh can notkeep up with sealevel rise
Strongly productive vegetation allows the marsh
to keep up with sea level rise
2. Modeling the effect of wind acting on the shoals
Two distinct effects:
ii) The second: generation of currents driven by the surface setup induced by the shear stress acting on the free surface (ENGELUND, 1986)
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)
Morphological implications of wind resuspension in shoals.
What can we envisage on purely physical ground ?
ηlocal and instantaneous laterally averaged bed elevation
qs total sediment flux per unit width
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!
Sea level rise3.5 mm/year
Waves and currents interactions
The role of wave breaking
Biofilm role on salt marsh stability