N. Tambroni , G. Seminara
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Transcript of N. Tambroni , G. Seminara
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N. Tambroni, G. Seminara
A one-dimensional eco-geomorphic model of marsh
response to sea level rise: Wind effects, dynamics of the marsh
border and equilibrium*
A one-dimensional eco-geomorphic model of marsh
response to sea level rise: Wind effects, dynamics of the marsh
border and equilibrium*
Trieste, OGS, 22 Luglio 2014
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
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WHAT ‘S WETLAND FATE IN A CENTURY OF GLOBAL WARMING?
CAN THEY SURVIVE SEA LEVEL RISE?
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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
wetlands (velme e barene) surface: 435.68 km2 (≈ 80% of the lagoon territory)
Venice lagoon wetlands
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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.
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 typical view of the lagoon at low tide. (archivio Alinari).
A view of the lagoon during an extreme event of low tide occurred in January 2002
(-0,7 m). (courtesy of G. Cecconi- CVN)
Salt marshes have undergone siltation for the last years
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Day et al., 1999
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Sea
Canals
Tida
l Fla
ts
Wet
land
s
MECHANISM GOVERNING WETLANDS LONG TERM EVOLUTION
Eustatism and subsidence
Sediment availability
Mineralogenic Organic
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TIDAL CHANNEL+ TIDAL FLATS
TIDAL CHANNEL
TIDAL CHANNEL+ TIDAL FLATS +SALTMARSHES
1D numerical model
THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS
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TIDAL CHANNEL+ TIDAL FLATS
TIDAL CHANNEL+ TIDALFLATS+SALTMARSHES
1D numerical model
THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS
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Bott
om
Evolu
tion
• M2 tidal forcing at the inlet and channel closed at the other end.
Morphodynamics of tidal channels, Lanzoni and Seminara’s model, JGR 2002
• 1D numerical model: De S.Venant + Exner.
Main features:
• Sediment transport equal to local transport capacity
Main results:
M.S.L.
initial bottom
80 cycles200 cycles
2000 cycles500000 cycles
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Summarizing……on the long term morphodynamic evolution of straight
tidal channels• 1D Numerical model
(Lanzoni & Seminara, JGR 2002 )
It exists a bottom equilibrium
configuration
• Laboratory observations
•(Tambroni et al., 2005)
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i) VEGETATION
ii) SEA LEVEL RISE
iii) WIND
Developments
Novel Ingredients:
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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)
1. Modelling vegetation
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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
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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)
Organic sediments are produced in proportion
to aboveground biomass B(kg/m2)
(Randerson, 1979, Day et al., 1999)
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
EFFECTS OF VEGETATION
SEDIMENT PRODUCTION OPPOSING RESUSPENSION
1.2 Modelling the effects of vegetation
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Morphology, vegetation andsea level rise:
the fate of tide dominated salt marshes
Sea level rise 0, 3.5, 20 mm/yr
NO WIND
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Bmax=1kg/m2; u sea rise =0 mm/y
Marsh aggrades and slowly progrades seaward
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Bmax=1kg/m2; u sea rise =3.5 mm/y
Marsh keeps up with sealevel rise but slowly retreats
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Bmax=1kg/m2; u sea rise =20 mm/y
Marsh can not keep up with sealevel rise
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Bed profiles after 1000 yrs :
sea level 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
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2. Modeling the effect of wind acting on the shoals
Two distinct effects:i) 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) wind
Uwind
Set-up
D
z
Wind stress driven Wind
setup driven
ĉĉ
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Sediment Flux
ĉĉ
wind
Utidal
Uwind
Set-up
i) The first: advection by tidal currents
Two distinct contributions:
qs tidal
ii) The second: advection by wind currents (driven by wind stress and wind setup)
z
D
s dzUcq0
D
qs wind
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!
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0 100 200 300 400 500 600 700 800 900 1000
x[m]
w=0.5fww(Hs)2/(TSh(kD))2
Hs
w=0.5fww(Hs)2/(TSh(kD))2
Morphological implications of wind resuspension in shoals.
Wind directionqs wind
erosion
deposition
What can we envisage on purely physical ground ?
m.h.w.l.
m.s.l.
w=0.5fww(Hs)2/(TSh(kD))2
Sh(kD)
w=0.5fww(Hs)2/(TSh(kD))2
w
η local and instantaneous
laterally averaged bed elevation
p sediment porosity
qs total sediment flux per unit
width
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EROSION
DEPOSIT
Bed profiles after 100 yrs :
no sea level rise
-in the presence of vegetation with Bmax= = 1 Kg/m2
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…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!
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Sea level rise3.5 mm/year
NO WIND
An example of competition among different species:
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Future Developments
The role of wave breaking
Waves and currents interactions
Thank you
Biofilm role on salt marsh stability