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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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slide1

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

slide4

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

slide5

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

slide7

Wetlands

Tidal Flats

Canals

Sea

MECHANISM GOVERNING WETLANDS LONG TERM EVOLUTION

Eustatism and subsidence

Sediment availability

Mineralogenic

Organic

slide8

THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS

TIDAL CHANNEL

TIDAL CHANNEL

+ TIDAL FLATS

TIDAL CHANNEL

+ TIDAL FLATS +

SALTMARSHES

1D numerical model

slide9

THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS

TIDAL CHANNEL

+ TIDAL FLATS

TIDAL CHANNEL

TIDAL CHANNEL

+ TIDALFLATS+

SALTMARSHES

1D numerical model

slide10

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

slide11

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
slide12

i) VEGETATION

ii) SEA LEVEL RISE

iii) WIND

Developments

Novel Ingredients:

slide13

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)
slide14

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

slide15

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)
morphology vegetation and sea level rise the fate of tide dominated salt marshes
Morphology, vegetation andsea level rise:the fate of tide dominated salt marshes

Sea level rise 0, 3.5, 20 mm/yr

NO WIND

slide17

Bmax=1kg/m2; u sea rise =0 mm/y

Marshaggrades and slowlyprogradesseaward

slide18

Bmax=1kg/m2; u sea rise =3.5 mm/y

Marshkeeps up with sealevel rise butslowlyretreats

slide19

Bmax=1kg/m2; u sea rise =20 mm/y

Marsh can notkeep up with sealevel rise

slide20

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

slide21

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

slide22

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

slide23

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

slide24

Bed profiles after 100 yrs :

  • no sea level rise
  • in the presence of vegetation with Bmax= = 1 Kg/m2

DEPOSIT

EROSION

slide25

…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!

slide27

Future Developments

Waves and currents interactions

The role of wave breaking

Biofilm role on salt marsh stability

Thank you