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

WHAT ‘S WETLAND FATE IN A CENTURY OF GLOBAL WARMING?

CAN THEY SURVIVE SEA LEVEL RISE?

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

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

Day et al., 1999

Sea

Canals

Tida

l Fla

ts

Wet

land

s

MECHANISM GOVERNING WETLANDS LONG TERM EVOLUTION

Eustatism and subsidence

Sediment availability

Mineralogenic Organic

TIDAL CHANNEL+ TIDAL FLATS

TIDAL CHANNEL

TIDAL CHANNEL+ TIDAL FLATS +SALTMARSHES

1D numerical model

THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS

TIDAL CHANNEL+ TIDAL FLATS

TIDAL CHANNEL+ TIDALFLATS+SALTMARSHES

1D numerical model

THE SIMPLIEST MODEL CONTAINING ALL THE RELEVANT MECHANISMS

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

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)

i) VEGETATION

ii) SEA LEVEL RISE

iii) WIND

Developments

Novel Ingredients:

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

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

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

Morphology, vegetation andsea level rise:

the fate of tide dominated salt marshes

Sea level rise 0, 3.5, 20 mm/yr

NO WIND

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

Marsh aggrades and slowly progrades seaward

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

Marsh keeps up with sealevel rise but slowly retreats

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

Marsh can not keep up with sealevel rise

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

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

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!

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

EROSION

DEPOSIT

Bed profiles after 100 yrs :

no sea level rise

-in the presence of vegetation with Bmax= = 1 Kg/m2

…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!

Sea level rise3.5 mm/year

NO WIND

An example of competition among different species:

Future Developments

The role of wave breaking

Waves and currents interactions

Thank you

Biofilm role on salt marsh stability