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* 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: /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: km 2 (≈ 80% of the lagoon territory) Venice lagoon wetlands

Progressive loss of salt marshes areas from about 110 Km 2 in 1790 to 30 Km 2 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 Tidal Flats Wetla nds MECHANISM GOVERNING WETLANDS LONG TERM EVOLUTION Eustatism and subsidence Sediment availability MineralogenicOrganic

TIDAL CHANNEL + TIDAL FLATS 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

Bottom Evolution M 2 tidal forcing at the inlet and channel closed at the other end. Morphodynamics of tidal channels, Lanzoni and Seminara’s model, JGR D numerical model: De S.Venant + Exner. Main features: Sediment transport equal to local transport capacity Main results: M.S.L. initial bottom 80 cycles 200 cycles 2000 cycles 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/m 2 ) (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 and sea level rise: the fate of tide dominated salt marshes Sea level rise 0, 3.5, 20 mm/yr NO WIND

Bmax=1kg/m 2 ; u sea rise =0 mm/y Marsh aggrades and slowly progrades seaward

Bmax=1kg/m 2 ; u sea rise =3.5 mm/y Marsh keeps up with sealevel rise but slowly retreats

Bmax=1kg/m 2 ; 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 B max = 1 Kg/m 2 - in the presence of vegetation with B max = 3 Kg/m 2 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 U wind Set-up D z Wind stress driven Wind setup driven ĉ

Sediment Flux ĉ wind U tidal U wind Set-up i) The first: advection by tidal currents Two distinct contributions: q s tidal ii) The second: advection by wind currents (driven by wind stress and wind setup) z D q s 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!

 w =0.5f w  w (  H s ) 2 /(TSh(kD)) 2 Hs  w =0.5f w  w (  H s ) 2 /(TSh(kD)) 2 Morphological implications of wind resuspension in shoals. Wind direction q s wind erosion deposition What can we envisage on purely physical ground ? m.h.w.l. m.s.l.  w =0.5f w  w (  H s ) 2 /(TSh(kD)) 2 Sh(kD)  w =0.5f w  w (  H s ) 2 /(TSh(kD)) 2 w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 B max= = 1 Kg/m 2

…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