Critical bifurcation of shallow microtidal landforms in tidal flats and salt marshes

Abstract

Shallow tidal basins are characterized by extensive tidal flats and salt marshes that lie within specific ranges of elevation, whereas intermediate elevations are less frequent in intertidal landscapes. Here we show that this bimodal distribution of elevations stems from the characteristics of wave-induced sediment resuspension and, in particular, from the reduction of maximum wave height caused by dissipative processes in shallow waters. The conceptual model presented herein is applied to the Venice Lagoon, Italy, and demonstrates that areas at intermediate elevations are inherently unstable and tend to become either tidal flats or salt marshes.intertidal landforms T he distribution of elevations in shallow tidal basins such as the Venice Lagoon in Italy (Fig 1, tidal range of 0.7 m) shows that tidal flats have differences in elevation of few tens of centimeters, with an average elevation between Ϫ0.50 and Ϫ1.00 m above mean sea level (MSL), whereas salt marshes lie at an average elevation higher than ϩ0.20 m, with some variability dictated by local sedimentological and ecological conditions (1-4). Few areas are located at intermediate elevations (i.e., between Ϫ0.50 and ϩ0.20 m), suggesting that the processes responsible for sediment deposition and erosion produce either tidal flats or marshes but no landforms located at intermediate elevations. In the relatively pristine northern part of the Venice Lagoon, the most frequent bottom elevation is around Ϫ0.50 m (Fig. 2), similar to natural conditions in 1901 in the Southern Lagoon ( Fig. 1 A). During the last century, anthropogenic causes produced consistent bottom erosion in the Southern Lagoon, leading to a median elevation of approximately Ϫ1.00 m above MSL (Figs. 1B and 2). Nevertheless, all three distributions of elevations show a relatively low frequency of elevations between 0 and Ϫ0.5 m.Typical conceptual and numerical models of salt-marsh formation envision a gradual transformation of sand flats and mudflats in response to sediment buildup and plant colonization (5-7). However, the evidence points to abrupt transitions to one of two distinct stable outcomes. Salt marshes emerge from tidal flats in locations where sedimentation is enhanced by lower tidal velocities, higher sediment concentrations, or the sheltering effects of splits and barrier islands (1,8). Alternatively, in areas with consistent sediment resuspension caused by a combination of tidal fluxes and wind waves, tidal flats are dominant. In tidal flats, sediment deposition is balanced by erosion, and the bottom elevation is constantly maintained below MSL (9). Sediment resuspension by wind waves is decisive, because tidal fluxes alone are unable to produce the bottom shear stresses necessary to mobilize tidal-flat sediments (10).On the basis of a simplified model for wave generation in shallow water (10), we developed a conceptual model to study the distribution of bottom shear stress as a function of elevation. The results are used to explain the bimodal distribution of bathymetry in the Ve...

Influence of storm surges and sea level on shallow tidal basin erosive processes

“…If the erosion rate in the tidal flats is greater than the increased deposition rate, then a global positive feedback will be established and marsh retreat will accelerate until they disappear, while tidal flats will keep on deepening. A stable condition will eventually occur only when tidal flats become deep enough to eliminate the effect of wave induced bottom shear stress [Fagherazzi et al, 2006;Marani et al, 2007;Carniello et al, 2009b].…”

“…The numerical values of the parameters used to solve equation (5) are reported by Carniello et al [2005] and Fagherazzi et al [2006].…”

“…Two distinct erosional mechanisms are associated with wind waves. Wave-generated shear stresses, combined with tidal currents, are the main mechanism responsible for sediment resuspension on tidal flats [Carniello et al, 2005;Fagherazzi et al, 2006Fagherazzi et al, , 2007Marani et al, 2007], and regulate both sediment concentration in the water column (and hence light availability at the bed) [e.g., Lawson et al, 2007] and sediment export to salt marshes and to the ocean [Mariotti and Fagherazzi, 2010]. Waves impacting salt marsh boundaries produce intermittent forces that promote marsh edge erosion and salt marsh regression.…”

Understanding and predicting wave erosion of marsh edges

“…As a consequence, neglecting temporal and spatial variations, a local equilibrium of wave energy sources and sinks is assumed, i.e., [Fagherazzi et al, 2006]:…”

“…[8] The model used to simulate the wind waves is a version of the model developed by Fagherazzi et al [2006] and Carniello et al [2005Carniello et al [ , 2011. Using an approach similar to the one described by Young and Verhagen [1996] and Breugem and Holthuijsen [2007], we first estimate the wave period T = 3.5(gD/U 2 ) 0.35 · U/g, on the basis of the observed wind speed, U, and the instantaneous value of the water depth, D.…”