Luca Carniello

Biologically-controlled multiple equilibria of tidal landforms and the fate of the Venice lagoon

Looking across a tidal landscape, can one foresee the signs of impending shifts among different geomorphological structures? This is a question of paramount importance considering the ecological, cultural and socio-economic relevance of tidal environments and their worldwide decline. In this Letter we argue affirmatively by introducing a model of the coupled tidal physical and biological processes. Multiple equilibria, and transitions among them, appear in the evolutionary dynamics of tidal landforms. Vegetation type, disturbances of the benthic biofilm, sediment availability and marine transgressions or regressions drive the bio-geomorphic evolution of the system. Our approach provides general quantitative routes to model the fate of tidal landforms, which we illustrate in the case of the Venice lagoon (Italy), for which a large body of empirical observations exists spanning at least five centuries. Such observations are reproduced by the model, which also predicts that salt marshes in the Venice lagoon may not survive climatic changes in the next century if IPCCs scenarios of high relative sea level rise occur. Copyright 2007 by the American Geophysical Union.

Sea level rise, hydrologic runoff, and the flooding of Venice

Reference ECHO-ARTICLE-2008-002doi:10.1029/2008WR007195View record in Web of Science Record created on 2009-06-22, modified on 2016-08-08

The importance of being coupled: Stable states and catastrophic shifts in tidal biomorphodynamics

We describe and apply a point model of the joint evolution of tidal landforms and biota which incorporates the dynamics of intertidal vegetation; benthic microbial assemblages; erosional, depositional, and sediment exchange processes; wind-wave dynamics, and relative sea level change. Alternative stable states and punctuated equilibria emerge, characterized by possible sudden transitions of the system state, governed by vegetation type, disturbances of the benthic biofilm, sediment availability, and marine transgressions or regressions. Multiple stable states are suggested to result from the interplay of erosion, deposition, and biostabilization, providing a simple explanation for the ubiquitous presence of the typical landforms observed in tidal environments worldwide. The main properties of accessible equilibrium states prove robust with respect to specific modeling assumptions and are thus identified as characteristic dynamical features of tidal systems. Halophytic vegetation emerges as a key stabilizing factor through wave dissipation, rather than a major trapping agent, because the total inorganic deposition flux is found to be largely independent of standing biomass under common supply-limited conditions. The organic sediment production associated with halophytic vegetation represents a major contributor to the overall deposition flux, thus critically affecting the ability of salt marshes to keep up with high rates of relative sea level rise. The type and number of available equilibria and the possible shifts among them are jointly driven and controlled by the available suspended sediment, the rate of relative sea level change, and vegetation and microphytobenthos colonization. The explicit description of biotic and abiotic processes thus emerges as a key requirement for realistic and predictive models of the evolution of a tidal system as a whole. The analysis of such coupled processes finally indicates that hysteretic switches between stable states arise because of differences in the threshold values of relative sea level rise inducing transitions from vegetated to unvegetated equilibria and vice versa.

Morphological evolution of the Venice lagoon: Evidence from the past and trend for the future

[1] During the last century, the Venice lagoon, Italy, has been experiencing a general degradation consisting of the deepening of tidal flats and the reduction of salt marsh areas. A conceptual model describing the long-term evolution of such lagoons has recently been proposed. According to the model, the long-term degradation consists of two steps: an initial salt marsh deterioration phase followed by a tidal flat erosion phase. In this work we test the long-term evolution model through the analysis of four different bathymetries of the Venice lagoon during the last century (1901, 1932, 1970, and 2003). The result of the analysis confirms that the recent past morphological evolution of the Venice lagoon has actually followed the proposed model and highlights a slower erosive trend characterizing the northern part of the lagoon compared to the moderately rapid erosion affecting the central southern part. This result enables us to infer the likely future evolution of the Venice lagoon as long as the present forcing conditions are maintained.

Sediment dynamics in shallow tidal basins: In situ observations, satellite retrievals, and numerical modeling in the Venice Lagoon

The morphological evolution of shallow tidal systems strongly depends on gradients in transport that control sediment erosion and deposition. A spatially refined quantitative description of suspended sediment patterns and dynamics is therefore a key requirement to address issues connected with dynamical trends, responses, and conservation of these systems. Here we use a combination of numerical models of sediment transport dynamics, high temporal resolution point observations, and high spatial resolution remote sensing data to overcome the intrinsic limitations of traditional monitoring approaches and to establish the robustness of numerical models in reproducing space-time suspended sediment concentration (SSC) patterns. The comparison of SSC distributions in the Venice Lagoon (Italy) computed with a numerical model with SSC retrievals from remote sensing data allows us to define the ability of the model to properly describe spatial patterns and gradients in the SSC fields. The use of point observations similarly allows us to constrain the model temporally, thus leading to a complete space-time evaluation of model abilities. Our results highlight the fundamental control exerted on sediment transport intensity and patterns by the sheltering effect associated with artificial and natural intertidal landforms. Furthermore, we show how the stabilizing effect of benthic vegetation is a main control of sediment dynamics at the system scale, confirming a notion previously established in the laboratory or at small field scales.

Spatially integrative metrics reveal hidden vulnerability of microtidal salt marshes

Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. However, lateral erosion can lead to rapid marsh loss as marshes build vertically. Marsh sediment budgets represent a spatially integrated measure of competing constructive and destructive forces: a sediment surplus may result in vertical growth and/or lateral expansion, while a sediment deficit may result in drowning and/or lateral contraction. Here we show that sediment budgets of eight microtidal marsh complexes consistently scale with areal unvegetated/vegetated marsh ratios (UVVR) suggesting these metrics are broadly applicable indicators of microtidal marsh vulnerability. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability. These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability.

A comparative study of physical and numerical modeling of tidal network ontogeny

We investigate the initiation and long-term evolution of tidal networks by comparing controlled laboratory experiments and their associated scaling laws with outputs from a numerical model. We conducted numerical experiments at both the experimental laboratory scale (ELS) and natural estuary scale (NES) and compared these simulations with experimental data and field observations. Sensitivity tests show that initial bathymetry, frictional parametrization, sediment transport, and bed slope terms play an important role in determining the morphodynamic evolution and the final landscape. Consistent with experimental observations, the morphodynamic feedbacks between flow, sediment transport, and bathymetry gradually lead the system to a less dynamic state, finally reaching a stable network configuration. In both the ELS and NES simulations, the initially planar lagoon with large intertidal areas is subject to erosion, indicating ebb-dominance. Based on quantitative analyses of the ELS and the NES simulations (e.g., geometric characteristics and relationship between modified tidal prism and cross-sectional area), we conclude that numerical simulations are consistent with laboratory experiments and show that both type of models provide a realistic, albeit simplified, representation of natural systems. The combination of laboratory and numerical experiments also allowed us to explore the possibility of reaching a long-term morphodynamic equilibrium. Both the physical and numerical models approach a dynamic equilibrium characterized by negligible gradients in sediment fluxes. The equilibrium configuration appears to be consistent with traditional relationships linking tidal prism and cross-sectional area of the inlet. Finally, this contribution highlights the significance of complementary research between experimental and numerical modeling in investigating long-term morphodynamics of tidal networks.

An ecogeomorphic model of tidal channel initiation and elaboration in progressive marsh accretional contexts

The formation and evolution of tidal networks have been described through various theories which mostly assume that tidal network development results from erosional processes, therefore emphasizing the chief role of external forcing triggering channel net erosion such as tidal currents. In contrast, in the present contribution we explore the influence of sediment supply in governing tidal channel initiation and further elaboration using an ecogeomorphic modeling framework. This deliberate choice of environmental conditions allows for the investigation of tidal network growth and development in different sedimentary contexts and provides evidences for the occurrence of both erosional and depositional channel-forming processes. Results show that these two mechanisms in reality coexist but act at different time scales: channel initiation stems from erosional processes, while channel elaboration mostly results from depositional processes. Furthermore, analyses suggest that tidal network ontogeny is accelerated as the marsh accretional activity increases, revealing the high magnitude and prevalence of the depositional processes in governing the morphodynamic evolution of the tidal network. On a second stage, we analyze the role of different initial topographic configurations in driving the development of tidal networks. Results point out an increase in network complexity over highly perturbed initial topographic surfaces, highlighting the legacy of initial conditions on channel morphological properties. Lastly, the consideration that landscape evolution depends significantly on the parameterization of the vegetation biomass distribution suggests that the claim to use uncalibrated models for vegetation dynamics is still questionable when studying real cases.

An approximate solution to the flow field on vegetated intertidal platforms: Applicability and limitations

Tidal wetland evolution is governed by interactions between topography, vegetation, and the flow field. Aiming to provide an appropriate hydrodynamic tool within a long-term geomorphic model of vegetated wetlands, we describe an approximate procedure to model the depth-averaged flow field on vegetated intertidal platforms. The procedure is tested by a qualitative comparison with laboratory experiments and quantitatively comparing with a numerical model, focusing on the influence of spatial variations in friction on the flow field. Overall, satisfactory comparisons are obtained. Nevertheless, some limitations of the approach are apparent. These are discussed in the light of the model assumptions. We analyze the impact of the observed limitations on the ability of the approximate solution to describe the morphodynamic evolution of the bed elevation. This is performed by evaluating the changes in the bed elevation after one tidal cycle on the intertidal platform based on flow velocities obtained with a numerical model and those of the simplified procedure. It is found that the bed evolution on the platform is reasonably described with the approximate solution, even though the accumulation of sediment is underestimated near the watershed divide by the approximate model. Taking into account the computationally economic character of the approximate procedure, the analysis indicates that the model provides a suitable tool to investigate the long-term morphodynamic evolution of tidal wetlands.