No 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...

[1] The drainage density of a network is conventionally defined as (proportional to) the ratio of its total channelized length divided by the watershed area, and in practice, it is defined by the statistical distribution and correlation structure of the lengths of unchanneled pathways. In tidal networks this requires the definition of suitable drainage directions defined by hydrodynamic (as opposed to topographic) gradients. In this paper we refine theoretically and observationally previous analyses on the drainage density of tidal networks developed within tidal marshes. The issue is quite relevant for predictions of the morphological evolution of lagoons and coastal wetlands, especially if undergoing rapid changes owing, say, to combined effects of subsidence and sea level rise. We analyze 136 watersheds within 20 salt marshes from the northern lagoon of Venice using accurate aerial photographs and field surveys taken in different years in order to study both their space and time variability. Remarkably, the tidal landforms studied show quite different physical and ecological characteristics. We find a clear tendency to develop characteristic watersheds described by exponential decays of the probability distributions of unchanneled lengths, and thereby a pointed absence of scale-free distributions which instead usually characterize fluvial settings. We further find that total channel length relates well to watershed area rather than to tidal prism, a somewhat counterintuitive result on the basis of dynamical considerations. Finally, we show that in spite of the apparent site-specific features of morphological variability, conventional measures of drainage density appear to be quite constant in space and time, indicating a similarity of form. We show that such similarity is an artifact of the Hortonian measure. Indeed, important morphological differences, most notably in stream (or link) frequency reflecting the true extent of branching innervating the marshes and the sinuosity of tidal meandering, may only be captured by introducing measures of the extent of unchanneled flow paths based on hydrodynamics rather than topography and geometry.

Abstract. A new set of two-dimensional shallow flow equations is developed in order to deal with partially wet and very irregular domains. The bottom irregularities, which in many practical cases strongly affect the dynamics and the continuity, are accounted for statistically. Assuming hydrostatic approximation, the three-dimensional Reynolds equations are suitably averaged over a representative elementary area and then integrated over the depth. The resulting subgrid model for ground irregularities is tested by resolving two sample problems. The first concerns the wetting and drying of tidal flats; the second deals with overland flow on an irregular plane surface. Numerical simulations show that the proposed equations are a useful tool for modelers who have to cope with partially dry domains.

[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.