Andrea Defina

On the drainage density of tidal networks

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.

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

Two-dimensional shallow flow equations for partially dry areas

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.

Modeling of channel patterns in short tidal basins

We model branching channel patterns in short tidal basins with two methods. A theoretical stability analysis leads to a relationship between the number of channels and physical parameters of the tidal system. The analysis reveals that width and spacing of the channels should decrease as the slope of the bottom profile and the Shields parameter increase and as the mean water depth decreases. In general, the channel depth should halve at every bifurcation. These theoretical results agree well with the field data from the Dutch Wadden Sea. A numerical model based on Delft3D, a software system of WL/Delft Hydraulics, is used to simulate the time evolution of a channel network in a geometrically simplified basin of similar dimensions as the Wadden Sea basins. The resulting channel network displays a three-times branching behavior, similar to the three- to four-times branching patterns observed in the Wadden Sea. The simulated channel pattern satisfies the relation derived from the theoretical analysis. The results of this pattern analysis provide for additional validation of two-dimensional/three-dimensional process-based morphodynamic models of tidal basins.

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.

Mathematical modeling of tidal hydrodynamics in shallow lagoons: A review of open issues and applications to the Venice lagoon

Although considerable progress has been made in the application of two- and three-dimensional shallow water models to simulate flow in estuaries and coastal lagoons, a number of outstanding problems still remain in this branch of computational fluid dynamics. These problems mainly deal with the proper representation of physical processes and arise when dealing with very shallow flow, temporary submergence and time-dependent flow domains, and complex morphology, and are often related to inaccuracies in modeling the geometry of the domain. Among others, wetting and drying of salt marshes and tidal flats, inclusion of the small scale drainage networks which often strongly affect hydrodynamics, turbulence closure schemes, and accurate friction evaluation are discussed in this paper. To better illustrate these problems and their possible solutions some examples are presented which use the Venice lagoon as a benchmark shallow tidal basin characterized by a complex interplay of channels and marshes.

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.

Hysteretic behavior of the flow under a vertical sluice gate

In the paper, the phenomenon of hysteresis which can develop in a supercritical channel flow approaching an obstacle is analyzed, and a simple theory to predict the occurrence of hysteresis is described. The results of an in-depth theoretical and experimental study of the case of flow under a vertical sluice gate in a rectangular channel are then presented. Possible flow regimes in the vicinity of a gate are classified on the basis of the nondimensional gate opening and the Froude number of the undisturbed approaching flow. It is shown that within a wide range of flow parameters both undisturbed and free outflow conditions may exist for the same gate opening. Within this range, the actual regime depends on the previous history of the flow, thus implying the hysteretic character of the flow. It is worth noting that a subcritical approaching flow may also exhibit such a hysteretic behavior provided the Froude number is greater than approximately 0.8. This occurrence, which has not previously been reported in the literature, is probably a result of the contraction affecting the flow issuing from under the gate. The results of an extensive series of experiments, performed over a wide range of flow parameters, are detailed in the paper and confirm theoretical predictions.

Diffusion of floating particles in flow through emergent vegetation: Further experimental investigation

[1] In this paper we present the results of a new laboratory investigation aimed at providing a better understanding of the transport and diffusion processes of floating particles (e.g., buoyant seeds) in open channel flow with emergent vegetation. The experiments are designed primarily to study the influence of vegetation density and flow velocity on the relevant interaction mechanisms between particles and vegetation. The aim is also to ascertain the validity of a stochastic model recently proposed by Defina and Peruzzo (2010). We find that (1) the proper definition of plant spacing is given as 1/npdp, with dp being the plant diameter and np being the number of plants per unit area, (2) the particle retention time distribution can be satisfactorily approximated by a weighted combination of two exponential distributions, (3) flow velocity has a significant influence on the retention time and on the efficiency of the different trapping mechanisms, and (4) vegetation pattern and density have a minor influence on the probability of capture and on the retention time of particles. Indeed, the comparison between model predictions and experimental results is satisfactory and suggests that the observed relevant aspects of the particle-vegetation interaction processes are properly described by the model.

Open channel flow through a linear contraction

In this work we study the open channel flow through a linear contraction. We focus on the flow configuration with a two-dimensional jump in the contraction discussed by Akers and Bokhove [Phys. Fluids 20, 056601 (2008)] and propose a simple quasi-two-dimensional model to assess equilibrium conditions and explore their stability. We show that both bed friction and bed slope contribute to the stability of this flow configuration, which otherwise would be unstable. Results of the theoretical model are compared to both experimental data and the results of numerical simulations, and the comparison confirms the reliability of the proposed model. We also show that channel flow through a linear contraction can experience two different nested hysteretic loops.