Giovanni Seminara

[1] Enhanced understanding of information propagation by waves in rivers is essential in the context of fluvial hydraulics and morphodynamics. Lanzoni et al. [2006] are complimented for their interesting contribution, which would help to understand the long waves in fluvial flows with sediment transport and morphological evolution. The investigation is formulated on the basis of a set of hyperbolic equations under the framework of shallow water hydrodynamics. However, the continuity equation of the fluid phase features a formal flaw. This comment presents the correct continuity equations, identifies the flaw, and briefly addresses its potential impact on the analysis of fluvial processes.

The Committee Firenze 2016, on the occasion of the 50th anniversary of the tragic 1966 flood, invited six engineers and scientists to form an International Technical Scientific Committee (ITSC) to assess the current status of flood protection for the city of Florence and identify steps to reduce the risk of flooding facing the city. In this final Report, ITSC concludes that Florence remains at risk to significant flooding and this risk grows each day. It is not a question of whether a flood of the magnitude of 1966 or greater will occur, but when. In fact, the level of protection that exists in Florence now is not on a level appropriate to the citizens and treasures that rest within the city. If, under current conditions, a 1966-like flood occurred, the consequences to human lives, treasures, properties and community infrastructure could be much more catastrophic than they were in 1966.

We survey the problem of the response of coastal wetlands to sea level rise. Two opposite views have traditionally been confronted. According to the former, on the geological time scale, coastal lagoons would be ‘ephemeral’ features. The latter view maintains that marshes would keep pace with relative sea level rise as, increasing the rate of the latter, the sedimentation rate would also increase. In any case, the timescale of morphodynamic evolution is of the order of centuries, which makes it not easily perceived. For example, in Venice, the diversion of the rivers debouching into the lagoon undertaken in the Renaissance has taken centuries to display its consequences (shift from depositional to erosional environment). This process accelerated in the last two centuries due to effects of the industrial revolution and of an enhanced sea level rise. Recent research has employed powerful computational techniques and advanced models of marsh vegetation. Zero-order modeling suggests tha...

Evaluating the implications of common features that we observe in meandering landforms on Mars and on the Earth in a variety of hydrodynamic and geomorphic environments is crucial for our understanding of the relevant dynamic and morphologic processes, and possibly relevant to the ongoing debate over martian waters. A theoretical framework and martian data from accurate geomorphic analyses, jointly with the results of a famous dataset on earth meandering forms, provide striking evidence for common structures of meanders regardless of gravity, basic erosional properties, relevant dynamics and fluid properties. Evidence for a link between the martian and earth landforms, and for the immaturity of martian meanders, appears to suggest the past existence on Mars of massive and ephemeral outbursts of fluid (not necessarily water) driven by gravity or density gradients.

Stability of salt marshes is a very delicate issue depending on the subtle interplay among hydrodynamics, morphodynamics and ecology. In fact, the elevation of the marsh platform depends essentially on three effects: i) the production of soil associated with sediments resuspended by tidal currents and wind waves in the adjacent tidal flats, advected to the marsh and settling therein; ii) production of organic sediments by the salt marsh vegetation; iii) soil 'loss' driven by sea level rise and subsidence. In order to gain insight into the mechanics of the process, we consider a schematic configuration consisting of a salt marsh located at the landward end of a tidal channel connected at the upstream end with a tidal sea, under different scenarios of sea level rise. We extend the simple 1D model for the morphodynamic evolution of a tidal channel formulated by Lanzoni and Seminara (2002, Journal of Geophysical Research-Oceans, 107, C1) allowing for sediment resuspension in the...

Evaluating the implications of common features that we observe in meandering landforms on Mars and on the Earth in a variety of hydrodynamic and geomorphic environments is crucial for our understanding of the relevant dynamic and morphologic processes, and possibly relevant to the ongoing debate over martian waters. A theoretical framework and martian data from accurate geomorphic analyses, jointly with the results of a famous dataset on earth meandering forms, provide striking evidence for common structures of meanders regardless of gravity, basic erosional properties, relevant dynamics and fluid properties. Evidence for a link between the martian and earth landforms, and for the immaturity of martian meanders, appears to suggest the past existence on Mars of massive and ephemeral outbursts of fluid (not necessarily water) driven by gravity or density gradients.

ABSTRACT We explore the problem of morphodynamic equilibrium of tidal channels, bounded seaward by a tidal sea and shoaling landward as observed in coastal lagoons and estuaries. These channels are typically landward converging, meandering and bounded by tidal flats periodically flooded by the tidal wave. We then attempt to provide an answer to the following questions. How is equilibrium defined for tidal channels? Do tidal channels have an equilibrium length with a predictable bed profile? Why are channels typically converging? What fundamental differences exist between lagoon channels and estuaries? We identify three distinct cases. The first (coastal) case concerns the 'short' tidal channels observed in coastal wetlands and lagoons: their dis-tinct feature is the absence of a fluvial supply of fresh water and sediments. This case has been fully explored. In particular, Seminara et al. (2010) showed that rigorous conditions of static equilibrium exist and require that the sediment flux must vanish at each instant throughout the tidal cycle. The equilibrium length is proportional to the inlet depth and decreases as convergence, roughness or tidal amplitude increase. These channels satisfy the so called O'Brien law. Results have been substantiated by detailed laboratory measurements of Tambroni et al. (2005a). The second (fluvial) case concerns the transition of a river into a tidal channel characterized by fairly 'small' tidal oscillations. We derive a perturbation solution for flow and bed topog-raphy showing that equilibrium arises from a balance between the aggrading effect of channel divergence and the opposite effect of the residual sediment flux driven by tide propagation. The third (estuarine) case concerns the transition of a river into a tidal channel characterized by fairly 'large' tidal oscillations. We derive a numeri-cal solution for flow and bed topography able to describe conditions intermediate between the two limit cases discussed previously. Results show that the model is able to describe a wide class of settings: ranging from fluvial dominated estuaries to tidally dominated estuaries, where the equilibrium profile tends to the 'coastal' profile, with some correction needed in order for the hydrodynamics to accommodate the fluvial transport.

Recent works, both theoretical (Schuttelaars and de Swart, Eur. J. Mech., B/Fluids, 1996; Schuttelaars and de Swart, J. Geophys. Res., 2000; Lanzoni and Seminara, J. Geophys. Res., 2002; Seminara et al., J. Fluid Mech., 2010) and experimental (Tambroni et al., J. Geophys. Res 2005; Garotta et al., Proc. 5th IAHR RCEM Symposium, 2007) have investigated the long term morphodynamic equilibrium of tidal channels. The main outcome of such works is the prediction of the existence of an equilibrium bed profile, which is established through the propagation of a sediment wave leading to the emergence of the bed in the landward part of the channel. The above works have ignored the role of tidal flats, permanently submerged areas bordering laterally the main channel. More recently, Canestrelli et al. (5th IAHR RCEM Symposium 2007) and Tambroni and Seminara (Scientific Research and Safeguarding of Venice, 2008) have investigated numerically the morphodynamic evolution of tidal channels flanked ...

ABSTRACT The paper reports on a laboratory investigation of the morphodynamic evolution of tidal erodible channels. The issue of the possible existence of a long term longitudinal equilibrium profile of tidal channels has been recently pursued theoretically by Schuttelars &\ de Swart (2000), Lanzoni &\ Seminara (2002) and Bolla Pittaluga &\ Seminara (2002). The latter work suggests that tidal channels closed at one end and connected at the other end with a tidal sea, can reach a morphodynamic equilibrium characterized by vanishing net sediment flux in a tidal cycle and the formation of a beach close to the landward end of the channel. The latter findings have been verified through a controlled laboratory experiment performed at the laboratory of the Department of Environmental Engineering of the University of Genoa (Italy). In the first set of experiments a straight rectangular channel characterised by constant width has been employed, while the effects of a weak channel convergence have been introduced in the second experiment. Experimental results confirm that sediment dynamics is essentially controlled by the marked distortion and asymmetry of tidal currents: the latter lead, in flood dominated channels, to a net, landward directed, sediment flux within a tidal cycle. As a consequence sediments are removed from the seaward portion of the channel and transported landward where they deposit. The morphodynamic evolution of the longitudinal profile observed during the experiments shows that, starting from an initially flat bed, tidal channels do evolve towards an equilibrium configuration upward concave seaward and convex landward, curvature increasing as the estuary convergence increases. Observations concerning the morphodynamic evolution of the channel inlet and the formation of bedforms, are also reported. Bolla Pittaluga, M. e Seminara, G., Depth integrated modelling of suspended sediment transport submitted to Water Resour. Res. (2002). Lanzoni, S.e Seminara,G., Long term evolution and morphodynamic equilibrium of tidal channels, J.Geoph.Res., 107, C1,1-13, 2002. Schuttelaars, H.M. e de Swart, H., Multiple morphodynamic equilibria in tidal embayment. J.Geoph.Res., 105, C10, 24105-24118, 2000.

... con sezione rettangolare di larghezza B* che si riduce esponenzialmente nella direzione longitudinale x* (Friedrichs & Aubrey, 1994) secondo la ... La formulazione matematica del problema è completata dall'equazione di Exner (1925), che esprime il principio di conservazione ...

We develop a three dimensional non linear asymptotic theory for flow and bed topography in meandering channels able to describe finite amplitude perturbations of bottom topography. The model extends a previous analysis on the equilibrium finite bed deformations, accounting here for arbitrary, yet slow, variations of channel curvature. This approach then allows us to formulate a non-linear bend instability theory, which predicts several characteristic features of the actual meandering process and extends results obtained by classical linear bend theories. In agreement with previous weakly non linear findings and consistently with field observations, the bend growth rate turns out to have a peak at some value of the meander wavenumber, typically larger than the resonant value of linear stability theory. Moreover, a feature typical of non linear waves arises: the selected wavenumber depends on the amplitude of the initial perturbation and, in particular, larger wavelengths are associat...

... Res. 33, 519 (1998). LCVan Rijn, “Sediment transport, part II: Suspended load transport,” J. Hydraul. ... The coordinate measures the distance from the inlet. Multiple row bars in the Waimakariri River, New Zealand (courtesy of Bianca Federici). Sketch of the channel and notations. ...

Perturbations of channel geometry (like variations of channel curvature or channel width) in meandering rivers give rise to morphodynamic effects which display themselves through the development of large-scale perturbations of bottom topography in the form of stationary bars developing in the longitudinal direction. The latter may then drive the lateral migration of the channel by enhancing bank erosion at bar pools: through this mechanism local perturbations of channel geometry may affect the planimetric development of meandering rivers on large timescales. The problem tackled herein is whether such morphodynamic influence is invariably felt downstream as the commonly employed model of river meandering would suggest.In order to solve this problem, we derive the exact solution of the linearized form of the mathematical problem of river morphodynamics. Linear analysis had pointed out the existence of a resonance phenomenon: in a linear (hence ideal) context, resonance occurs when the...

We investigate the basic mechanism whereby bars form in tidal channels or estuaries. The channel is assumed to be long enough to allow neglect of the effects of end conditions on the process of bar formation. In this respect, the object of the present analysis differs from that of Schuttelaars & de Swart (1999) who considered bars of length scaling with the finite length of the tidal channel. The channel bottom is assumed to be cohesionless and consisting of uniform sediments. Bars are shown to arise from a mechanism of instability of the erodible bed subject to the propagation of a tidal wave. Sediment is assumed to be transported both as bedload and as suspended load. A fully three-dimensional model is employed both for the hydrodynamics and for sediment transport. At the leading order of approximation considered, the effects of channel convergence, local inertia and Coriolis forces on bar instability are shown to be negligible. Unlike fluvial free bars, in the absence of mean cur...

Do tidal channels have a characteristic length? Given the sediment characteristics, the inlet conditions and the degree of channel convergence, can we predict it? And how is this length affected by the presence of tidal flats adjacent to the channel? We answer the above questions on the basis of a fully analytical treatment, appropriate for the short channels typically observed in coastal wetlands. The equilibrium length of non-convergent tidal channels is found to be proportional to the critical flow speed for channel erosion. Channel convergence causes concavity of the bed profile. Tidal flats shorten equilibrium channels significantly. Laboratory and field observations substantiate our findings.

ABSTRACT River meandering has been extensively investigated. Two fundamental features to be explored in order to make further progress are nonlinearity and unsteadiness. Linear steady models have played an important role in the development of the subject but suffer from a number of limits. Moreover, rivers are not steady systems; rather their states respond to hydrologic forcing subject to seasonal oscillations, punctuated by the occurrence of flood events. We first derive a classification of river bends based on a systematic assessment of the various physical mechanisms affecting their morphodynamic equilibrium and their evolution in response to variations of hydrodynamic forcing. Using the database by Lagasse et al. (2004) we also show that natural meanders are typically mildly curved and long, i.e. such that both the centrifugal and the topographic secondary flows are weak, but they are almost invariably nonlinear. We then review some recent developments which allow us to treat analytically the flow and bed topography of mildly curved and long nonlinear bends subject to steady forcing, taking advantage of the fact that flow and bed topography in mildly curved long bends are slowly varying. Results show that nonlinearity has a number of consequences: most notably damping of the morphodynamic response and upstream shifting of the location of the nonlinear peak of the flow speed. Next we extend the latter model to the case of unsteady forcing. Results are found to depend crucially on the ratio between the flood duration and a morphodynamic timescale. It turns out that, in a channel subject to a repeated sequence of floods, the system reaches a dynamic equilibrium. We conclude the paper discussing how the present assessment relates to the debate on meander modelling of the late 1980s and suggesting what we see as promising lines of future developments.

ABSTRACT Experimental measurements of the collision and rebound of a spherical particle with a solid wall in both newtonian and non-newtonian flow are provided. Experiments are carried out in a cubic Plexiglas tank where spheres of different materials, with a di- ameter of 5 and 10 mm, are free to settle in a still fluid. The latter consists of water, in the first set of observations, and a solution of Carboxylmethylcellulose (CMC) in wa- ter, in the second set. The rheology of the solution has been investigated and found to behave as an Hershel-Bulkley fluid. Different viscosities have been obtained varying CMC concentrations, providing a transparent media for Particle Image Velocimetry acquisition. The position and the velocity of the particle are estimated measuring in time the position of its centroid. Images have been acquired by IDT Cross correlation camera with a frame rate of 30Hz synchronized with a Nd:Yag laser by New Wave Re- search. Simultaneous measurements of both particle velocity and flow field allow for the identification of flow structures triggered by the sedimentation of the sphere and its impact on the horizontal wall. Comparison between the intensity of the ejections of the two different fluids observed whenever the particle approaches the wall can shed some light on particle wall collision mechanism in non-newtonian flow. Implications can be sought in the mechanism of collision of coarse particles in a muddy matrix, a process known to play an important role in debris flow dynamics.

Bedrock rivers link climate, tectonics, and topography, and drive landscape evolution. Sediment in bedrock channels plays a major role in their dynamics, because bedrock abrasion due to impacts from saltating bedload particles is an important, and sometimes dominant, erosive mechanism. However, because bedrock channels are characterized by conditions where sediment supply is less than the channel's transport capacity, morphodynamic theory that has provided valuable insights on alluvial rivers cannot readily be applied to bedrock systems. Here I present some recent work aimed at improving our ability to model and understand the morphodynamics of bedrock rivers, particularly the effects of sediment on channel evolution. I will present a theoretical framework for the morphodynamics of bedrock-alluvial channels that overcomes the restrictive assumption of sufficient sediment supply by reformulating the sediment continuity equation to account for temporal changes in the areal concent...

We review the state of knowledge on the bio-fluid dynamic mechanisms involved in the transmission of the infection from SARS-CoV-2. The relevance of the subject stems from the key role of airborne virus transmission by viral particles released by an infected person via coughing, sneezing, speaking or simply breathing. Speech droplets generated by asymptomatic disease carriers are also considered for their viral load and potential for infection. Proper understanding of the mechanics of the complex processes whereby the two-phase flow emitted by an infected individual disperses into the environment would allow us to infer from first principles the practical rules to be imposed on social distancing and on the use of facial and eye protection, which to date have been adopted on a rather empirical basis. These measures need compelling scientific validation. A deeper understanding of the relevant biological fluid dynamics would also allow us to evaluate the contrasting effects of natural ...

Tidal meanders (Marani et al, Water Resour Res, 2002) display similarities as well as important differences from fluvial meanders (Seminara, J Fluid Mech, 2006). Like fluvial meanders they have characteristic wavelengths scaling with channel width: this is why the convergent character of tidal channels leads to meander wavelengths decaying landward. Unlike fluvial meanders, the typical curvature spectra of tidal meanders contain even harmonics: hence, meander skewing does non display any distinct correlation with the flow direction and the known Kinoshita curve, which approximates the shape of fluvial meanders, is not appropriate to tidal meanders. Additional constraints are brought up by the spatial gradients of the basic bed profile connected to the finite length of tidal channels at equilibrium. In fact, it has been theoretically established (Schuttelaars and De Swart, Eur J Mech, B/Fluids, 1996, Seminara et al, J Fluid Mech submitted, 2009) and confirmed by controlled laboratory...

Although tidal channels typically exhibit a meandering pattern, their morphodynamics still awaits to be fully explored. Theoretical findings (Solari et al., 2002) suggest that in a sequence of 'sine generated' tidal meanders, oscillations associated with the basic flow give rise to symmetric oscillations of the bar-pool pattern around the locations of maximum curvature with no net migration in a tidal cycle. The above authors also analyzed the bottom evolution showing that it reaches eventually a state of morphodynamic equilibrium. In order to shed further light on the process, a physical model has been constructed, consisting of a sequence of five sine generated meanders with constant width of 0.4 m. The channel is 21.3 m long, it is closed at one end and connected at the other with a basin, representing the sea, where a tidal wave is generated. The channel width was chosen, on the basis of previous experimental observations referring to straight channels (Tambroni et al., ...

ABSTRACT Some recent work on the mechanics of formation and non linear development of bars is reviewed. It is shown that many characteristic features of non linear systems, namely non linear dispersion, damped oscillations, resonance, instabilities and bifurcations are displayed by bars. Some novel results on non linear behaviour of resonant meanders are outlined, which appear to open new paths to model the development of meander bends of finite amplitudes.