Showing papers by "Giovanni Seminara published in 2006"

On the nature of meander instability

Abstract: [1] Bend instability is the process whereby perturbations of the planform distribution of a channel relative to a straight configuration may grow, driven by erosion at concave banks and deposition at convex banks, leading to the development of a meandering pattern. Here we investigate the nature of this instability; that is, we ascertain under what conditions bend instability is convective or absolute. In the former case, an initial nonpersistent, small perturbation localized in space is convected away, eventually leaving the flow domain unperturbed. In the latter case, perturbations spread in both the upstream and downstream directions, eventually affecting the whole flow domain. In convective instabilities, the spatial-temporal development of perturbations is somewhat dependent on the characteristics of the initial perturbation which is required to be persistent in time. On the contrary, absolute instabilities are able to amplify perturbations, even if triggered at some initial time and then ceased. If bend instability is convective, planform information migrates only in one direction, while in the absolute regime, information is propagated in both directions. We show that bend instability is most often, though not invariably, convective at both a linear and nonlinear level. Moreover, the group velocity of perturbations changes sign as the width to depth ratio of the channel crosses some threshold value (the resonant value of Blondeaux and Seminara (1985)): Below (above) resonance, information is propagated downstream (upstream). We discuss the implications that these findings have on the morphological characteristics of meandering rivers (in particular, the sense of skewing of meander bends and the direction of meander migration). We also clarify how the choice of appropriate boundary conditions in numerical simulations of planform evolution is crucially dependent on the nature of bend instability and on its subresonant or superresonant regime.

Long waves in erodible channels and morphodynamic influence

Abstract: [1] Like most media, open channel flows propagate information through waves When the channel boundary is fixed, the vectors of information consist typically of surface gravity waves In the less straightforward case of channels with cohesionless bed and possibly erodible banks, other types of waves arise from the erodible nature of the boundaries and the ability of the stream to transport sediments In this paper we restrict our attention to the important case of long waves, which can be described by employing the shallow water approximation for the flow field and a quasi-equilibrium assumption for sediment transport on weakly sloping beds We focus on a major issue: In which direction is information propagated? This is a problem raised and partially solved by de Vries in the context of one-dimensional morphological modeling as early as 1965 We review some of the available knowledge on this subject, viewed in a more general context where vectors of information can be a variety of waves: purely longitudinal one-dimensional sediment waves, two-dimensional waves driven by large-scale bed forms (bars), and plan form waves carrying information related to the planform shape of the channel Both linear and nonlinear, migrating and stationary waves are considered It turns out that the role played by the Froude number in determining the direction of one-dimensional perturbations of bed topography is somewhat taken by the aspect ratio of the channel when large-scale two-dimensional bed forms as well as planform waves are considered

Are inlets responsible for the morphological degradation of Venice Lagoon

Abstract: [1] Through the centuries, Venice Lagoon has undergone morphological changes that can be attributed to both natural events and human actions. The lagoon has progressively deepened, and it is claimed to lose roughly one million cubic meters of sediments each year. In the ongoing debate concerning the possible means to counteract this morphodynamic degradation, inlet geometry is considered a major factor controlling the exchange of sediments. Our aim is to explore the causes of this loss. We focus first on sand, as this is the type of sediment present on the bottom of the near-inlet regions. We employ a simple model of the inlet hydrodynamics to estimate the net exchange of sand associated with the sequence of tidal events recorded for several years. Results suggest that in the absence of an excess supply from the sea, the yearly loss of sand through Venice inlets is an order of magnitude smaller than the total sediment loss usually claimed. We then show that this estimate is only slightly affected by the sand supply from wave resuspension in the far field whose effect is simply to store sediments in the near-inlet region. We finally argue that most of the sediment loss is wash load carried by the ebb currents overloaded by very fine sediments resuspended by wind in the inner lagoon and unable to settle within the channel network.

Effect of suspended load on sandbar instability

Abstract: [1] We analyze bars in sandy rivers when a significant portion of the sediment load is carried in suspension. First, we investigate the ability of a depth-averaged formulation of both the hydrodynamics and sediment transport recently developed for slowly varying flows to capture the main mechanisms of sandbar formation. The exercise turns out to be successful and opens the way to the development of analytical and numerical models of the nonlinear development of sandbars requiring a feasible computational effort. Second, we make use of the depth-averaged approach to analyze the nature of bar instability in sandy rivers applying Briggs criterion to distinguish between the convectively and absolutely unstable asymptotic response to an initial boundary value perturbation of bed topography. This analysis allows us to conclude that in the presence of suspended load, instability remains convective. The practical implications of such findings are discussed.

On the migration of tidal free bars

Abstract: We derive and employ a depth-averaged model for the formation of free bars in infinitely long tidal channels in order to investigate the mechanism whereby tidal bars may experience a net migration over a tidal cycle. The flux of the suspended sediment is modeled by means of an analytical relationship derived by Bolla Pittaluga and Seminara [M. Bolla Pittaluga and G. Seminara, Water Resour. Res. 39, 5 (2003)] for slowly varying flows. The model is validated by performing a linear stability analysis of flow and bed topography in a rectangular channel with an erodible bed, subject to the propagation of a symmetric tidal wave of small amplitude. The results of the present depth averaged model show a fairly satisfactory agreement with previous results based on a three-dimensional model [G. Seminara and M. Tubino, J. Fluid Mech. 440, 49 (2001)]. We then investigate the role of overtides, showing that a flood or ebb asymmetry of the basic flow gives rise to a net migration of bars. The mechanism is due to the no...