Giampaolo Di Silvio

A one-dimensional model for the transport of a sediment mixture in non-equilibrium conditions

The non-stationary one-dimensional continuity equations of each granulometric class of sediments are written for the waterstream (suspended transport), the bottom layer (bedload transport) and the mixing layer below the bottom surface. Proper closure equations have been introduced to express the transversal sediment (luxes between these three layers, accounting for the different adaptation-length of each grainsize class. A number of examples are given, showing the importance of non-equilibrium for non-uniform particles in erosion-deposition processes at short time-scale.

Control of macroalgae blooms in the Lagoon of Venice

Abstract The present work is part of the program financed by the Italian Government to save Venice and its lagoon. The aim of the program is the identification, design and construction of the interventions needed both to protect the territory from flood events and to improve the quality of the ecosystem. This paper reports on the progress of the work concerning the second point. Extensive blooms of macroalgae of the type Ulva rigida leading to anoxia in large parts of the Venice lagoon are rapidly reducing the quality of the ecosystem and affecting its use as a natural resource for fishing, acquiculture and recreation. The uncontrolled eutrophication of the lagoon is caused mainly by the presence of high loads of nutrients and the particular morphological and hydro-dynamic setting of the lagoon. Overlaying techniques using GIS capabilities were adopted to assess the relative importance of the physical and chemical factors for each part of the lagoon and the most vulnerable areas were identified. To formalize the cause-effect chains in the Venice lagoon an ecological model was set up based on a fine-grid hydrodynamic flow field. The ecological model focused on the life cycle of the macroalgae. It includes the processes of overwintering, growth and decomposition as well as the horizontal transport due to wind and the circulation induced by the tide. Combined use of the GIS and the model has allowed the definition of the most promising strategies based on the present understanding of the functioning of the ecosystem.

Modelling the impact of large impoundments on the Lower Zambezi River

The Lower Zambezi River is influenced by the presence of two very large hydropower impoundments which have modified the natural seasonal flows, the sediment balance, the morphology of the river and the pattern of the riparian vegetation. Downstream of these large impoundments, appreciable local effects are reported to take place, such as scour, bank collapse and shoreline degradation. In order to quantify the sediment transport along the Lower Zambezi River and to predict the effects of the dams on morphology we have developed a simplified 1-D hydro-morphodynamic model capable to deal with the scarcity of available data. Besides the systematic flow records at the dam gauging station and few occasional measurements of turbidity and grain size of the river bed, only the Digital Elevation Model (DEM) and some recent satellite images of the river have been used as the input data of the model. The results confirm the expected qualitative response of the river to the constructions of dams: the reduction of water flow has an immediate effect downstream of the reservoirs by initially fostering the sediment deposition; afterwards, the total interception of sediment by the dams slowly takes over and inverts this tendency; a smaller aggradation rate with respect to the natural conditions without dams, seems to represent the dominant effect of damming in the long-term evolution of the river. The morphological effects of damming over the transversal cross-section of the river have been studied by a simplified transversal quasi 2-D sub-model, coupled to a 1-D longitudinal model. Besides the evolution of the bottom longitudinal profile and grain size composition, the model simulates also the evolution of the cross-section shape and the growth of the riparian vegetation. A first application confirms the qualitative observations reported in the literature on the recent planimetric evolution of the Lower Zambezi.

Modeling Desiltation of Reservoirs by Bottom-Outlet Flushing

The problem of preventing the total and irreversible obstruction of the outlet has been examined on the model. Two typical configurations have been considered: (a) short tunnel through the dam, and (b) long tunnel by-passing the dam. After a qualitative description of the flushing operations and of the behaviour of the outlet, the model laws and similitude criteria have been discussed in the paper. Some quantitative results obtained from the model of type (b) are also provided.

Modeling of River Width Variations Based on Hydrological, Morphological, and Biological Dynamics

AbstractOne-dimensional (1D) models can be applied, for engineering purposes, to long-term and large-scale morphodynamic simulations of entire river systems only if appropriate simplifications are introduced. This paper proposes an improvement of an existing simplified 1D model based on the local uniform flow hypothesis, coupled with a synthetic description of the transverse profile, which provides the active river width by analyzing the total and the vegetated widths of a watercourse, with assumed variables of the water flow. The overall density of the riparian vegetation, expressed in terms of biological carrying capacity, is predicted as a function of the local climate and some stresses due to interaction between hydrology, morphology, and biology. The constitutive equations have been deduced and the relevant parameters have been calibrated and validated against various hydrological and geometrical data, taken by satellite imagery covering two large watercourses located in tropical and subtropical area...

Floods and sediment dynamics in mountain rivers

While floods in plain rivers are solely controlled by water flow, severe inundations in mountain rivers are always related to extremely large inputs of sediment from landslides and debris flow, followed by sudden deposition along the hydrographic network and consequent bank overflowing. To describe the space — and time — dependent process in relatively large and morphologically complex watersheds, an adequate model for water flow and sediment transport is proposed. Since a fundamental role in the overaggradation mechanism is played by the grain size distribution of sediment input, an approximate method is described how to estimate the composition of input material by the composition of material at the stream bottom.

Morphological Consequences of the Nodal Modulation on a Tidal Lagoon

AbstractDespite its relatively small amplitude, the nodal tide can play a relevant role in several environmental processes, spanning from ecology to morphodynamics; in particular, field studies and numerical investigations indicate the nodal tide as the dominant factor in long-term and large-scale net sediment transport in some estuaries. Nevertheless, while some important results have been obtained about its morphological effect on estuary channels, the response of tidal flats and salt marshes is still to be investigated. In the present work, a two-dimensional (2D), vertically averaged, long-term morphodynamic model based on the concepts of intertidal dispersion and transport concentration has been applied to the case of a schematic lagoon forced by a nodally-modulated tidal range. The effect of such a periodic oscillation has been studied in terms of sediment fluxes, suspended sediment concentration, and morphology. The results of simulations, carried out starting from different equilibrium conditions i...

Validity and Limitations of Different Transport Models with Particular Reference to Sediment Transport

Abstract The consequences of successive simplifications (Fig. 1) made on the transport equations in order to obtain simpler and less detailed models, are examined. After a review of surface flow models (transport of mass and momentum) with constant density (Fig. 2), the transport models of a neutrally buoyant tracer are discussed (Fig. 3). Groundwater flow and groundwater tracer transport models are also considered and the approaches for modeling transport processes in stratified flows are mentioned. Afterwards, a survey of sediment transport models is made, stressing the effects of space and time averaging. Starting from the most general three-dimensional model of suspension, a one-dimensional unsteady model is obtained, aimed at simulating short time-scale processes both with uniform and non-uniform grainsize distribution. The quasi-steady onedimensional model (averaged over the hydrological cycle) is discussed, considering at first different grainsize classes and then all the classes altogether in the overall transport.