Paolo Ghilardi | University of Pavia (original) (raw)
Papers by Paolo Ghilardi
Various tests are run using two different laboratory flumes with rectangular cross section and tr... more Various tests are run using two different laboratory flumes with rectangular cross section and transparent walls. The grains used in a single experiment have an almost constant grain sizes; mean diameter ranges from 5 mm to 20 mm. In each test various measurements are taken: hydrograms, velocity distribution near the transparent walls and on the free surface, average flow concentration. Concentration values are measured taking samples. Velocity distributions are obtained from movies recorded by high speed video cameras capable of 350 frames per second; flow rates and depth hydrograms are computed from the same velocity distributions. A gate is installed at the beginning of one of the flumes; this gate slides normally to the bed and opens very quickly, reproducing a dam-break. Several tests are run using this device, varying channel slope, sediment concentration, initial mixture thickness before the gate. Velocity distribution in the flume is almost constant from left to right, except for the flow sections near the front. The observed discharges and velocities are less than those given by a classic dam break formula, and depend on sediment concentration. The other flume is fed by a mixture with constant discharge and concentration, and is mainly used for measuring velocity distributions when the flow is uniform, with both rigid and granular bed, and to study erosion/deposition processes near debris flow dams or other mitigation devices. The equilibrium slope of the granular bed is very close to that given by the classical equilibrium formulas for debris flow. Different deposition processes are observed depending on mixture concentration and channel geometry.
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Fluvial Hydraulics of Mountain Regions, 1991
We present a numerical model whose goal is to describe the unsteady behavior of supercritical ope... more We present a numerical model whose goal is to describe the unsteady behavior of supercritical open channel flows on a loose granular bed as far as short term aggradation/degradation phenomena are concerned. Following the usual approach, the problem is formulated in terms of De Saint Venant equations plus a sediment continuity relationship. The presence of an unerodible bed underlying the granular layer is taken into account; to this purpose the sediment balance relationship is re-formulated using an original approach. The numerical solution is achived via a generalized Preissman scheme. A test simulation is discussed which gives rise to a solid transport mechanism essentially based on an intermittent shedding of sediment bursts; the example shows how a pulsating non periodic behavior may arise within a strictly deterministic framework this due to the strongly non linear formulation of the problem.
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Water Resources Research, 1990
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Journal of Hydrology, 2010
The objectives of this contribution are to analyse and quantify the segregation process of water ... more The objectives of this contribution are to analyse and quantify the segregation process of water saturated mixtures flowing down steep chutes.Experiments were carried out using two granular mixtures, a steep chute of variable length and a collecting box divided by sectors, in which the flowing material freely falls down at the chute outlet. The flow characteristics, the filling in time of the box itself, the grain-size composition of the box content with varying the distance from the chute outlet are examined by means of laser level transducers, image analysis and sieving.Segregation is effective and rapid along the flow direction, showing a modest dependence on mixture grain-size composition. The grain-size composition profile in time is reconstructed, showing coarser particles concentrated at the flow front, on the flow surface and at the sides, whereas fine fractions prevail at the tail and at the bottom.
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Owing to their destructive power, debris flow are the subjects of extensive investigations aiming... more Owing to their destructive power, debris flow are the subjects of extensive investigations aiming at characterizing and modeling their inner nature. Laboratory experiments on simulated dam breaks are one important way to trigger and study debris-flow waves. High-speed recordings of granular flows arising from a dam-break-like event can be processed to extract useful information about the flow dynamics. Although the techniques for measuring of velocities in liquid and gas flows are well established, they cannot be used directly on the flows arising from a dam break: this paper presents two required additions that were made to compute such flows. All the resulting quantities are being used to tune a mathematical model describing the observed flows.
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This research deals with the deposition of a debris flow in a laboratory flume made by two reache... more This research deals with the deposition of a debris flow in a laboratory flume made by two reaches with different slopes. The flume is fed by a steady gravel flow and a steady water flow, and tests are run varying concentration from 0.30 to 0.45. Deposition starts far downstream the slope decrease, triggering the development of a new granular bed which grows in the upstream direction while no local temporal increase of the bed thickness is observed. This last result differs from those of analogous experiments described in the literature. This difference in bed evolution is related to the different flow dynamics near the "solid wave front" which travels in the direction opposite to that of the debris flow.
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Debris flow is modelled using the equations governing the dynamics of a liquid-solid mixture. An ... more Debris flow is modelled using the equations governing the dynamics of a liquid-solid mixture. An upwind finite volume scheme is applied to solve the resulting differential equations in one dimension. These equations have a structure similar to those of the monophasic water flow, differing from them by the presence of some terms characteristic of the bifasic nature of the mixture,
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Pattern recognition and image …, 2005
The analysis focus on dam breaks stems from their ability to offer a simplified, yet effective wo... more The analysis focus on dam breaks stems from their ability to offer a simplified, yet effective workbench for debris flow waves, which in turn are helpful in gaining a deeper understanding of the highly destructive debris flows. High-speed recordings of granular flows arising from a dam-break-like event can be processed to extract useful information about the flow dynamics. Gradient-based optical-flow techniques cannot compute the correct velocity field as they detect the flow induced by the boundary evolution. Methods that are based on cross-correlation, such as particle imaging velocimetry (PIV), are able to capture the micro-scale flow, but, as they are designed for flows within fixed boundaries, they cannot deal directly with dam-break-caused flows because such flows, by their own nature, exhibit a fast moving boundary. This paper presents a procedure that is able to compute the evolving background and supply it to a PIV program as a masking region that should be excluded from the computation of the flow velocity field. This improvement leads to reliable results, while reusing existing software. All the resulting quantities are being used to tune a mathematical model describing the observed flows.
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Journal of Hydrology, 1996
Porous media whose grain sizes are distributed according to the law proposed by Turcotte (1986, J... more Porous media whose grain sizes are distributed according to the law proposed by Turcotte (1986, J. Geophys. Res., 91(B2): 1921–1926) are investigated in order to check if the void volume is a fractal object. It is shown that a power-law distribution of the particle sizes implies a scaling structure of the voids only within a given range of values of the porosity and of the distribution exponent.
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Physics and Chemistry of the Earth, Part C: …, 2001
A mathematical model is applied to simulate two debris flows occurred in 1997 and 1998 in Italian... more A mathematical model is applied to simulate two debris flows occurred in 1997 and 1998 in Italian Alpine valleys. The flows caused a casualty and significant damages to buildings and roads. The model considers a two-phase mixture of coarse sediments and interstitial fluid. The concentration of the finer solid fraction in the interstitial fluid is assumed to be negligible, so that this fluid acts as clean water. Assuming that the solids and the interstitial fluid move downstream with the same velocity, the flow of the mixture is described using a two dimensional depth averaged model with a unique 2D momentum equation and two mass balance equations for the mixture and the sediments, respectively. Erosion and deposition rates are computed with a modified version of the relationship developed by Egashira-Ashida. Differential equations are integrated with an upwind explicit finite-difference scheme. The total discharge hydrograph at the upstream sections of the alluvial fans was estimated by assuming equilibrium solid concentration values.
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Water Resour. Res, 1990
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Journal of Hydraulic Research, 2000
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Water resources …, 1993
Bookmarks Related papers MentionsView impact
Various tests are run using two different laboratory flumes with rectangular cross section and tr... more Various tests are run using two different laboratory flumes with rectangular cross section and transparent walls. The grains used in a single experiment have an almost constant grain sizes; mean diameter ranges from 5 mm to 20 mm. In each test various measurements are taken: hydrograms, velocity distribution near the transparent walls and on the free surface, average flow concentration. Concentration values are measured taking samples. Velocity distributions are obtained from movies recorded by high speed video cameras capable of 350 frames per second; flow rates and depth hydrograms are computed from the same velocity distributions. A gate is installed at the beginning of one of the flumes; this gate slides normally to the bed and opens very quickly, reproducing a dam-break. Several tests are run using this device, varying channel slope, sediment concentration, initial mixture thickness before the gate. Velocity distribution in the flume is almost constant from left to right, except for the flow sections near the front. The observed discharges and velocities are less than those given by a classic dam break formula, and depend on sediment concentration. The other flume is fed by a mixture with constant discharge and concentration, and is mainly used for measuring velocity distributions when the flow is uniform, with both rigid and granular bed, and to study erosion/deposition processes near debris flow dams or other mitigation devices. The equilibrium slope of the granular bed is very close to that given by the classical equilibrium formulas for debris flow. Different deposition processes are observed depending on mixture concentration and channel geometry.
Bookmarks Related papers MentionsView impact
Fluvial Hydraulics of Mountain Regions, 1991
We present a numerical model whose goal is to describe the unsteady behavior of supercritical ope... more We present a numerical model whose goal is to describe the unsteady behavior of supercritical open channel flows on a loose granular bed as far as short term aggradation/degradation phenomena are concerned. Following the usual approach, the problem is formulated in terms of De Saint Venant equations plus a sediment continuity relationship. The presence of an unerodible bed underlying the granular layer is taken into account; to this purpose the sediment balance relationship is re-formulated using an original approach. The numerical solution is achived via a generalized Preissman scheme. A test simulation is discussed which gives rise to a solid transport mechanism essentially based on an intermittent shedding of sediment bursts; the example shows how a pulsating non periodic behavior may arise within a strictly deterministic framework this due to the strongly non linear formulation of the problem.
Bookmarks Related papers MentionsView impact
Water Resources Research, 1990
Bookmarks Related papers MentionsView impact
Journal of Hydrology, 2010
The objectives of this contribution are to analyse and quantify the segregation process of water ... more The objectives of this contribution are to analyse and quantify the segregation process of water saturated mixtures flowing down steep chutes.Experiments were carried out using two granular mixtures, a steep chute of variable length and a collecting box divided by sectors, in which the flowing material freely falls down at the chute outlet. The flow characteristics, the filling in time of the box itself, the grain-size composition of the box content with varying the distance from the chute outlet are examined by means of laser level transducers, image analysis and sieving.Segregation is effective and rapid along the flow direction, showing a modest dependence on mixture grain-size composition. The grain-size composition profile in time is reconstructed, showing coarser particles concentrated at the flow front, on the flow surface and at the sides, whereas fine fractions prevail at the tail and at the bottom.
Bookmarks Related papers MentionsView impact
Owing to their destructive power, debris flow are the subjects of extensive investigations aiming... more Owing to their destructive power, debris flow are the subjects of extensive investigations aiming at characterizing and modeling their inner nature. Laboratory experiments on simulated dam breaks are one important way to trigger and study debris-flow waves. High-speed recordings of granular flows arising from a dam-break-like event can be processed to extract useful information about the flow dynamics. Although the techniques for measuring of velocities in liquid and gas flows are well established, they cannot be used directly on the flows arising from a dam break: this paper presents two required additions that were made to compute such flows. All the resulting quantities are being used to tune a mathematical model describing the observed flows.
Bookmarks Related papers MentionsView impact
This research deals with the deposition of a debris flow in a laboratory flume made by two reache... more This research deals with the deposition of a debris flow in a laboratory flume made by two reaches with different slopes. The flume is fed by a steady gravel flow and a steady water flow, and tests are run varying concentration from 0.30 to 0.45. Deposition starts far downstream the slope decrease, triggering the development of a new granular bed which grows in the upstream direction while no local temporal increase of the bed thickness is observed. This last result differs from those of analogous experiments described in the literature. This difference in bed evolution is related to the different flow dynamics near the "solid wave front" which travels in the direction opposite to that of the debris flow.
Bookmarks Related papers MentionsView impact
Debris flow is modelled using the equations governing the dynamics of a liquid-solid mixture. An ... more Debris flow is modelled using the equations governing the dynamics of a liquid-solid mixture. An upwind finite volume scheme is applied to solve the resulting differential equations in one dimension. These equations have a structure similar to those of the monophasic water flow, differing from them by the presence of some terms characteristic of the bifasic nature of the mixture,
Bookmarks Related papers MentionsView impact
Pattern recognition and image …, 2005
The analysis focus on dam breaks stems from their ability to offer a simplified, yet effective wo... more The analysis focus on dam breaks stems from their ability to offer a simplified, yet effective workbench for debris flow waves, which in turn are helpful in gaining a deeper understanding of the highly destructive debris flows. High-speed recordings of granular flows arising from a dam-break-like event can be processed to extract useful information about the flow dynamics. Gradient-based optical-flow techniques cannot compute the correct velocity field as they detect the flow induced by the boundary evolution. Methods that are based on cross-correlation, such as particle imaging velocimetry (PIV), are able to capture the micro-scale flow, but, as they are designed for flows within fixed boundaries, they cannot deal directly with dam-break-caused flows because such flows, by their own nature, exhibit a fast moving boundary. This paper presents a procedure that is able to compute the evolving background and supply it to a PIV program as a masking region that should be excluded from the computation of the flow velocity field. This improvement leads to reliable results, while reusing existing software. All the resulting quantities are being used to tune a mathematical model describing the observed flows.
Bookmarks Related papers MentionsView impact
Journal of Hydrology, 1996
Porous media whose grain sizes are distributed according to the law proposed by Turcotte (1986, J... more Porous media whose grain sizes are distributed according to the law proposed by Turcotte (1986, J. Geophys. Res., 91(B2): 1921–1926) are investigated in order to check if the void volume is a fractal object. It is shown that a power-law distribution of the particle sizes implies a scaling structure of the voids only within a given range of values of the porosity and of the distribution exponent.
Bookmarks Related papers MentionsView impact
Physics and Chemistry of the Earth, Part C: …, 2001
A mathematical model is applied to simulate two debris flows occurred in 1997 and 1998 in Italian... more A mathematical model is applied to simulate two debris flows occurred in 1997 and 1998 in Italian Alpine valleys. The flows caused a casualty and significant damages to buildings and roads. The model considers a two-phase mixture of coarse sediments and interstitial fluid. The concentration of the finer solid fraction in the interstitial fluid is assumed to be negligible, so that this fluid acts as clean water. Assuming that the solids and the interstitial fluid move downstream with the same velocity, the flow of the mixture is described using a two dimensional depth averaged model with a unique 2D momentum equation and two mass balance equations for the mixture and the sediments, respectively. Erosion and deposition rates are computed with a modified version of the relationship developed by Egashira-Ashida. Differential equations are integrated with an upwind explicit finite-difference scheme. The total discharge hydrograph at the upstream sections of the alluvial fans was estimated by assuming equilibrium solid concentration values.
Bookmarks Related papers MentionsView impact
Water Resour. Res, 1990
Bookmarks Related papers MentionsView impact
Journal of Hydraulic Research, 2000
Bookmarks Related papers MentionsView impact
Water resources …, 1993
Bookmarks Related papers MentionsView impact