Computational Methods and Applications to Simulate Water-Related Natural Hazards

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SPH Modeling of Solid Boundaries Through a Semi-Analytic Approach

Abstract:This paper presents a general semi-analytic approach for modeling solid boundaries in the SPH method: boundaries are here considered as a material continuum with a suitable distribution of velocity and pressure; their contributions to each term of the SPH mass and momentum equations can be expressed in terms of a suitable integral extended to the part of the sphere of influence of the particle delimited by the boundary surface. Analytical details with reference to a slightly compressible viscous Newtonian fluid in three dimensions are given. The validity of the method is checked by comparing the obtained numerical results with available experimental data in a benchmark flow case

Combining noisy well data and expert knowledge in a Bayesian calibration of a flow model under uncertainties: an application to solute transport in the Ticino basin

Groundwater flow modeling is commonly used to calculate groundwater heads, estimate groundwater flow paths and travel times, and provide insights into solute transport processes within an aquifer. However, the values of input parameters that drive groundwater flow models are often highly uncertain due to subsurface heterogeneity and geologic complexity in combination with lack of measurements/unreliable measurements. This uncertainty affects the accuracy and reliability of model outputs. Therefore, parameters' uncertainty must be quantified before adopting the model as an engineering tool. In this study, we model the uncertain parameters as random variables and use a Bayesian inversion approach to obtain a posterior,data-informed, probability density function (pdf) for them: in particular, the likelihood function we consider takes into account both well measurements and our prior knowledge about the extent of the springs in the domain under study. To keep the modelistic and computational complexities under control, we assume Gaussianity of the posterior pdf of the parameters. To corroborate this assumption, we run an identifiability analysis of the model: we apply the inversion procedure to several sets of synthetic data polluted by increasing levels of noise, and we determine at which levels of noise we can effectively recover the "true value" of the parameters. We then move to real well data (coming from the Ticino River basin, in northern Italy, and spanning a month in summer 2014), and use the posterior pdf of the parameters as a starting point to perform an Uncertainty Quantification analysis on groundwater travel-time distributions.Comment: First submissio

Combining the Morris Method and Multiple Error Metrics to Assess Aquifer Characteristics and Recharge in the Lower Ticino Basin, in Italy

Groundwater flow model accuracy is often limited by the uncertainty in model parameters that characterize aquifer properties and aquifer recharge. Aquifer properties such as hydraulic conductivity can have an uncertainty spanning orders of magnitude. Meanwhile, parameters used to configure model boundary conditions can introduce additional uncertainty. In this study, the Morris Method sensitivity analysis is performed on multiple quantities of interest to assess the sensitivity of a steady-state groundwater flow model to uncertain input parameters. The Morris Method determines which of these parameters are less influential on model outputs. Uninfluential parameters can be set constant during subsequent parameter optimization to reduce computational expense. Combining multiple quantities of interest (e.g., RMSE, groundwater fluxes) when performing both the Morris Method and parameter optimization offers a more complete assessment of groundwater models, providing a more reliable and physically consistent estimate of uncertain parameters. The parameter optimization procedure also provides us an estimate of the residual uncertainty in the parameter values, resulting in a more complete estimate of the remaining uncertainty. By employing such techniques, the current study was able to estimate the aquifer hydraulic conductivity and recharge rate due to rice field irrigation in a groundwater basin in Northern Italy, revealing that a significant proportion of surficial aquifer recharge (approximately 81-94%) during the later summer is due to the flood irrigation practices applied to these fields.Comment: second submission after minor revision

WCSPH with Limiting Viscosity for Modeling Landslide Hazard at the Slopes of Artificial Reservoir

This work illustrated an application of the FOSS code SPHERA v.8.0 (RSE SpA, Milano, Italy) to the simulation of landslide hazard at the slope of a water basin. SPHERA is based on the weakly compressible SPH method (WCSPH) and holds a mixture model, consistent with the packing limit of the Kinetic Theory of Granular Flow (KTGF), which was previously tested for simulating two-phase free-surface rapid flows involving water-sediment interaction. In this study a limiting viscosity parameter was implemented in the previous formulation of the mixture model to limit the growth of the apparent viscosity, thus saving computational time while preserving the solution accuracy. This approach is consistent with the experimental behavior of high polymer solutions for which an almost constant value of viscosity may be approached at very low deformation rates near the transition zone of elastic–plastic regime. In this application, the limiting viscosity was used as a numerical parameter for optimization of the computation. Some preliminary tests were performed by simulating a 2D erosional dam break, proving that a proper selection of the limiting viscosity leads to a considerable drop of the computational time without altering significantly the numerical solution. SPHERA was then validated by simulating a 2D scale experiment reproducing the early phase of the Vajont landslide when a tsunami wave was generated that climbed the opposite mountain side with a maximum run-up of about 270 m. The obtained maximum run-up was very close to the experimental result. Influence of saturation of the landslide material below the still water level was also accounted, showing that the landslide dynamics can be better represented and the wave run-up can be properly estimated

A LAGRANGIAN PARTICLE MODEL FOR LANDLSIDE DYNAMICS AND WATER-SEDIMENT INTERACTION

The dynamic analysis of free-surface, rapidly varied dense granular flows is of great concern in several field of the engineering. In particular, this topic is relevant when dealing with rainfall-induced shallow landslides that represent the most common natural hazards in some areas of the world (Bordoni et al, 2015). Beside the assessment of the triggering zones for defining maps of shallow landslides' susceptible areas, could be of interest also the analysis of their propagation for assessing landslide run-out associated risk with respect to sensible targets (i.e. civil structures and infrastructures). Here are illustrated the basic aspects of a lagrangian, mesh-free particle method (SPH) that has been successfully adopted to treat landslide motion and the hydrodynamic action on non-cohesive sediment. SPH could be extended to the dynamic analysis of rainfall-induced shallow landslides, especially those ones classified as complex landslides that, starting as shallow rotational-translational failures, change into earth flows owing to the large water content and behave like dense granular flow. The load caused by the landslide impact on fixed obstacles (e.g. bridge piles or retaining walls) can be also evaluated through the SPH approach (Di Monaco et al., 2011)

Standard WCSPH for Free-Surface Multi-Phase Flows with a Large Density Ratio

The standard weakly compressible Smoothed Particle Hydrodynamics (WCSPH) is successfully applied to multi-phase problems involving fluids with similar densities, but when density ratio increases at some order of magnitude, serious instability phenomena occur at the interface. Several remedies have been proposed based on numerical correctives that deviate from standard formulation, increasing the algorithm complexity and, sometimes, the computational cost. In this study, the standard SPH has been adapted to treat free-surface multi-phase flows with a large density ratio through a modified form of the governing equations which is based on the specific volume (i.e. the inverse of particle volume) instead of density: the former is continuous across the fluid interface while the latter is not and generates numerical instability. Interface sharpness is assured without cohesion forces; kernel truncation at the interface is avoided. The model, relatively simple to implement, is tested by simulating two-phase dam breaking for two configurations: kinematic and dynamic features are compared with reference data showing good agreement despite the reduced computational effort

Dynamic Analysis of an Offshore Wind Turbine: Wind-Waves Nonlinear Interaction

An offshore wind turbine can be considered as a relatively complex structural system since several environmental factors (e.g. wind and waves) affect its dynamic behavior by generating both an active load and a resistant force to the structure’s deformation induced by simultaneous actions. Besides the stochastic nature, also their mutual interaction should be considered as nonlinear phenomena could be crucial for optimal and cost-effective design. Another element of complexity lies in the presence of different parts, each one with its peculiar features, whose mutual interaction determines the overall dynamic response to non-stationary environmental and service loads. These are the reasons why a proper and safe approach to the analysis and design of offshore wind turbines requires a suitable technique for carrying out a structural and performances decomposition along with the adoption of advanced computation tools. In this work a finite element model for coupled windwaves analysis is presented and the results of the dynamic behavior of a monopiletype support structure for offshore wind turbine are shown

WCSPH with Limiting Viscosity for Modeling Landslide Hazard at the Slopes of Artificial Reservoir

This work illustrated an application of the FOSS code SPHERA v.8.0 (RSE SpA, Milano, Italy) to the simulation of landslide hazard at the slope of a water basin. SPHERA is based on the weakly compressible SPH method (WCSPH) and holds a mixture model, consistent with the packing limit of the Kinetic Theory of Granular Flow (KTGF), which was previously tested for simulating two-phase free-surface rapid flows involving water-sediment interaction. In this study a limiting viscosity parameter was implemented in the previous formulation of the mixture model to limit the growth of the apparent viscosity, thus saving computational time while preserving the solution accuracy. This approach is consistent with the experimental behavior of high polymer solutions for which an almost constant value of viscosity may be approached at very low deformation rates near the transition zone of elastic–plastic regime. In this application, the limiting viscosity was used as a numerical parameter for optimization of the computation. Some preliminary tests were performed by simulating a 2D erosional dam break, proving that a proper selection of the limiting viscosity leads to a considerable drop of the computational time without altering significantly the numerical solution. SPHERA was then validated by simulating a 2D scale experiment reproducing the early phase of the Vajont landslide when a tsunami wave was generated that climbed the opposite mountain side with a maximum run-up of about 270 m. The obtained maximum run-up was very close to the experimental result. Influence of saturation of the landslide material below the still water level was also accounted, showing that the landslide dynamics can be better represented and the wave run-up can be properly estimated