Giorgio Galeati

Coupled and partially coupled Eulerian-Lagrangian Model of freshwater-seawater mixing

The problem of density-dependent transport of salt in unconfined coastal aquifers is solved numerically by means of an implicit Eulerian-Lagrangian finite element formulation. Such a formulation leads to symmetric positive definite finite element matrices which are ideally suited for efficient solution by preconditioned conjugate gradient methods. Additional known advantages of the formulation are unconditional stability, reduced numerical dispersion and suitability for parallel computation. The method has been used to study the effect of dewatering on seawater intrusion within a vertical cross section through an aquifer in southern Italy, related to the construction of a thermoelectric power plant. To investigate the extent to which the dependence of fluid density on salt concentration affects the numerical solution, the flow and advection-dispersion equations were solved in coupled (iterative), partially coupled (noniterative) and completely decoupled modes. Partial coupling was found to yield results very close to those obtained by full coupling but at great savings in computer time; the less rigorous decoupled approach led to results substantially different from those obtained through coupling and partial coupling. Effects of aquifer heterogeneity and the construction of a cutoff wall on seawater intrusion are discussed.

Global optimization techniques for the calibration of conceptual rainfall-runoff models

Abstract In this study we present the results of the comparison of three different algorithms: the Genetic Algorithm coupled with Sequential Quadratic Programming (GA-SQP), the Pattern Search also coupled with SQP (PS-SQP) and the Shuffled Complex Evolution (SCE-UA). The analyses were conducted using a conceptual rainfall-runoff model applied both to a single basin and to a complex basin. For both types of basin, a theoretical case without model and data errors was considered, in which the true values of the parameters are known a priori, and several real-world cases where model and data errors exist. With reference to the single basin, the SCE-UA algorithm was the most reliable while the other two algorithms gave solutions equivalent to those of the SCE-UA in the theoretical case, but in the real-world cases they showed an increasing tendency (particularly the PS-SQP) to be trapped in local minima. With reference to the complex basin, none of the three algorithms identified the exact solution in the theo...

Comparing several genetic algorithm schemes for the calibration of conceptual rainfall-runoff models

Abstract The Genetic Algorithm (GA) is often associated with local search optimisation techniques in the calibration process of Conceptual Rainfall-Runoff Models (CRRMs) (Wang, 1991; Franchini, 1996), i.e. the GA is used for approaching the region encompassing the global solution and then its results are used as a starting point for the local optimizer in the subsequent “fine-tuning” process. However, the GA can be formulated in very many ways. This study analyses various GA structures and their robustness and efficiency. In addition, a sensitivity analysis of the various schemes to their own parameters is performed. The analysis is conducted using an 11-parameter CRRM, called A Distributed Model (ADM), applied to both a theoretical case without model and data errors and two cases of the real world in which there are both model and data errors. Finally, assuming the same role as the GA for the “Pattern Search” (PS) method in a two-step optimisation technique (Hendrickson et al., 1988), the results of the ...

A comparison of parametric and non-parametric methods for runoff forecasting

Abstract The paper evaluates the performance of a non-parametric scheme, the nearest neighbour (NN) method, to predict the daily mean discharge in a mountain basin supplying a hydroelectric reservoir in northeastern Italy. The results are compared with those of an autoregressive model with exogenous input (ARX), coupled with a previously developed snow cover evolution model. Both methods give good performances, but the NN prediction requires a much simpler simulation structure. In the case investigated, for example, the snowpack accumulation-melting model can be completely eliminated. This greater simplification assumes considerable importance in the Electric Load Distribution Institutes of the Italian National Electricity Board (ENEL), where many hydroelectric basins are managed every day.

Estimating the index flood using indirect methods

Abstract Three indirect techniques for index flood estimation are analysed in order to evaluate their applicability and effectiveness. These indirect techniques, based on both statistical and conceptual approaches, are applied to a set of 33 hydrometric stations, located in a large area in northern-central Italy. The results show that the statistical model, due to its flexible structure, has a better descriptive ability than the physically-based models, which are rigidly structured as they conceptualize the rainfall-runoff transformation. However, the rigid structure of the conceptual approaches reduces their dependence on the specific information of the single stations and therefore increases their robustness. Finally, the results highlight that direct estimation techniques could be advisable for catchments with peculiar geomorphoclimatic properties; that is to say properties which differ substantively from those of the majority of the basins considered in the identification of the indirect models. This conclusion seems to hold even when a very limited amount of hydrometric information is available.

Flood risk assessment using an integrated hydrological and hydraulic modelling approach: a case study

Abstract This paper describes an integrated hydrological and hydraulic modelling approach for the risk assessment of a flood-prone area and its application to analysing the effects of extreme flood events on the Montalto di Castro thermoelectric power plant. The approach is based on four major steps. The first step entails a detailed analysis of available critical events as well the collection of hydro-meteorological and cartographic data to perform a statistical evaluation of extreme rainfall events and an estimation of the probable maximum precipitation (PMP). The second step involves the calibration of a rainfall—runoff model for the upper catchment area based on the data observed during a recent flood event. The third step involves the calibration of a two-dimensional hydraulic model for simulating the flood plain inundation using the previously reconstructed runoff and a comparison of the results with the maximum flood levels observed during the same event. The fourth and final step concerns the simulation by the two-dimensional hydraulic model of the flood wave obtained via the rainfall—runoff model using the extreme and PMP values of rain defined in the first step. The results of this approach appear to be extremely useful and easily transferable to other areas.

Optimization of a snow network by multivariate statistical analysis

ABSTRACT The aim of the present work is that of analysing the data of snow-cover measurements carried out in the northeastern Alps by the Italian National Electricity Board (ENEL) since 1966, in order to select a reduced number of measurement stations from which to evaluate the water equivalent amount. The selection was based on cluster methods for identification of homogeneous areas and afterwards on principal component analysis for individualization of representative stations. The results confirm that by employing a set of reduced input data the output is only moderately affected.

3-D finite element transport models by upwind preconditioned conjugate gradients

Publisher Summary This chapter discusses 3-D finite element transport models by upwind preconditioned conjugate gradients. Numerical solutions to the dispersion–convection equation in subsurface systems have relied extensively on the Galerkin approach, mainly because of its high versatility in handling irregular geometries. However, in convection-dominated problems, this approach leads to: 1) Unstable solution with oscillating behavior 2) artificial dispersion, which yields the smearing of the simulated contaminant plume. The oscillations can be dampened out by weighting unsymmetrically the trial functions while the numerical dispersion can be controlled by an appropriate selection of the grid Peclet and Courant numbers. This usually requires a refined mesh with a large number of nodes and some difficulty is easily met for a cost-effective solution in 3-D finite element simulations. The purpose of this chapter is to extend to 3-D problems a class of iterative solvers, based on the preconditioned conjugate gradients, which proved quite robust and efficient in 2-D dispersion-convection models.

Upwind Preconditioned Conjugate Gradients for Finite Element Transport Models

The iterative weighted residual solution of diffusive-convective equations may easily fall to converge in convection-dominated models. If the classical Galerkin approach is used, oscillations may also occur. To cope with this problem, an upwind weighting technique may prove appropriate. In the present paper an upwind finite element model is developed to analyze the 2-D transport of subsurface contaminants and is solved by preconditioned generalized conjugate residual schemes (GCR) especially designed for unsymmetric matrices. Three different algorithms, the ORTHOMIN(k), the GCR(k), and the Minimal Residual are compared on test problems whose size is up to 1500 for a wide range of Peclet and Courant numbers. The numerical results show that these numerical schemes are all robust, efficient, and reliable. The performance of the Minimal Residual is usually better, and particularly so for large-sized problems.

New Approaches and Applications in Subsurface Flow Modeling: 3-D Finite Element Analysis of Dewatering for an Electro-Nuclear Plant

The foundations of the 2.000 MW electro-nuclear plant to be built at Trino Vercellese, 70 km west of Milan, Italy, require the dewatering of a large area located amidst a productive agricultural land primarily dedicated to the culture of rice. To reduce the impact on the environment and particularly on the existing hydrologic regime, a number of remedial measures are planned including the construction of a concrete cut-off wall entirely encompassing the dewatered site and the excavation of recharging ditches to reinject the water withdrawn. To assist in the design of the dewatering system and the emergency wells to cope with a possible accident in the refrigeration circuit, a 3-D finite element model of subsurface flow is developed and applied using the data derived from the “ad hoc” reconnaissance study, soil borings and pumping tests. The model fully takes into consideration the artificial elements (wall and ditches) of the simulated system and provides the necessary information as to the overall quantity of water to be pumped out to dewater the site and the amount which can be safely reinjected. It explores the effectiveness of the low permeable barrier and recharging trenches to contain the propagation of the water table decline beyond the area directly involved in the project. The model also allows for the correct design of the emergency wells whose exceptional use is planned during a hypothetical accident in the cooling scheme of the nuclear reactors.