1,721,280 research outputs found
Parallel block iterative method for multiaquifer flow models
No full textFlow in a multiaquifer porous system can be simulated by the so-called “quasi three-dimensional” models. When heterogeneous and/or aquitards with nonlinear hydrogeologic behavior are considered, a fully numerical approach is required for the model solution. If the finite element method is used to integrate the partial differential flow equations, the final solution of large systems is required. In the present article, an original iterative solution strategy is developed. The global system is decoupled into a number of smaller subsystems consistent with the geologic structure (aquitards and aquifers) of the multiaquifer system. The aquifer and the aquitard equations are solved separately with the modified conjugate gradient and the Thomas algorithms, respectively, while the final coupled solution is obtained with an iterative procedure equivalent to a Block Jacobi scheme. The procedure can be efficiently implemented on a parallel super-computer distributing the computational load, so that two successive blocks (related to an aquifer and the underlying aquitard) are solved on the same processor. The procedure is analyzed with linear porous media, where the convergence is theoretically ensured. The results obtained with a realistic linear multiaquifer system, employing a massively parallel computer like the CRAY T3D, have pointed out the high degree of parallelization of the algorithm. Comparison with the parallel implementation of the Block SOR and Block Gauss-Seidel schemes shows that parallel Block Jacobi performs significantly better with a reduction of the elapsed times, which depends on the rate of leakage between neighboring aquifers
Geomechanics of subsurface water withdrawal and injection
Full text linkLand subsidence and uplift, ground ruptures, and induced seismicity are the principal geomechanic effects of groundwater withdrawal and injection. The major environmental consequence of groundwater pumping is anthropogenic land subsidence. The first observation concerning land settlement linked to subsurface
processes was made in 1926 by the American geologists Pratt and Johnson, who wrote that ‘‘the cause of subsidence is to be found in the extensive extraction of fluid from beneath the affected area.’’ Since then, impressive progress has been made in terms of: (a) recognizing the basic hydrologic and geomechanic principles underlying the occurrence; (b) measuring aquifer compaction and ground displacements, both vertical and horizontal; (c) modeling and predicting the past and future event; and (d) mitigating environmental impact through aquifer recharge and/or surface water injection. The first milestone in the theory of pumped aquifer consolidation was reached in 1923 by Terzaghi, who introduced the principle of ‘‘effective intergranular stress.’’ In the early 1970s, the emerging computer technology facilitated development of the first mathematical model of the subsidence of Venice, made by Gambolati and Freeze. Since then, the comprehension, measuring, and simulation of the occurrence have improved dramatically. More challenging today are the issues of ground ruptures and induced/triggered seismicity, which call for a shift from the classical continuum approach to discontinuous mechanics. Although well known for decades, anthropogenic land subsidence is still threatening large urban centers and deltaic areas worldwide, such as Bangkok, Jakarta, and Mexico City, at rates in the order of 10 cm/yr
A block iterative finite element model for non-linear leaky aquifer systems
No full textA new quasi three-dimensional finite element model of groundwater flow is developed for highly compressible multiaquifer systems where aquitard permeability and elastic storage are dependent, on hydraulic drawdown. The model is solved by a block iterative strategy, which is naturally suggested by the geological structure of the porous medium and can be shown to be mathematically equivalent to a block Gauss-Seidel procedure. As such it can be generalized into a block overrelaxation procedure and greatly accelerated by the use of the optimum overrelaxation factor. Results for both linear and nonlinear multiaquifer systems emphasize the excellent computational performance of the model and indicate that convergence in leaky systems can be improved up to as much as one order of magnitude
Residual land settlement near abandoned gas fields raises concern over Northern Adriatic coastland
No full textTerraSAR-X reveals the impact of the mobile barrier works on Venice coastland stability
No full textLand subsidence and eustacy concurred to make the relative sea level in Venice (Italy) 23 cm higher over the last century. In order to protect the city and its lagoon environment from increased flooding, a series of mobile barriers are under construction at the three inlets of Lido, Malamocco, and Chioggia connecting the Adriatic Sea to the inner water body. Since 2003 work has been proceeding with the reinforcement and extension of the existing jetties and the construction of breakwaters, harbors, and a small island within the Lido inlet. We detected significant local settlements of a few centimeters between March 2008 and January 2009 at the three inlets induced by the construction works through an interferometric analysis of 30 satellite radar images acquired by the new German TerraSAR-X mission. On a more regional scale we observe that the city of Venice and the other major urban settlements on the lagoon littorals are not impacted by subsidence during this period. The very high spatial resolution of 3 m and the short repeat-time interval of 11 days of
TerraSAR-X enable the investigation of displacements with an unprecedented observed level of details, opening new perspectives to geodynamic's research and civil engineering sectors for the monitoring of large infrastructures with potential vulnerability to terrain motion
Can gas withdrawal from Chioggia Mare field affect the stability of the Venetian littoral?
No full textResidual land subsidence over depleted gas fields in the Northern Adriatic basin, Environ. Eng. & Geosciences, V(4), 389-405, 1999.
No full textAn original nonlinear three-dimensional finite element model is developed to predict the residual land subsidence adjacent to depleted gas fields, as a delayed response from an active aquifer which may keep on compacting for a long time after the field abandonment. The pore pressure recovery within the reservoir and the depletion of the lateral/bottom aquifer are simulated by a subsurface flow model coupled with the equation of state of the residual gas repressurized by the ground water which floods the field. The resulting pore pressure distribution is used as input data in a pore-elastic structural model of land subsidence. The modeling approach is nonlinear because of both the dynamic coupling between the flux from the aquifer and the reservoir gas pressure response, and the dependence of the porous medium elastic properties on the effective intergranular stress and the loading/unloading conditions. The model is applied to the 3,000 m deep gas reservoir of Dosso degli Angeli, one of the major fields in the Northern Adriatic sedimentary basin, made of three major gas pools. Representative basin-scale mechanical parameters have been obtained from laboratory triaxial and oedometric tests, density logs, and recent measurements of in situ compaction by the use of radioactive markers. In 1992, at the end of 21 year production life the maximum pore pressure drawdown in the depleted pools approached 300 kg/cm(2). The largest land settlement from the modeling simulation turns out to be 31 cm, in good agreement with the available leveling records. Numerical predictions suggest that a residual land sinking of about 10 cm is yet to be expected in 2042, i.e., 50 years after the field abandonment, close to the areas of Porto Garibaldi and Casal Borsetti a few kilometers south and north of the field, respectively, namely between two and three times the subsidence experienced by those areas during the field development. Gas pressure recovery in 2042 ranges from 50 to 130 km/cm(2) according to gas pool, and with the cone of depression still expanding toward the far outer boundary of the adjacent aquifer
Water-gas dynamics and coastal land subsidence over Chioggia Mare field, Northern Adriatic sea
No full textA coupled MFE poromechanical model of a large-scale load experiment at the coastland of Venice
No full textA three-dimensional fully coupled mixed finite element (MFE) model based on Biot’s consolidation equations is implemented to simulate the geomechanical
response of a large-scale 5-year long loading/unloading test performed at the Venice coastland, Italy. The model uses linear piecewise polynomials and the lowest order Raviart–Thomas mixed space to represent the porous medium motion and the groundwater flow rate, respectively. The approach ensures an element-wise mass conservative formulation while preserving the stability of the numerical solution and providing at the same time an accurate calculation of the flow field. With the aim of characterizing the Late Pleistocene and Holocene deposits above which the MoSE project, i.e. the mobile barriers to protect Venice from acqua alta, is under implementation, a 20-m radius, 6.7-m tall vertically walled cylinder was built from September 2002 to March 2003 and removed in June 2007. The maximum load exerted on the ground at the completion of the building activity was 0.105 MPa. The land displacements were accurately monitored at various depths, the
center and outer boundary of the embankment by sliding deformeters, leveling, global positioning system, and persistent scatterer interferometry. Moreover, in situ tests and standard lab tests were performed to define the hydrological and geomechanical properties of the soil underlying the cylinder. The model addresses the actual lithostratigraphy of the subsurface down to 50-m depth below the embankment and prescribes the land surface loading versus time as an external source of strength. A hysteretic elastic constitutive law, with the Young modulus E in the loading phase between 2 to 36 Mpa according to lithology, a ratio s = 15 of loading to unloading cycle E, and a small adjustment of the hydrological parameters allow to predict quite satisfactorily most of the observed pressure behavior, together with vertical and horizontal displacements
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