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See detailThe Hybrid Finite-Element Mixing-Cell method: a candidate for modelling groundwater flow and transport in karst systems
Brouyère, Serge ULg; Wildemeersch, Samuel ULg; Orban, Philippe ULg et al

in Bertrand, C.; Carry, N.; Mudry, J. (Eds.) et al Proc. H2Karst, 9th Conference on Limestone Hydrogeology (2011, September)

Groundwater flow and contaminant transport modelling in karst systems remains a challenge because of the complexity of the geology made of caves, voids, conduits of various sizes and forms and interacting ... [more ▼]

Groundwater flow and contaminant transport modelling in karst systems remains a challenge because of the complexity of the geology made of caves, voids, conduits of various sizes and forms and interacting matrix. Such heterogeneous structures cause complex hydraulic conditions for groundwater flow and transport processes. Despite the progresses in field investigation techniques and experiments, detailed knowledge and characterization of the karst system geometry and connectivity remains inaccessible and pragmatic modelling approaches have to be used. Groundwater models of different complexities have been developed for karst systems, ranging from transfer functions and linear reservoir models to spatially distributed models. Here, a new flexible modelling approach, the Hybrid Finite-Element Mixing-Cell method (HFEMC), has been developed that allows combining in a single model, and in a fully interacting way, different mathematical approaches of various complexities for groundwater modelling in complex environments. This includes linear reservoirs, distributed reservoirs, groundwater flow in variably saturated equivalent porous media, with possibilities to consider by-pass flows along preferential flow paths, internal boundary conditions between the karstic features and the surrounding rock mass matrix background and drainage by surface waters. This method has been implemented in the groundwater flow and solute transport numerical code SUFT3D. The objective of this communication is to present the modelling concepts and to discuss the potentials and advantages of the HFEMC method for modelling groundwater flow in karst systems over existing more classical modelling approaches. The discussion is supported by illustrative “synthetic” examples representative of karst systems and a real modelling application to the case of groundwater rebound and water inrush in a closed underground coal mine which presents a very similar geometrical and hydrological context to a karst, with cavities, drains and interacting rock mass. [less ▲]

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See detailRegional transport modelling for nitrate trend assessment and forecasting in a chalk aquifer
Orban, Philippe ULg; Brouyère, Serge ULg; Batlle-Aguilar, Jordi et al

in Journal of Contaminant Hydrology (2010), 118

Regional degradation of groundwater resources by nitrate has become one of the main challenges for water managers worldwide. Regulations have been defined to reverse observed nitrate trends in groundwater ... [more ▼]

Regional degradation of groundwater resources by nitrate has become one of the main challenges for water managers worldwide. Regulations have been defined to reverse observed nitrate trends in groundwater bodies, such as the Water Framework Directive and the Groundwater Daughter Directive in the European Union. In such a context, one of the main challenges remains to develop efficient approaches for groundwater quality assessment at regional scale, including quantitative numerical modelling, as a decision support for groundwater management. A new approach combining the use of environmental tracers and the innovative ‘Hybrid Finite Element Mixing Cell’ (HFEMC) modelling technique is developed to study and forecast the groundwater quality at the regional scale, with an application to a regional chalk aquifer in the Geer basin in Belgium. Tritium data and nitrate time series are used to produce a conceptual model for regional groundwater flow and contaminant transport in the combined unsaturated and saturated zones of the chalk aquifer. This shows that the spatial distribution of the contamination in the Geer basin is essentially linked to the hydrodynamic conditions prevailing in the basin, more precisely to groundwater age and mixing and not to the spatial patterns of land use or local hydrodispersive processes. A three-dimensional regional scale groundwater flow and solute transport model is developed. It is able to reproduce the spatial patterns of tritium and nitrate and the observed nitrate trends in the chalk aquifer and it is used to predict the evolution of nitrate concentrations in the basin. The modelling application shows that the global inertia of groundwater quality is strong in the basin and trend reversal is not expected to occur before the 2015 deadline fixed by the European Water Framework Directive. The expected time required for trend reversal ranges between 5 and more than 50 years, depending on the location in the basin and the expected reduction in nitrate application. To reach a good chemical status, nitrate concentrations in the infiltrating water should be reduced as soon as possible below 50mg/l; however, even in that case, more than 50 years is needed to fully reverse upward trends. [less ▲]

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See detailApplication of the Hybrid Finite Element Mixing Cell method to an abandoned coalfield in Belgium
Wildemeersch, Samuel ULg; Brouyère, Serge ULg; Orban, Philippe ULg et al

in Journal of Hydrology (2010), 392(3-4), 188-200

The Hybrid Finite Element Mixing Cell (HFEMC) method is a flexible modelling technique particularly suited to mining problems. The principle of this method is to subdivide the modelled zone into several ... [more ▼]

The Hybrid Finite Element Mixing Cell (HFEMC) method is a flexible modelling technique particularly suited to mining problems. The principle of this method is to subdivide the modelled zone into several subdomains and to select a specific equation, ranging from the simple linear reservoir equation to the groundwater flow in porous media equation, to model groundwater flow in each subdomain. The model can be run in transient conditions, which makes it a useful tool for managing mine closure post-issues such as groundwater rebound and water inrushes. The application of the HFEMC method to an abandoned underground coal mine near the city of Liege (Belgium) is presented. The case study zone has been discretized taking advantage of the flexibility of the method. Then, the model has been calibrated in transient conditions based on both hydraulic head and water discharge rate observation and an uncertainty analysis has been performed. Finally, the calibrated model has been used to run several scenarios in order to assess the impacts of possible future phenomena on the hydraulic heads and the water discharge rates. Among others, the simulation of an intense rainfall event shows a quick and strong increase in hydraulic heads in some zones coupled with an increase in associated water discharge rates. This could lead to stability problems in local hill slopes. These predictions will help managing and predicting mine water problems in this complex mining system. [less ▲]

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See detailApplication of the HFEMC method to an abandoned coalfield in Belgium: From conceptualisation to scenario simulations
Wildemeersch, Samuel ULg; Brouyère, Serge ULg; Orban, Philippe ULg et al

Conference (2009, October 20)

The Hybrid Finite Element Mixing Cell (HFEMC) method is a flexible modelling technique particularly suited to mining context (Brouyère et al., 2009). The principle of this method is to subdivide the ... [more ▼]

The Hybrid Finite Element Mixing Cell (HFEMC) method is a flexible modelling technique particularly suited to mining context (Brouyère et al., 2009). The principle of this method is to subdivide the modelled zone into several subdomains and to select a specific equation, ranging from the simple linear reservoir equation to the groundwater flow in porous media equation, to model groundwater flow in each subdomain. The model can be run in transient conditions, which makes it a useful tool for managing mine closure post-issues such as groundwater rebound and water inrushes. An application of the HFEMC method to an abandoned underground coal mine near the city of Liège (Belgium) is presented. The case study zone has been discretized taking advantage of the flexibility of the method. Then, the model has been calibrated in both steady-state and transient flow regimes based on hydraulic head and water discharge rate observations. Finally, the calibrated model has been used to run several scenarios in order to assess the impacts of possible future phenomena on the hydraulic heads and the water discharge rates. Among others, the simulation of a strong rainfall event shows a quick and strong increase in hydraulic heads in some exploited zones coupled with a strong increase in associated water discharge rates. This could lead to stability problems in the hill slopes near the exploited zones. This kind of predictions can greatly help managing and predicting mine water problems in this particularly complex mining system. [less ▲]

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See detailGround water flow simulation in mining works: Application of the Hybrid Finite Element Mixing Cell method
Wildemeersch, Samuel ULg; Orban, Philippe ULg; Brouyère, Serge ULg et al

Conference (2009, April 22)

Hydrogeological models can greatly help simulating groundwater flow in the particular context of mining works. However, classical modeling techniques are not so suited in this context because the medium ... [more ▼]

Hydrogeological models can greatly help simulating groundwater flow in the particular context of mining works. However, classical modeling techniques are not so suited in this context because the medium is strongly disturbed by shafts and networks of galleries. Consequently, specific modeling techniques have been developed. Box model techniques require only few parameters but they calculate only mean water levels in exploited zones and do not consider interactions between exploited and unexploited zones. Conversely, physically-based and spatially-distributed techniques represent explicitly both exploited and unexploited zones and take into account interactions between them but they require a lot of parameters sometimes difficult to estimate. The Hybrid Finite Element Mixing Cell (HFEMC) method is a new modeling technique constituting a compromise between simple and complex techniques used in mining context. The principle of this method is to fully couple a representation of exploited zones by a group of mixing cells possibly interconnected by pipes and a representation of unexploited zones by classical finite elements. Interactions between mixing cells and finite elements of the mesh are taken into account through 3rd type (Fourier) internal boundary conditions. With this method, exploited zones are characterized by mean water levels while spatially-distributed hydraulic heads are calculated for the entire adjacent and overlaying unexploited zones. Additionally, thanks to the coupling between mixing cells and finite elements, water exchanges between exploited and unexploited zones are explicitly taken into account. The HFEMC method allows also simulating groundwater rebound following mine closure and associated phenomena such as water inrushes. Concepts and equations of the HFEMC method are presented and illustrated using test cases. First results of an application on an abandoned coalfield in the region of Liege (Belgium) are also proposed. [less ▲]

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See detailThe Hybrid Finite Element Mixing Cell Method: A New Flexible Method for Modelling Mine Ground Water Problems
Brouyère, Serge ULg; Orban, Philippe ULg; Wildemeersch, Samuel ULg et al

in Mine Water & the Environment (2009), 28(2), 102-114

Dewatering operations often stop at mine closure. The ground water rebound can have undesirable consequences, which numerical models can help one understand and manage. However, classical modelling ... [more ▼]

Dewatering operations often stop at mine closure. The ground water rebound can have undesirable consequences, which numerical models can help one understand and manage. However, classical modelling techniques are relatively unsuitable to these contexts. While spatially-distributed and physically-based models suffer difficulties due to the lack of data and the complexity of geological and hydrogeological conditions, black-box models are too simple to deal with the problems effectively. A new modelling method is proposed to simulate ground water environments in which water flows through mined (exploited) and unmined (unexploited) areas. Exploited zones are simulated using a group of mixing cells possibly interconnected by pipes. Unexploited zones are simultaneously simulated using classical finite elements. This combined approach allows explicit calculation of ground water flows around the mine and mean water levels in the exploited zones. Water exchanges between exploited zones and unexploited zones are simulated in the model using specifically-defined internal boundary conditions. The method is tested on synthetic cases of increasing complexity, and first results from a real case study are presented. [less ▲]

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See detailThe Hybrid Finite-Element Mixing-Cell method: a new flexible method for large scale groundwater modelling
Orban, Philippe ULg; Brouyère, Serge ULg; Wildemeersch, Samuel ULg et al

Conference (2008)

Interest of end-users and policy makers for understanding and managing water systems at the regional scale has increased for years. At this scale, groundwater models of different complexity ranging from ... [more ▼]

Interest of end-users and policy makers for understanding and managing water systems at the regional scale has increased for years. At this scale, groundwater models of different complexity ranging from black-box models to physically based distributed models have been used in various hydrogeological conditions. Black-box models, such as transfer functions, have been applied for example to model groundwater in large scale hydrological models, to model karstic systems, in particular for the interpretation of isotopic data. Their concepts are simple and attractive because they require relatively few data. The main drawbacks are however that modelling results are not spatially distributed and their predictive capability is questionable due to the semi-empirical nature of process descriptions. On the contrary, due to a more advanced description of ongoing processes, physically-based distributed models are expected to have better predictive capabilities than black-box models. However, because such models require more data, they are generally applied for case studies that are better characterized from a hydrogeological point of view, for which the distribution of water levels or solute concentrations in the groundwater systems are needed. For large-scale modelling purposes, black-box models and physically-based distributed models have both proved their utilities and have their own justifications, advantages and disadvantages. However, few attempts have been made to combine the advantages of these two categories of approaches in a unified modelling application. A new flexible modelling approach, the Hybrid Finite-Element Mixing-Cell method (HFEMC), has been developed that allows combining in a single model, and in a fully integrated way, different mathematical approaches of various complexities for groundwater modelling in complex and large scale environments. This method has been implemented in the groundwater flow and solute transport numerical code SUFT3D. The approach has been first tested and illustrated using basic and advanced “synthetic” examples that allow validating and discussing its advantages over existing modelling concepts. The HFEMC approach is now applied for the development of a large scale groundwater flow and solute transport model in different groundwater basins in Belgium. [less ▲]

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See detailThe Hybrid Finite-Element Mixing-Cell method: a new flexible method for modelling mine water problems
Brouyère, Serge ULg; Orban, Philippe ULg; Wildemeersch, Samuel ULg et al

in Rapantova, N.; Hrkal, Z. (Eds.) Mine Water and the Environment Proceedings of IWMA 2008 (2008)

Mine closure is often accompanied by a stop in dewatering operations. This generally induces groundwater rebound in the mined rock system with short and long term consequences that may be disastrous: soil ... [more ▼]

Mine closure is often accompanied by a stop in dewatering operations. This generally induces groundwater rebound in the mined rock system with short and long term consequences that may be disastrous: soil instabilities such as landslides and subsidence reactivation, flooding, flooded basement and acid mine drainage. Modelling tools can be very advantageous and efficient in helping understanding and managing such problems, however, classical modelling approaches have proved to be relatively unsuited to such contexts. Because of the former mining operations, the underground geological system is strongly disturbed (excavated, fractured and collapsed zones, galleries, large shafts, etc.). Using complex spatially distributed modelling approaches such as 3D finite elements usually lead to strong difficulties related to the lack of data, the complexity of geological and hydrogeological conditions (complex geometry, non Darcian fluxes…). On the other hand, using a simplified approach such as black-box models often leads to oversimplification of the reality: particularly when interactions between the mined system and its surrounding geological and hydrogeological environment are very important. A new modelling approach is developed for simulation of the groundwater flow in such complex environments. It combines, in a single fully integrated simulator, a representation of the unmined area by a classical finite element modelling technique, together with conceptualisation of the worked areas and galleries by a group of mixing cells connected by pipes. The whole assembled groundwater flow model allows an accurate estimation and representation of (a) water infiltration (precipitations, river losses …) through the unsaturated zone reaching the exploited area (recharge of boxes) and (b) water exchanges with adjacent aquifers. The model can estimate the flow of groundwater in and around the minefield and the mean water level in the boxes. It is also capable of considering water exchanges between different mined zones, through connection pathways such as old roadways galleries and shafts. Modelling concepts and equations are described and illustrated using basic and advances validation examples. A real case application corresponding to an abandoned coalfield in the region of Liège (Belgium) is used to illustrate the suitability and efficiency of the approach. [less ▲]

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