Groundwater flow equation | EPFL Graph Search

Used in hydrogeology, the groundwater flow equation is the mathematical relationship which is used to describe the flow of groundwater through an aquifer. The transient flow of groundwater is described by a form of the diffusion equation, similar to that used in heat transfer to describe the flow of heat in a solid (heat conduction). The steady-state flow of groundwater is described by a form of the Laplace equation, which is a form of potential flow and has analogs in numerous fields. The groundwater flow equation is often derived for a small representative elemental volume (REV), where the properties of the medium are assumed to be effectively constant. A mass balance is done on the water flowing in and out of this small volume, the flux terms in the relationship being expressed in terms of head by using the constitutive equation called Darcy's law, which requires that the flow is laminar. Other approaches are based on Agent Based Models to incorporate the effect of complex aquifer

Stream temperature modeling using fiber optic distributed temperature measurements in a sub-Saharan watershed, Burkina Faso

Malik Matthey

High spatial and temporal resolution water temperature measurements, using DTS, allow to model and to control stream temperature evolution in the Tambarga’s basin (Burkina Faso). A wireless sensor network recorded solar radiation, air temperature, air humidity, wind speed (among others) on different sites on Tambarga’s basin (B.F.) in 2009. These data are compiled and used as inputs to force the model and to assess simulated river temperatures in the basin. Local parameters as shade coefficient, stream bed heat losses as well as land coverage coefficient are estimated from on field knowledge. Purpose but also need for this study, stream discharge estimation in the Tambarga’s river is paramount. Discharges are formed by subsurface flow, groundwater flow and runoff during rain events. Assuming that all three water discharge components have a different thermal signature leads to imagine that they can be identified with the DTS optical fiber, once they reach the main stream. The inflow emplacements are referred to as springs in this study. Finally, although this study leads to significant results, the last sections propose the installation of some more measuring instruments for controlling and improving the model accuracy.

2010

Contribution à l'étude du comportement thermo-hydraulique du stockage de chaleur en aquifère

Stéphane Dupasquier

In this work, a parametric study is done with computational methods for groundwater flow and heat transport in order to describe the dominant physical phenomena in an Aquifer Thermal Energy Storage (ATES). Optimum operating conditions and geological parameters are deduced from it. The intended use of the ATES is the domestic water or space heating. The stock is seasonal and functions without heat pump. Parametric study is organized as a series of reference simulations relating to the thickness of the aquifer and the injection flow rate. The other parameters and operating conditions of are kept constant. Other simulations are done and compared to the reference simulations in order to describe the influence of injection temperature, hydraulic conductivity, anisotropicity, injection frequencies, phases of rest, outflow rate and doublet wells. The cells of natural convection are strongly affecting the performances of an ATES. The thermal losses by conduction trough bedrock and caprock is an unfavourable factor as well, but to a lesser extent however. The increase of inflow rate always improves the efficiency, whereas an optimum aquifer thickness exists for each inflow rate. This optimum is characterised by weak natural convection phenomena. A greater thickness means strong density-viscosity effects, while a thinner aquifer forms a horizontally stretched stock, with increased conductive losses through the bedrock and cap rock. A maximum thermal recovery rate exists for a given flow rate. This maximum increases with inflow rate. The injection temperature behaves on the stock in the same way as the aquifer thickness for a given inflow: there is an optimal temperature. However it is in general preferable to increase the injection temperature instead of the inflow rate. In terms of optimization of the thermal power at disposal, the most favorable scenario is an increasing flow during the discharge. The precise scenario is different for each problem. The performances of a ATES are the complex result of the many parameters which enter in plays. One will recommend an inflow rate of at least 100m33/d during 6 months, and even rather 500 - 1000m3/d with an injection temperature of 80°C at least. An injection temperature of 60°C will have to be associated with an inflow of 1000m3/d at least. The geological targets proposed in Switzerland are the quaternary alluvial deposits filling the former glaciated valleys, fluvioglacial deposits and alluvial fans.

EPFL2001

Laminar ground water flow through stochastic channel networks in rock

The flow through fractured rock is of great importance for many civil engineering projects, for example, when considering the safe storage of nuclear waste or the stability and profitability of dams, tunnels and slopes. How the fluid actually flows through a fractured rockmass is still a matter of vivid discussion within the hydrogeologic community. Various approaches to calculate the flow through a fractured rockmass exist. Which approach is best depends on the rockmass properties and the size of the rockmass under investigation. This study presents a new model which assumes that groundwater flow will take place preferably at the intersection of fractures. Study of the scientific literature showed that flow through intersections of fractures exists and can play a major role in flow through a fractured rockmass. Fieldwork has been conducted at outcrops close to Granada, Spain. At these outcrops fossilized flow paths have been observed at the intersections of fractures. During this fieldwork, fracture data have been gathered, which could be used to reconstruct the fracture geometry. As part of this research a mathematical model has been developed to simulate flow along the intersections of fractures, neglecting flow within the fractures or through the rock matrix. To this end a computer program, CPA, developed for a different purpose, has been modified and completed in order to generate a stochastic tubular network of fracture intersections and calculate flow rates through this network. The new version of the CPA code has been applied to a number of problems to test the correctness of the model and investigate the effects of the different input parameters. The application part of this study can be roughly divided into: network generation, sensitivity analysis, and the comparison with a model, Joint-OKY, that assumes flow to take place through the fractures. The field data gathered during fieldwork in Spain have been used to confirm the geometric modelling capabilities of the CPA program. A comparison between the network model generated by CPA and the observations in the field showed good agreement. The use of an eigenvec- tor approach to represent fracture orientation distribution has proven to be a good and simple method. To better understand the influence of various input parameters in the CPA model, sensitivity analyses were performed on the models generated by the CPA code. The following parameters have been investigated: fracture density, size of the model area, fracture size and anisotropy of conductivity. Finally a comparison between the CPA code and the Joint-OKY code, which assumes flow to take place within the plane of the fractures, has been made. This comparison showed that a model assuming flow within the fractures is more conductive. Further studies are needed to better include the transport of contaminants in the current CPA program.

EPFL2003

Contribution à l'étude du comportement thermo-hydraulique du stockage de chaleur en aquifère

Stéphane Dupasquier

EPFL2001

Laminar ground water flow through stochastic channel networks in rock

EPFL2003