Faculté des sciences

Towards improved numerical modeling of karst aquifers : coupling turbulent conduit flow and laminar matrix flow under variably saturated conditions

Rooij, Rob de ; Perrochet, Pierre (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2007 ; Th.1984.

Numerical models for simulating groundwater flow based on full saturation do not account for moving water tables and the infiltration and storage processes in unsaturated zones. Strictly speaking these models are based on the concept of a confined aquifer with well known boundary conditions. Nonetheless, models based on full saturation are also used for more complicated problems such as the... Plus

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    Summary
    Numerical models for simulating groundwater flow based on full saturation do not account for moving water tables and the infiltration and storage processes in unsaturated zones. Strictly speaking these models are based on the concept of a confined aquifer with well known boundary conditions. Nonetheless, models based on full saturation are also used for more complicated problems such as the groundwater flow in unconfined karst aquifers. An important reason for this is that variably saturated groundwater flow is a non-linear problem which is difficult to solve numerically. Ideally, if variable saturation is not accounted for, arguments should be provided that the infiltration and storage processes in the unsaturated zones are not of significant importance. However, conceptually karst aquifers are highly influenced by the processes in the unsaturated zone. Voids in the cave system provide storage potential. Storage, infiltration and drainage in the epikarst layer are believed to be important processes that determine the evolution of karst water resources. With a numerical model that accounts for variable saturation in karst aquifers it is possible to test the hypothetical ideas about the processes in the unsaturated zone. The development of such a numerical model is the main objective of the research presented in this thesis. The presented numerical model permits the simulation of turbulent conduit flow coupled with laminar matrix flow under variably saturated conditions. A variety of numerical techniques is discussed. For simulating variably saturated flows upstream weighting and positivity preserving schemes may be needed. Other discussed numerical challenges are the coupling of conduit-matrix flow and the treatment of drying/wetting fronts in the conduits. Based on new insights into the flow equations and an in-depth discussion of numerical stability, new arguments are formulated for using a positivity preserving scheme to compute free-surface flows in the conduits. It is shown that this scheme makes the code more reliable as well as more economical, especially if coupled conduit-matrix flow is simulated. The numerical code is verified by considering simple simulation scenarios. Simulations on hypothetical karst aquifers provide interesting new insights into the hydrodynamic behavior of karst aquifers and into the application of classical modeling approaches. Temporal storage in the conduit network by filling and emptying of conduits can have pronounced effect on the spring hydrograph. It is shown that this temporal storage can result in tailing effects on the spring hydrographs. Models based on laminar and turbulent conduit flow give significantly different results. Finally, simulation results confirm that the epikarst layer plays an important role in concentrating recharge from precipitation into the conduit network. It is also illustrated that spring hydrographs depend significantly on the storage and drainage processes in the epikarst. These findings are important conclusions that have implications for the evolution of water resources in karst aquifers and for the interpretations of spring hydrographs.