Physics-based fine-scale numerical model of a karst system (Milandre Cave, Switzerland)

In karst aquifers, groundwater flow is often turbulent and occurs in variably saturated conduits. This implies a nonlinear response to recharge events which cannot be reproduced with the tools commonly used for groundwater flow modeling. Recent studies have shown the usefulness of using conduit flow models to simulate flow in karst systems. However existing models are built on abstract representations of the actual conduit network or at a rather coarse resolution. Such models cannot provide information on local flow conditions in the conduits or be used to simulate mass transport. In the catchment of the Milandre Cave, in the Jura Mountains of Switzerland, a good portion of the active drainage network is accessible by caving, which opens the possibility for a distributed fine-scale numerical model. This report presents the development of a hydraulic model of the downstream part of this system (1.5 km × 0.5 km) using EPA SWMM 5. The network geometry is based on a detailed cave survey. Calibration is achieved by fitting the observed hydraulic head vs. flow-rate curves. The model performs well when compared against the hydraulic heads that were monitored throughout the system. Simulated transit times are also in line with tracer test results. This confirms that the model reproduces well the physics of flow in this karst system, while being spatially distributed at a fine scale (median element size of 6 m). It can thus provide information on local flow conditions in the conduits at various water stages and be used to address mass transport problems.