MODFLOW-Based Karst Modeling - Recent Development

Karst aquifers can be conceptualized as dual flow systems composed of low conductive matrix and highly conductive conduits. Discharge in conduits is highly variable ranging from slow and therefore laminar base-flow to rapid and turbulent peak-flow. Commonly employed continuum models such as MODFLOW are not designed to reflect the dual karst characteristics i.e. turbulent flow in discrete conduits is not implemented into the code. The modular structure of MODFLOW allows to overcome these limitations by adding karst-specific modules representing laminar and turbulent flow in discrete conduits.

A number of numerical models are available that can be used as versatile tools to deal with numerous karst related processes. This contribution intends to provide an overview about the recent development as well as application of MODFLOW-based models with respect to karst related problems.

The Carbonate Aquifer Void Evolution model (CAVE, Liedl et al. 2003) computes flow in discrete conduits with the Darcy-Weisbach approach, which accounts for laminar and turbulent flow in fully filled conduits. Implementing carbonate-dissolution kinetic processes, this hybrid-model is able to simulate the evolution of karst conduits in geological time, i.e. speleogenesis. CAVE was integrated into the MODFLOW-2005-based hybrid-model Conduit Flow Process (CFP) Mode1 (Shoemaker et al. 2008).

Beyond Mode1, CFP is able to account for turbulent flow processes by employing an innovative continuum approach termed Mode2. This allows the computation of turbulent flow without the additional effort of hybrid-models, i.e. parameter demand. Hence, CFP Mode2 still allows the use of existing MODFLOW-based packages such as transport models.

For highly transient processes in large conduits, i.e. during flood events, it is necessary to take into account other effects like wave propagation and storage in partly filled conduits. The coupled overland- and groundwater flow model MODBRANCH is therefore adapted to consider the above spectrum of processes in karst conduits.