Addition and Enhancement of Flow and Transport Processes to the MODFLOW-2005 Conduit Flow Process

Conduit Flow Process (CFP) couples a pipe flow model to MODFLOW-2005. In this hybrid model the karst aquifer is conceptualized as a dual flow system consisting of highly conductive conduits and a less conductive porous/fissured rock matrix. The code is partly based on the hybrid model Carbonate Aquifer Void Evolution (CAVE). The existing CFP functionality is enhanced by processes and boundary conditions to facilitate the practical applicability for karst characterization and management. Modifications comprise (A) enhancements to flow routines and (B) addition of heat and solute transport.

Enhancements to flow routines are intended to improve CFP capabilities for water abstraction from karst aquifers, e.g., large scale hydraulic tests. These applications require one to consider fast-responding storage in connection with the conduit network. Hence, conduit-associated drainable storage was implemented in CFP. Beyond this, specific boundary conditions were added, namely a constraint for the fixed head boundary that limits in- or outflow from karst conduits by a user-defined threshold. It is demonstrated that the enhanced CFP flow routines can qualitatively reproduce water abstraction scenarios.

Implementation of mass and heat transport routines allows one to apply CFP to tracer test analysis. Existing heat and solute transport routines from CAVE were updated, enhanced, and converted to CFP. With this, CFP computes convective heat transport with matrix conduction for laminar and turbulent flow. A conduit interacts with the matrix via a thermal boundary layer and matrix conduction is considered by a localized 1D radial approach perpendicular to the conduits. Solute transport considers advection and dispersion for laminar and turbulent flow. Additionally, solutes in conduits can interact with the surrounding rock, resulting in matrix diffusion. The correct implementation of heat and solute transport is demonstrated by schematic model studies. Likewise, these model studies illustrate the sensitivity of different transport processes useful for future field application.