TURBULENT GROUNDWATER FLOW IN SINGLE-CONTINUUM MODELS

Due to solution processes, karst aquifers feature highly conductive structures such as conduits within which groundwater flow can be turbulent. Some numerical groundwater flow models explicitly account for turbulence, e.g. by coupling the continuum model with a pipe network that represents the conduit system using nonlinear flow equations. In contrast, the recently published Conduit Flow Process Mode 2 (CFPM2) for MODFLOW-2005 considers turbulent flow in highly porous layers by adjusting the hydraulic conductivity of a laminar continuum model.

We modified the existing CFPM2 by implementing a general nonlinear flow equation and by removing the iterative solution for adjustment of the hydraulic conductivity. Finally, a user-defined exponent is introduced that increases or decreases turbulent flow nonlinearity. With this, the modified CFPM2 is able to reflect conduit type flow in continuum cells.

The modified CFPM2 is verified by evaluating the relationship between hydraulic gradient and discharge resulting from simulations with single conduits. In addition, a coupled conduit-matrix system is considered to investigate effects of turbulent flow on the response of karst springs to rainfall events using the modified CFPM2 approach, existing hybrid models, and MODFLOW-2005. The new single-continuum approach CFPM2 represented laminar and turbulent spring-flow equally well as the more complex hybrid models that require additional programming and parameterization. Further, the results illustrate the importance of turbulent flow in karst conduits: Laminar models overestimate maximum spring discharge and underestimate hydraulic gradients within the conduit. The modified CFPM2 represents a new method that makes it possible to account for these effects with reduced practical and numerical efforts as compared to hybrid models.