SIMULATING LAMINAR AND TURBULENT GROUNDWATER FLOW IN FRACTURED OR CARBONATE AQUIFERS

The U.S. Geological Survey is developing a Conduit Flow Process (CFP) for the modular finite-difference ground-water flow model, MODFLOW-2005. The CFP is designed to simulate both laminar and turbulent groundwater flow occurring within the primary and complex secondary porosity of carbonate or fractured aquifers. Laminar flow is a flow regime in which streamlines are parallel, and shear stresses within the water are overcome by the viscous forces of the water. Turbulent flow is characterized by streamlines flowing in random complex patterns (eddies) created by shear stresses overcoming the viscous forces of the water. The ultimate fate and rate of chemical migration in the subsurface is affected by the occurrence of laminar or turbulent groundwater flow. Rapid transport of pollutants toward springs or production wells is more likely to occur within turbulent, preferential flow zones that exchange water with diffuse flow zones acting mostly as groundwater storage reservoirs.

The CFP simulates both laminar and turbulent groundwater flow by (1) coupling the traditional ground-water flow equation with formulations for a discrete network of cylindrical pipes (CFPM1), (2) inserting a high-conductivity layer that can transition between laminar and turbulent flow (CFPM2), or (3) simultaneous coupling of a discrete-pipe network while inserting a high-conductivity layer that can transition between laminar and turbulent flow (CFPM3). Pipes may represent dissolutional caverns, burrowed features, and/or fractures that are fully or partially saturated under laminar or turbulent flow conditions. Conduit flow layers may represent either (1) a single secondary porosity subsurface feature, such as a well-defined laterally extensive underground cave (for example, the Woodville Karst Plain, Florida), or (2) a horizontal preferential flow layer consisting of many interconnected tubes, such as a burrowed limestone with interconnected vugs greater than 10 millimeter in diameter (for example, the Biscayne aquifer, Florida). Benchmark testing has shown that calculations from the CFP compare accurately with several laminar and turbulent analytical groundwater flow solutions.