USGS Releases Conduit Flow Process (CFP) for MODFLOW-2005

A Conduit Flow Process (CFP) for MODFLOW-2005 is available from the U.S. Geological Survey (http://water.usgs.gov/software/ground_water.html). The CFP simulates conduit flow in karst aquifers by: (1) coupling the traditional ground-water flow equation with a discrete network of cylindrical pipes (Conduit Flow Process Mode 1 [CFPM1]—an update of the Carbonate Aquifer Void Evolution code); (2) inserting a high-conductivity, preferential-flow layer that can switch from laminar to turbulent flow (CFPM2); or (3) simultaneously coupling a discrete-pipe network while inserting a high-conductivity layer that can switch from laminar to turbulent flow (CFPM3). The pipe network represents secondary conduit porosity under laminar or turbulent flow conditions. Preferential flow layers simulated with CFPM2 may represent either: a single secondary porosity subsurface feature, such as a well-defined laterally extensive underground cave; or a horizontal preferential flow layer consisting of many interconnected tubes, such as burrowed limestone with interconnected vugs greater than .01m in diameter. CFPM1 requires more data than CFPM2. Pipe locations, lengths, diameters, tortuosity, internal roughness, ground-water temperature, critical Reynolds numbers, and exchange permeability are required for CFPM1. Layer top and bottom, ground-water temperature, and critical Reynolds number are required for CFPM2. CFPM1 solves the pipe-network equations in a matrix independent of the porous-media equation matrix, which may avoid numerical instability for some problems. Large pipe networks, however; can result in systems of equations that are either slow to converge or will not converge. Water is exchanged between the rock matrix and conduits via head-dependent flux terms. With CFPM1, the Hagen-Poiseuille equation is used when flow is laminar (Reynolds number less than critical Reynolds number), and Darcy-Weisbach equation is used when flow is turbulent (Reynolds number greater than critical Reynolds number). With CFPM2, energy losses due to turbulent flow are approximated by lowering laminar hydraulic conductivity once the critical Reynolds number is exceeded.