The pane for the Conduit Flow Process is on the MODFLOW Packages and Programs dialog box under Conduit Flow Process.
The Conduit Flow Process is used to simulate non-Darcian flow that occurs when the voids are large enough and the flow is fast enough to become turbulent. For example, it can be used in karst terrain or with lava tubes or mine dewatering.
The Conduit Flow Process is described in Shoemaker, W.B., Kuniansky, E.L., Birk, S., Bauer, S., and Swain, E.D., 2007, Documentation of a Conduit Flow Process (CFP) for MODFLOW-2005: U.S. Geological Survey Techniques and Methods, Book 6, Chapter A24, 50 p. Chapter 5 of that report gives guidance on assignment of Conduit Flow Process parameters.
An updated version of the Conduit Flow Process is described in Boyce and others (2020). The updated version of the Conduit Flow Process is included in MODFLOW-OWHM version 2.
The Conduit Flow Process pane has two tabs: "CFP" and "CRCH, COC."
The Conduit Flow process can simulate both individual mapped conduits or entire layers that have abundant conduits that are not individually specified. The MODE determines whether individual conduits, conduit layers, or both are simulated.
CFP tab
Pipe Flow Parameters
As you might expect, specifying individual mapped conduits requires more data than conduit layers. Individual conduits are specified using objects that intersect two or more adjacent cells and are not polygons. In addition the objects that specify the conduits must have only one formula for its X, Y, or Z formula. The data for individual conduits is specified on the CFP_Conduit_Flow_Process_Pane of the Object Properties dialog box. If one or more nodes of a conduit is a fixed head node, that is specified using a separate object on the CFP: Boundary Conditions or Fixed Heads Pane of the Object Properties dialog box.
If individual conduits are specified, additional data must be specified that are not specific to any one conduit pipe.
The TEMPERATURE of the water in the conduit pipes must be specified because it helps determine the viscosity and density of the water.
The elevation of the nodes (GEOHEIGHT) can be specified in either of two ways. Either the elevation of the pipe at each cell (node) can be specified individually or all the nodes can be specified as being a fixed distance above or below the center of the cell. For nodes with fixed heads, it is important that the fixed head be greater than or equal to the node elevation. A node will go dry if the head in the node is less than its elevation.
SA_EXCHANGE: In addition to flow within conduits, there is also flow between the conduits and the porous medium. The rate at which this occurs is controlled in part by the conductance of the pipes. The conductance of the pipes can be specified directly (SA_EXCHANGE = 0) or it can be calculated from a permeability term as described in equation 38 of Shoemaker and others (2007) p. 22 (SA_EXCHANGE = 1).
The Conduit Flow Process uses and iterative procedure to calculate the flow in the pipes. The pipe convergence criterion (EPSILON), Maximum number of iterations (NITER), and relaxation parameter (RELAX) are used to control the iterative procedure similarly to the way the various parameters in the Solvers control the calculation of the groundwater flow. However, EPSILON typically needs to be much smaller than the convergence criteria used in the solvers because in pipes, even small gradients can result in significant flow.
Information about the iterative solution can be printed if desired (P_NR).
Conduit Flow Layer Parameters
If Conduit Flow Layers are simulated, the user must specify the temperature (LTEMP) of the water in the layers. Additional data regarding the flow layers is specified on the Conduit Layers tab of the MODFLOW Layer Groups dialog box.
CRCH, COC tab
Conduit Recharge (CRCH)
The Conduit Flow Process allows the modeler to specify that some of the recharge in a cell goes directly to a conduit. This option is only available if individual conduits are simulated and the Recharge package is being used. If this option is selected, the modeler can specify the fraction of the recharge to divert to a conduit on the CFP: Conduit Recharge Pane of the Object Properties dialog box.
Conduit Output Control (COC)
The Conduit Flow Process allows the modeler to specify that data for individual pipes and nodes be saved to file. The frequency with which such data is saved is the output interval (N_NTS, T_NTS).
Conduit-Associated Drainable Storage (CADS)
Conduit-Associated Drainable Storage (CADS) is described in the documentation for MODFLOW-OWHM version 2 (Boyce and others, 2020). It can be used to simulate the effects of storage in large pipes or caves on groundwater flow. If CADS is used, the width of the CADS must be specified in the CFP: Conduit Flow Process pane in the Object Properties dialog box.
