The Seawater Intrusion package is used for simulating variable density flow with MODFLOW-2005. The pane for this package is on the MODFLOW Packages and Programs dialog box under Flow Packages.
In the Seawater Intrusion package, two or more zones of different density are simulated within each model cell. The zones can have either uniform densities or densities that vary linearly between their upper and lower surfaces. The Seawater Intrusion package will calculate updated positions of the interfaces between zones in each time step.
You should be aware of the following:
•Steady state positions of the interfaces can not be reached by just designating a stress period as a steady state stress period. Instead, enough multiple time steps in the stress period must be simulated for the interface positions to stop changing.
•Initial Heads must represent freshwater heads.
•All specified head boundaries must represent freshwater heads.
•All heads specified in head-dependant flow boundaries must represent freshwater heads.
Data Sets
There are four new data sets associated with the SWI2 Package.
•Active_Surface_Elevation[i] represents the initial elevation of an interface between zones [i] and [i+1] where [i] is a zone number.
•EffectivePorosity represents the effective porosity of the aquifer cell
•FluidSourceDensityZone is used to determine the density of water interacting with sources or sinks. If FluidSourceDensityZone is positive, FluidSourceDensityZone represents the zone number of any sources or sinks in a cell. If FluidSourceDensityZone is negative, the absolute value of FluidSourceDensityZone represents the zone number of any sources in a cell but sinks in a cell will have the density of the uppermost active zone in the cell. If FluidSourceDensityZone is zero, sources and sinks will the same fluid density as the active zone at the top of the aquifer.
•SWI_Observation_Name represents the name of observation locations. In any cell in which SWI_Observation_Name is not blank, The SWI2 package will save data regarding the interface positions in the cell for each time step. This data set will only be present if ISWIOBS is not zero. Observations at particular locations and times can be used for parameter estimation using PEST or UCODE. Such observations are defined on the SWI2: Seawater Intrusion Package pane of the Object Properties dialog box.
There are three tabs on the Seawater Intrusion Package pane: Basic, Solver, and Dimensionless Density.
Basic
•Number of surfaces (NSRF). This is the number of surfaces that separate zones of different density within a cell. Thus, if there is one surface there will be two zones.
•Density treatment (ISTRAT). The zones can either have constant density of the density can vary linearly from the top to the bottom of the zone.
•Save ZETA to file (ISWIZT). If checked the SWI2 package will save the elevations of the zone surfaces (ZETA) to a file at every time step at which head is written. The times at which head is saved are specified in the MODFLOW Output Control dialog box. ModelMuse can import the ZETA data and display it by coloring the grid, computing contour lines, or drawing cross sections.
•Observations (ISWIOBS). The elevation of the zone surfaces at particular locations at every time step can be saved to a file. The SWI_Observation_Name data set will be present if ISWIOB is not zero.
•Maximum slope of toe cells (TOESLOPE). The SWI2 package uses TOESLOPE to determine whether the toe of an interface between zones needs to more horizontally. The toe is where the interface intersects the base of the cell. The toe will be moved horizontally if the slope of the interface exceeds TOESLOPE.
•Maximum slope of tip cells (TIPSLOPE). The SWI2 package uses TIPSLOPE to determine whether the tip of an interface between zones needs to more horizontally. The tip is where the interface intersects the top of the cell. The tip will be moved horizontally if the slope of the interface exceeds TIPSLOPE.
•Fraction of threshold used to move the tip and toe (ALPHA). See Bakker and others (2013) for a more complete description.
•Fraction of threshold used to move the toe (BETA). See Bakker and others (2013) for a more complete description.
•Use adaptive time stepping (ADAPTIVE). The SWI2 package has an option to adjust the zone boundaries multiple times within a single MODFLOW time step. Checking this checkbox will activate the option. "The adaptive time step algorithm implemented in SWI2 can also be useful for determining an appropriate time step for subsequent SWI2 simulations. Initially, the adaptive SWI2 time step algorithm can be used to determine the SWI2 time step necessary to meet the desired ADAPTFCT value for movement of the tips and toes. After determining the number of SWI2 time steps needed to satisfy ADAPTFCT, the number of MODFLOW time steps in each stress period can be increased to match the number of SWI2 time steps determined dynamically by the adaptive SWI2 time step algorithm. The number of SWI2 time steps per MODFLOW time step for each stress period are written to the MODFLOW LST file as a summary table near the end of the file to facilitate setting final MODFLOW time steps based on interim adaptive SWI2 time step simulations."
•Maximum number of SWI2 time steps per MODFLOW time step (NADPTMX).
•Minimum number of SWI2 time steps per MODFLOW time step (NADPTMN).
•Factor controlling number of SWI2 time steps per MODFLOW time step (ADPTFCT). See Bakker and others (2013) p. 17 for a more complete description.
Solver
•Solver choice (NSOLVER). The SWI2 package can use the DE4 or PCG2 solvers to adjust the interface boundaries up or down. The modeler must decide which solver to use. It doesn't need to be the same solver as is used to solve the goundwater flow equation.
•Printout interval (IPRSOL). This determines how frequently information from the solver will be printed. The maximum ZETA change (positive or negative) and residual change are printed for each iteration of a time step whenever the time step is an even multiple of IPRSOL.
•Printing control (MUTSOL). MUTSOL determines what sort of information will be printed by the solver.
•Maximum number of outer iterations (MXITER). Maximum number of outer iterations in the PCG solver.
•Maximum number of inner iterations (ITER1). Maximum number of inner iterations in the PCG solver.
•Matrix preconditioning method (NPCOND). The PCG2 solver can use either of two preconditioning methods. The Modified Incomplete Cholesky method is almost always used.
•Max. abs. change in zeta (ZCLOSE). To achieve convergence, the maximum change in the zeta (the interface elevation) in an iteration must be less than ZCLOSE.
•Max. abs. residual (RCLOSE). The residual convergence criterion in units of Length3/Time.
•Relaxation parameter (RELAX). Usually RELAX is 1 but in some cases, setting RELAX to values such as 0.99, 0.98, or 0.97 may speed convergence. RELAX is only used if NPCOND = 1.
•Upper bound of the max. eigenvalue (NBPOL). NPBOL indicates whether the upper bound on the maximum eignevalue is calculated to simply set to 2. NBPOL is only used if NPCOND = 2.
•Damping factor (DAMP). DAMP is usually set to 1 but in some cases, setting it to value between zero and one may allow a model to converge that would otherwise not converge. In ModelMuse, DAMP only applies to steady-state stress periods.
•Transient damping factor (DAMPPCGT). DAMPPCGT is usually set to 1 but in some cases, setting it to value between zero and one may allow a model to converge that would otherwise not converge. DAMPPCGT only applies to transient stress periods.
Dimensionless Density
This tab contains a table in which the user specifies the fluid dimensionless densities in zones or at surfaces depending on the choice of ISTRAT. Dimensionless density equals (ρ - ρf)/ρf. where ρ is the density of the fluid and ρf is the density of freshwater.
