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SWI2 - Seawater Intrusion Package

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Abbreviation in Name file

SWI2

Purpose

The SWI2 Package allows three-dimensional vertically integrated variable-density groundwater flow and seawater intrusion in coastal multiaquifer systems to be simulated using MODFLOW-2005.

Documentation

Bakker, M., Schaars, F., Hughes, J.D., Langevin, C.D., Dausman, A.M., 2013, Documentation of the seawater intrusion (SWI2) package for MODFLOW: U.S. Geological Survey Techniques and Methods, book 6, chap. A46, 47 p., https://pubs.usgs.gov/tm/6a46/.

Supported in

MODFLOW-2005

MODFLOW-NWT

MODFLOW-OWHM

Other Notes

The SWI2 package is not intended for the simulation of the combined flow of freshwater and brines, because variations in viscosity are neglected.

It is important to recognize that the steady-state head option does not mean that the steady-state density distribution is computed. The steady-state density distribution can only be computed by running the simulation until the density distribution is no longer changing.

Starting heads (STRT) should be modified to represent the freshwater head at the top of each layer.

All standard MODFLOW packages may be used with the SWI2 Package. When the package requires specification of a head, for example the General Head Boundary (GHB) Condition Package, this head must be the freshwater head at the top of the layer.

Similar to constant- and initial-head (STRT) values in the Basic (BAS) Package, time-variant specified head values for cells representing fluid densities greater than freshwater should be modified to represent the freshwater head at the top of each cell.

The SWI2 documentation contains tips for designing MODFLOW-2005 models using the SWI2 package.

Simulation of vertically integrated variable-density groundwater flow using the seawater intrusion (SWI2) Package is activated by including a record in the MODFLOW name file using the file type (Ftype) “SWI2” to indicate that relevant calculations are to be made in the model and to specify the related input data file. The NAM file should also be modified to include the appropriate information for saving interface (ZETA) elevations, SWI2 cell-by-cell budget data, and (or) SWI2 observation well data, if necessary.

The SWI2 file contains solver variables, data values for the different zones, algorithm parameters, the initial positions of the surfaces, the type of sources and sinks, and ZETA observation locations. In the body of the report, the number of surfaces are defined as being equal to the number of zones plus one. This number includes overlying and underlying surfaces corresponding to the layer top and bottom. In the input and output described here, however, only active ZETA surfaces are read and written. Active surfaces include only those surfaces between zones, and do not include the surfaces corresponding to the layer top and bottom. Thus, the number of active surfaces is equal to the number of zones minus one. Optional variables are indicated in [brackets].

FOR EACH SIMULATION

Data Set 0

[#Text]

Item 0 is optional—“#” must be in column 1. Item 0 can be repeated multiple times.

Data Set 1

NSRF ISTRAT NOBS ISWIZT ISWIBD ISWIOBS [OPTIONS]

Data Set 2a

NSOLVER IPRSOL MUTSOL

IF NSOLVER = 2

Data Set 2b

MXITER ITER1 NPCOND ZCLOSE RCLOSE RELAX NBPOL DAMP [DAMPT]

Data Set 3a

TOESLOPE TIPSLOPE [ALPHA] [BETA]

IF OPTIONS = ADAPTIVE

Data Set 3b

NADPTMX NADPTMN ADPTFCT

Data Set 4

NU(ISTRAT=0: NSRF+2, ISTRAT=1: NSRF+1)

FOR EACH SURFACE (FROM 1 to NSRF)

FOR EACH LAYER

Data Set 5

ZETA(NCOL,NROW) U2DREL

FOR EACH LAYER

Data Set 6

SSZ(NCOL,NROW) U2DREL

FOR EACH LAYER

Data Set 7

ISOURCE(NCOL,NROW) U2DINT

IF NOBS > 0

FOR EACH NOBS

Data Set 8

OBSNAM LAYER ROW COLUMN