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SEAWAT Elder Model Tutorial

 

The following example walks through creating a simple numerical model with variable-density groundwater flow (using SEAWAT). The exercise is based on the well-known Elder model.

Objectives

Learn how to create a project and create a numerical grid for SEAWAT simulations

Learn how to define new property zones and boundary conditions

Define inputs/boundary conditions for SEAWAT contaminant transport

Translate the model inputs into SEAWAT input packages

Run SEAWAT engines

Understand the results by interpreting heads, drawdown, and concentrations in several views

Check the quality of the model by comparing observed heads to calculated heads, and observed vs. calculated concentrations

 

References

Guo, W., and Langevin, C.D., 2002. User’s Guide to SEAWAT: A Computer Program For Simulation of Three-Dimensional Variable-Density Grown-Water Flow. U.S. Geological Survey Open-File Report 01-232. Tallahassee, Florida.

Langevin, C.D., Shoemaker, W.B., and Guo, W. 2003. MODFLOW-2000, the U.S. Geological Survey Modular Ground-Water Model -Documentation of the SEAWAT2000 Version with the Variable-Density Flow Process (VDF) and the Integrated MT3DMS Transport Processes (IMT). U.S. Geological Survey. Tallahassee, Florida.

Introduction

Saltwater intrusion is a common occurrence along coastlines throughout the world. Saltwater is denser than freshwater, and consequently will tend to migrate inland underlying freshwater zones when groundwater resources are depleted or in areas where the water table is low. A static equilibrium is reached when the freshwater discharge force balances the buoyancy force due to density. Visual MODFLOW Flex includes the SEAWAT Engine to simulate saltwater intrusion.

SEAWAT is a computer program for simulating three-dimensional, variable-density, transient ground-water flow in porous media. SEAWAT was designed by combining a modified version of MODFLOW-2000 and MT3DMS into a single computer program. SEAWAT contains all of the processes distributed with MODFLOW-2000 and also includes the Variable-Density Flow Process (as an alternative to the constant-density Ground-Water Flow Process) and the Integrated MT3DMS Transport Process.

This tutorial illustrates a common application for SEAWAT and is based on the Elder Problem published in the User’s Guide to SEAWAT (Guo and Langevin, 2002). The basic design of this SEAWAT exercise is shown in the figure below.

 

 

The Elder model is commonly used as a benchmark problem for variable-density groundwater flow, where fluid density is a function of salt concentration. This problem simulates salt-water intrusion and proliferation through a freshwater aquifer. Molecular diffusion is the sole mechanism of hydrodynamic dispersion during the simulation, which runs for 20 years. Salt from the constant-concentration boundary at the top of the model diffuses into the model domain and initiates a series of complex vortices that redistribute the salt mass throughout the model. A constant-concentration boundary with a value of zero is specified for the lowest layer in the model. Two outlet cells with constant head values of zero are specified for the upper left and right boundaries. These constant head cells allow salt to diffuse into the model by providing an outlet for fluid and salt mass (Guo and Langevin, 2002).

Create the Project

Launch Visual MODFLOW Flex.

Select [File] then [New Project..]. The Create Project dialog will appear.

Type in project name 'SEAWAT_Elder'.

Click the [] button, and navigate to a folder where you wish your projects to be saved, and click [OK].

Define your coordinate system and datum (or just leave the Local Cartesian as defaults).

Most of the default units will be fine, but we will need to update the conductivity units.

Select [m/d] as the Conductivity units

Click [OK] button in the lower right corner of this window.

 

Note: It is recommended to select the folder “C:\Users\User_Name\Documents\VMODFlex” as the default location for your projects. You may then add a new folder to each individual modeling project. For the purposes of this project you may create the sub-folder SEAWAT_Elder. A new folder can be created automatically by selecting the ‘Create a folder for the project’ button under Data Repository.

 
Before clicking OK, the Create Project dialog should look like the image below:

 

The following window will then appear:

The Select Modeling Scenario allows you to choose whether to proceed with the Conceptual or Numerical modeling workflow. The conceptual modeling workflow allows you to import data objects into Visual MODFLOW Flex and to build a conceptual site model (CSM) that is grid independent. The CSM can then be used as a starting point for several different numerical models. In other words, numerical models (i.e. with different grid types, engines, etc.) can be quickly and easily created based on the same conceptual model. This makes it easy for you to manage several different numerical models with slight variations.

Conceptual modeling is not covered in this exercise, so we will proceed with the numerical modeling workflow

Select [Numerical Modeling] and the Numerical Modeling workflow will load.  

Proceeding with the numerical modeling workflow will bring you to the first step in the that workflow, which is to define your model objectives. This step allows you to specify whether you will be running a fully saturated or variably saturated model, whether it will be a constant or variable density model, whether contaminant transport will be included, which flow/transport engines will be utilized, etc.

The selection of these modeling objectives will determine which model- and species-specific transport properties must be defined (e.g. molecular diffusion coefficients, distribution coefficients, decay constants, etc.). It is also possible to define default property values at this time.

For the default project properties please input the values as shown figure below.

Select Flow type: Saturated (Variable Density)

Select Simulation type: Groundwater flow

Available Flow Engines: USGS SEAWAT from WH

Select Start Date: today

Type: Kx (m/d) = 0.411 in the Default Project Property Settings frame

Type: Ky (m/d) = 0.411 in the Default Project Property Settings frame

Type: Kz (m/d) = 0.411 in the Default Project Property Settings frame

Type: Initial Head = 1E-6 in the Default Project Property Settings frame

Type: Ss (1/m) = 1E-05 in the Default Project Property Settings frame

Type: Sy = 0.1 in the Default Project Property Settings frame

Type: Name = Salt in the Species Parameters table

Type: Description = Salt in the Species Parameters table

Type: DRHODC = 0.7 in the Species Parameters table

Type: MDCOEFF = 0.308 in the Species Parameters table

 

Note: MDCOEFF is the species-specific molecular diffusion coefficient. This parameter is only used if the multi-diffusion option is set to true in the DSP package advanced settings. While we have entered a value here, it will not be used. We will specify a model-wide molecular diffusion coefficient at a later step.

Now that the modeling objectives are defined you may advance to the next step of creating the numerical grid. Click the white right arrow in the blue circle at the top of the Numerical Model window to advance to the Define Numerical Model step.

Click [] (Next Step) to proceed to the Define Numerical Model workflow step

Click [Yes] to dismiss the warning message about the model start date

Now click on the [Create Grid] icon

Define Model Grid

We will now proceed with the model grid creation. You should now see the Create Grid workflow step, as shown in the image below:

We will create a grid with 1 row, 44 columns and 27 layers. Specifying the number of rows/columns and the overall grid extents horizontally will be quite easy, as these values can be specified directly in this window. To specify layer elevations for all 27 layers would be quite tedious though, so we will initially create the grid with a single layer with the elevations being the maximum and minimum final layer elevations. We will then refine the initial layer into 27 equally thick layers.

Please input the number of Rows and Columns and change the number of layers to 1. The top elevation of the first layer shall be set to 6 m, and the bottom elevation to -156 m.

Rows = 1 in the Grid Size frame

Columns = 44 in the Grid Size frame

X Min = 0 in the Grid Extents frame

X Max = 600 in the Grid Extents frame

Y Min = 0 in the Grid Extents frame

Y Max = 13.636 in the Grid Extents frame

Cell width = calculated this value can be adjusted as per project requirements

Cell height = calculated this value can be adjusted as per project requirements

Number of Layers = 1                

Layer1 - Top Elevation = 6

Layer1 - Bottom Elevation = -156

The screen should now look like the image below:

[Create Grid] button at the top-left of preview window

The Model Explorer (the lower left section of the interface) will be populated with the newly created model structure items. You will also see 2 new surfaces created in the Data Tree. These surfaces represent the top and bottom surfaces of the model layers and are consequently transformed automatically into ‘Horizons’ in the Model Explorer. Each pair of horizons in turn defines a zone, so you will also notice one new zone in the Model Explorer.

Your grid has been created and you’ll notice that the Properties section of the model structure is empty, so the next step will be to define boundary conditions and the model properties. The ‘View/Edit Grid’ portion of the workflow allows the user to make any necessary changes to the grid structure. This will be particularly useful for us, since we need to refine our initial grid vertically to have 27 layers (as defined by the original Elder model).

Click [] (Next Step) to proceed to the View/Edit Grid step

 

In the middle of the interface you will see a Toolbox. The buttons under this toolbox allow you to edit the grid in the horizontal direction (i.e. Edit Grid…), to edit the grid in the vertical direction (i.e. Edit layers…), to extract a subgrid from your model (i.e. Create subgrid…), or to assign model cells as active/inactive (i.e. the Inactive Cells frame). We will utilize the Edit layers functionality to refine our initial single layer grid, and then use the Inactive Cells feature to deactivate several flow cells in the top model layer.

Click the [Edit layers…] button under the Toolbox

Type: FinalGrid in the New grid name field

Type: 27 in the By a factor of field

Click the [Apply edit] button at the bottom left

The window should look like the following:

 

 

Finalize the layer edit by clicking [OK]

Once again you should see that the Model Explorer will be populated with the newly created model structure items, and a new workflow window will open. We will continue working with the FinalGrid workflow, so feel free to minimize the initial grid in the Model Explorer, and close the initial workflow window:

The final change to the grid is to specify two inactive areas in model layer #1. To make this change we will activate the Row view (Row 1) and assign inactive cells using the polyline drawing function:

þ Row in the Views frame, to activate Row view

Layer in the Views frame, to deactivate Layer view

Exaggeration: 5 in the field above the viewer to decrease the vertical exaggeration

[Assign] > [Using Polyline] button under the Toolbox, under Inactive Cells

Draw a polyline in layer 1, from Column 1 to 11 (click once to start, double-click to finish)

Draw another polyline in layer 1, from Column 34 to 44 (click once to start, double-click to finish)

[Finish] button under the Toolbox

[OK] in the Copy to Row(s) window that appears

The final grid should look like the image below:

We have now recreated the grid as required by the Elder model and are ready to proceed to the next step in the workflow, which is to define property values.

Click [] (Next Step) to proceed to the Define Properties step

Define Properties

You should now be viewing the Define Properties workflow step. You may want to inspect that the model properties are set according to the default property values that you defined during the Define Modeling Objectives step. Select any of the properties from the toolbox and click on the Edit… button, and you will see the values assigned to each cell of the model for the respective property. For example, the image below displays the Edit Property window for the conductivity property group:

Click [Edit...] in the Toolbox to open the Edit property window for a given property group

Click [OK] in the dismiss the Edit property window

Remember that at any step during the numerical workflow you have the possibility to inspect your grid in four different views: plan, two cross-section views and a 3D view. These views can be toggled on and off by checking the respective check-boxes in the Views area. Make sure you are looking at Row 1 in the Layer View window. If you have more viewer windows open, make sure that Row View is the active window, as shown in the image below:

Now we will assign property values as shown in the following image:

We will begin with the conductivity values for the top and bottom model layers:

Select [Conductivity] from the first menu in the Toolbox frame

Click [Assign] > [Polyline] buttons under the Toolbox

Type: 5 in the [Exaggeration] field above the viewer to decrease the vertical exaggeration

Draw a polyline across all of layer 1 (click once to start, double-click to finish)

Draw another polyline across all of layer 27 (click once to start, double-click to finish)

Click [Finish] button under the Toolbox

The New Property Zone window will open.

In this window we will assign the selected cells to a New property zone, and apply the desired Kx, Ky and Kz values (i.e. 1E-05 m/day).  

[New] button at the top-left of the window, to create a new property zone

Type: 1E-05 in the Value field for Kx, Ky and Kz

[OK] to accept inputs and create the new property zone

The New Property Zone window should look like the image below before clicking [OK]:

We will repeat this process for initial concentrations, assigning a value of 285,700 mg/L for layer 1 ONLY.

Select [Initial Concentration Salt] from the first menu in the Toolbox frame

Click [Assign] > [Polyline] buttons under the Toolbox

Draw a polyline across all of layer 1 (click once to start, double-click to finish)

Click [Finish] button under the Toolbox

The New Property Zone window will open

Click the [New] button at the top-left of the window, to create a new property zone

Type: 285700 in the Value field for Concentration

[OK] to accept inputs and create the new property zone

The remaining changes to property values will be made to the entire model, so we can use the Edit function rather than assigning new property zones using the drawing tools. We will update the effective and total porosity values (through the Storage property group) and longitudinal dispersivity (through the Dispersion property group).

Select [Storage] from the first menu in the Toolbox frame

Click [Edit...] button under the Toolbox

Type: 0.1 in the Effective Porosity column, then use [F2] to propagate through all cells

Type: 0.1 in the Total Porosity column, then use [F2] to propagate through all cells

[OK] to accept inputs and create the new property zone

 

Select [Dispersion] from the first menu in the Toolbox frame

Click [Edit...] button under the Toolbox

Type: 0 in the Longitudinal dispersivity (m) column, then use [F2] to propagate through all cells

[OK] to accept inputs and create the new property zone

Finally, one final change must be made to the model level dispersion parameters. These can be accessed by right-clicking the Dispersion property group under the Model Explorer and selecting Dispersion Parameters as shown in the image below:

Right-click Dispersion from the Model Explorer (under FinalGrid1 > Run1 > Inputs > Properties > Transport)

Click [Dispersion Parameters] from the menu that appears

 

 

The Dispersion Parameters window will open

Type: 0.308 in the Diffusion Coeff. (m^2/day) field, then use [F2] to propagate through all cells

Click [OK] to confirm and assign values

 

Note: this diffusion coefficient is the model-wide molecular diffusion coefficient. This is different than the species-specific diffusion coefficient applied during the Define Modeling Objectives step. The species-specific value is only used if the multi-diffusion option is set to true in the DSP package advanced translation settings.

Now is a good time to save the project.

Click [File] > [Save Project] from the main menu

 

Define Boundary Conditions

If you are satisfied with the property values assigned to the model, advance to the Define Boundary Conditions step in the numerical workflow. We will assign the model boundaries in Row view.

Click [] (Next Step) to proceed to the Define Boundary Conditions step

At this stage we will assign boundary conditions to our model, starting with a constant head boundaries at the top-left and top-right corners of the model (see the figure at the very beginning of this tutorial). We will do this by simply selecting the two desired cells.

[Constant Head] from the first menu under the Toolbox

[Assign] > [Cells] from the menu under the Toolbox

Left-click once in Layer 2, Row 1, Column 1

Left-click once in Layer 2, Row 1, Column 44

Click [Finish] under the Toolbox

The Define Boundary Condition window will open

Click [Next>>] to accept default name and proceed

We will assign a constant head value of 0 m to both of the selected cells. For both cells, type a value of 0 in the starting and ending head fields, select Specified Density as the density option, and apply a density of 1000 kg/m3.

Type: 1E-6 in the Starting Head (m) field, for both cells/rows

Type: 1E-6 in the Ending Head (m) field, for both cells/rows

Select Specified Density in the Density Option field, for both cells/rows

Type: 1000 in the Density (kg/m^3) field, for both cells/rows

Click the [Finish] button to finalize and create the constant head boundary condition

You will notice that both of the selected cells will be populated with red dots. These red dots represent the newly created boundary condition. Boundary conditions within Visual MODFLOW Flex are color-coded for quick identification.

We will now apply two constant concentration boundary conditions. In the top layer we will apply a high salt concentration (285.7 kg/m3), and in the bottom layer we will apply a constant concentration of 0 kg/m3. This concentration gradient is what will drive water flow and solute transport in this model.

[Constant Concentration] from the first menu under the Toolbox

[Assign] > [Polyline] from the menu under the Toolbox

Draw a polyline across active zone of layer 1 (click once to start, double-click to finish)

Click [Finish] under the Toolbox

The Define Boundary Condition window will open

Click [Next>>] to accept default name and proceed

Type: 285700 in the Salt (mg/L) field, then use [F2] to propagate through all cells

Click the [Finish] button to finalize and create the constant head boundary condition

 

Note: if the drawn polyline extends into the inactive zone, you may see a Resolve validation failures window appear. In this case, Flex is simply alerting you that some of the selected cells are in the inactive zone, and the boundary conditions for those cells can either be deleted or the cells can be reactivated. The default solution will be to Delete invalid cells. If this alert appears, simply click Apply then Close to resolve potential validation errors.

Now we will repeat the steps above to apply a constant concentration of 0 mg/L to the lowest model layer.

[Constant Concentration] from the first menu under the Toolbox

[Assign] > [Polyline] from the menu under the Toolbox

Draw a polyline across active zone of layer 27 (click once to start, double-click to finish)

Click [Finish] under the Toolbox

The Define Boundary Condition window will open

Click [Next>>] to accept default name and proceed

Type: 0 in the Salt (mg/L) field, then use [F2] to propagate through all cells

Click the [Finish] button to finalize and create the constant head boundary condition

Now all of the desired boundary conditions have been applied across the entire model domain. Your display should look something like the image below. Note the beige dots in layers 1 and 27, indicating the presence of the constant concentration boundaries. Also note the red dots indicating the constant head boundaries in layer 2 (top-left and top-right). Finally, note how each new boundary condition has been added into the Model Explorer.

We now have enough data to run this density-driven flow model, but first we have to define our model translation settings.

Translate and Run the SEAWAT Model

Navigate to the Select Run Type > Single Run step in the numerical workflow. You may click directly on this step as it is colored green, or you can navigate there step-by-step using the navigation arrows at the top of the workflow window.

Click [Single Run] from the Workflow Navigator, to proceed directly to the Single Run workflow step

You will be prompted to select which engines to run. By default, USGS SEAWAT from WH should already be selected. SEAWAT is the only flow engine within Visual MODFLOW Flex that where density-dependent flow modeling is possible; if you click on the Flow Engine menu you will see that 'USGS SEAWAT from WH' is the only available option.  The engine settings should look like the image below:

You will now advance to the Translate step of the workflow. This step will translate all the input data defined for the model into “packages” (i.e., MODFLOW input files) that can be run by SEAWAT.

Click [] (Next Step) to proceed to the Translate step

The translation settings will appear; this allows you to adjust solvers and their parameters (number of iterations, head-change criterion, damping factors), package settings, output control, etc.. In the Settings section of the SEAWAT branch you can change the Steady-State Simulation Time to 7300 by entering this value directly.

Go to the [SEAWAT] > [Settings]  section of the menu

Select BCF6 in the Property Package field.

Dismiss the warning about the VSC package which will be turned off later

Type: 7300 in the Steady-State Simulation Time field

Select Transient in the Run Type field

Type: 20 in the Time Steps field

Type: 1 in the Multiplier field

 

Go to the [SEAWAT] > [Solvers] section of the menu

Select Conjugate Gradient Solver (PCG) in the Selected Solver field

Type: 50 in the Max. Outer Iterations (MXITER) field

Type: 25 in the Max. Inner Iterations (ITER1) field

Type: 1.0E-7 in the Head Change Criterion (HCLOSE) field

Type: 1 in the Residual Criterion (RCLOSE) field

Type: 5 in the Printout Interval (IPRPCG) field

 

Go to the [SEAWAT] > [Layers] section of the menu

Apply layer type ‘0: Confined, constant S,T’ for ALL layers

 

Go to the [SEAWAT Transport] > [General] section of the menu

Select Total in the Porosity Options field

Type: 0.1 in the Courant Number field

Type: 0.05 in the Min. Sat. Thickness field

 

Go to the [SEAWAT Transport] > [Solution Method] section of the menu

Select Central Finite Difference (CFD) in the Advection Method field

Select Yes in the Use Implicit GCG Solver field

Type: 1E-08 in the Relative Convergence Criterion field

Type: 5 in the Concentration change printing interval field

Select Modified Incomplete Cholesky in the Preconditioners field

Type: 30 in the Initial step size (DT0) (days) field

Type: 0 in the Max step size (days) field

 

Go to the [SEAWAT Transport] > [Output Control] section of the menu

Type: 7300 in the Simulation time length (project time units) field

Type: 3000 in the Max number of transport steps field

Click [Add row] x6 press the Add row button six times to specify six output times

Type: 365 [press Tab], 730 [press Tab], 1095 [press Tab], 3650 [press Tab], 5475 [press Tab] and 7300 as the desired output times in each row

The solver settings window for the Output Control section should look like the image below:

 

It is also important to know that individual settings for the SEAWAT variable-density flow (VDF) and viscosity (VSC) packages can be set through the SEAWAT Transport > Advanced Settings menu.

The VDF translation menu allow you to specify how groundwater densities are calculated using the Density Option setting (i.e. using salt only, using multiple species, or using user-specified densities), how internodal density is calculated (i.e. central-in-space or upstream-weighted algorithm), whether a variable density water table correction is applied or not, and to specify constants such as minimum/maximum fluid density, and a reference fluid density.

Go to the [SEAWAT Transport] > [Advanced Settings] > [VDF] section of the menu

Select Up-stream weighted algorithm in the Intermodal density calculation algorithm field

Select Not applied in the Variable density water table correction field

Type: 0.001 in the Initial Time Step field

Select the Explicit coupling using one iteration option

The VSC translation settings allow you to specify how groundwater viscosities will be calculated in the simulation. The Viscosity Option setting allows you to choose whether viscosity is calculated using salt only (no temperature dependence), using temperature and additional species (e.g. salt), or whether specifies viscosities will be used. You can also specify constant values such as maximum, minimum and reference viscosities in this menu. If you access the VSC translation settings menu you should see the following:

However, we do not actually want to run the .VSC package in this simulation, assuming that there is no viscosity effects at higher concentrations. To remove the .VSC package from this model we will change both the ‘Run’ and ‘Translate’ fields to ‘No’.

Select [No] in the Run field

Select [No] in the Translate field

At this time the desired translation settings have all been specified and we can proceed to translating the model input files.Press the translate button to begin the translation process. As soon as you click the translate button Visual MODFLOW Flex will begin translating all the required input files for a SEAWAT model.

Click [] to create the SEAWAT input packages. Check the log to confirm the translation has finished.

Click [No] to dismiss any warnings about the RIV package being set to translate (and any similar messages)

Click [Yes] to dismiss an warning about initial heads

A translation log will run for a few moments and should terminate with the following message:  

######################## Translation Finished ########################

You should also see a series of new tabs at the top of the translation window, corresponding to the various input files which have been generated. You can click these tabs to view the contents of each input file. You can also click the open file button [] to open the folder containing the input files. If you aren’t happy with the translated files you can always return to the Settings tab, make changes and retranslate the files. For this model run you won’t need to make any further changes.

You may now proceed to the Run Numerical Engines step.

Click [] (Next Step) to proceed to the Run step

Click [] to run the SEAWAT model

The numeric engines will start running and display progress in the main window. SEAWAT will run first followed by any secondary engines selected during the Single Run workflow step (e.g. MODPATH or ZoneBudget; we haven’t selected any additional engines in this model). Each engine will have an information window that displays simulation results and progress. Clicking on the tab of the respective window will enable you to view detailed results of each run. When your model is finished running your results screen should look like the image below:

If you would like to inspect a record of the run process you may open the listing (.LST) file by clicking on the Open Engine Files… button [] at the top of the window. The model run may take several minutes to complete, since SEAWAT models are more complex than most contaminant transport models. A SEAWAT model may take 2-4 times longer to run than a MODFLOW model of the same dimensions due to the coupling of flow and transport processes.

Output Visualization

To see the results of the simulation navigate to View Results -> View Maps in the workflow tree. Equipotentials will be shown in plan view by default. You may want to change your map appearance by using the icons at the top of the window and/or right clicking on Heads in the Model Explorer and select Settings…. To see your equipotentials and the water table in cross-section activate the Row viewer (put a check-mark) and select the row number along which you would like to inspect the results.

Click [] (Next Step) to proceed to the View Results step

Click [View Maps] to proceed to the View Maps workflow step

þ Row in the Views frame, to activate Row view

Layer in the Views frame, to deactivate Layer view

Depending on your settings, you should see something like the figure below:

Note: you can maximize the viewer window by deactivating the Model Explorer and/or Workflow Navigator panels. Use the toolbar buttons [] to turn these panels on/off.

To better visualize the flow field in the model domain you can activate the Velocity outputs under the Model Explorer. The velocity flow field indicates that the diffusive flux of salt in the model domain is driving water flow in two recirculation cells, with the greatest flow velocities in the middle of the model and circulating upward toward the edge of the model where the constant heads boundaries have been applied.

Activate þ Velocity under the Outputs/Flow section of the Model Explorer

 

Note: if the velocities don’t match the figure above initially, try toggling the velocities and/or selecting 7300 from the time control bar in the toolbar.

 

Now you may activate the concentration results, which will display the diffusion of salt throughout the model domain at different output times:

Deactivate þ Heads under the Outputs/Flow section of the Model Explorer

Deactivate þ Velocity under the Outputs/Flow section of the Model Explorer

Activate þ Concentration (Salt) under the Outputs/Flow section of the Model Explorer

Press the last time step button [] in the time controls

 

Note: if the contours don’t match the figure above initially try right-clicking ‘Concentration (Salt)’ from the Model Explorer, accessing the contour line settings and updating the minimum contour level to 0 mg/L.

You can use the time-step picker buttons above the Flex viewer to display the concentration results at various output times. If you update the viewer to display the results at day 365 you should see something like the image below:

Press [] under the Outputs/Flow section of the Model Explorer

 

Using the cell-inspector tool, review salt concentrations throughout model layer 2 and you should notice some unusual results.

Press [] under the Outputs/Flow section of the Model Explorer

The Cell Inspector window will open, allowing you to select which information will be displayed on a cell-by-cell basis. We’re only interested in the results of the model run, so we can deactivate the position, properties, and boundary conditions categories:

Position to turn off position information

Properties to turn off properties information

Boundary Conditions to turn off boundary condition information

Budget turn off the flow budget information

Click once on Layer 2, Row 1, Column 11

The Cell Inspector window will now display the results of the simulation for the selected cell, as shown in the image below. You can see from these cell inspector results that the salt concentration in cell 2, 1, 11 is -43,772 mg/L. These results are clearly non-physical and can be attributed to truncation errors and/or numerical dispersion associated with the selected solution method for the transport equation (i.e. Central Finite Difference method).

You can improve the results by adjusting the solution method inputs. Return to the Translate workflow step and make the following changes:

Click [Translate] directly from the workflow navigator, to return directly to this workflow step

Go to the [SEAWAT] > [Solvers]  section of the menu

Type: 0.01 in the Residual criterion (RCLOSE) field

 

Go to the [SEAWAT Transport] > [Solution Method] section of the menu

Select Total Variation Diminishing (TVD) in the Advection Method field

Type: 20 in the Max. number of outer iterations field

Type: 100 in the Max. number of inner iterations field

Type: 1E-09 in the Relative Convergence Criterion field

 

Now re-translate and re-run the model

Click [] directly from the workflow navigator, to return directly to this workflow step

Click [No] to dismiss the warning(s) about No package data

Click [Yes] to dismiss the warning about initial heads

Click [] (Next Step) to proceed to the Run step

Click [] to run the SEAWAT model

 

When the model is finished running you should see the following results for salt concentrations at day 365 and 7300, respectively. You can also double-check salt concentrations throughout layer 2 to ensure that no negative concentrations are present:

 

Compare the contours at day 365 and 7300 to the following published results (Guo and Langevin, 2002):

Figure reference: Guo and Langevin (2002), page 74.

Note: concentrations in the published results are displayed as a percentage of the constant concentration at the top of the model. In the default Visual MODFLOW Flex output the concentrations are displayed as absolute values.

 

Now is a good time to save the project.  Click [File]>[Save Project] from the main menu.

*****This concludes the SEAWAT - Elder Model tutorial*****

 

 


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