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Define Particles

Introduction

Particle-tracking is a useful tool for Groundwater Modelers that provides insights and diagnostics in the modeling process. Two particle tracking engines are supported in Visual MODFLOW Flex:

MODPATH

MOD-PATH3DU

 

 

  MODFLOW-LGR models are not compatible with the particle tracking in Visual MODFLOW Flex

 

 

MODPATH

The computer program MODPATH was developed by the USGS (Pollock, 1998) to calculate three-dimensional particle tracking pathlines from steady-state and transient flow simulation output obtained using MODFLOW-2000, MODFLOW-2005, MODFLOW-NWT, MODFLOW-SURFACT, and SEAWAT.

MODPATH uses a semi-analytical particle-tracking scheme. The method is based on the assumption that each directional velocity component varies linearly within a grid cell in its own co-ordinate direction. This assumption allows an analytical expression to be obtained describing the flow path within a grid cell. Given the initial position of a particle anywhere in a cell, the co-ordinates of any other point along its path line within the cell, and the time of travel between them, can be computed. Documentation of the theoretical and numerical implementation of MODPATH are given in Pollock (1998), while source code and input documentation are available at the USGS MODPATH webpage.  MODPATH in Visual MODFLOW Flex is not currently compatible with MODFLOW-USG or MODFLOW-LGR.

 

MOD-PATH3DU

The computer program mod-PATH3DU was developed as part of a collaboration between S.S. Papadopulos & Associates, Inc. (SSPA) and the University of Waterloo (Muffels, et al, 2016) and calculates three-dimensional particle tracking pathlines from steady-state and transient flow simulation output.  

Similar to MODPATH, mod-PATH3DU uses a semi-analytical particle-tracking scheme based on the linear interpolation of the velocity field within a given grid cell. The major difference is that mod-PATH3DU supports intra-cell velocity field interpolation using the Waterloo method, which is based on a high-order Runge-Kutta scheme and is valid for any prismatic cell geometry. In addition to the flow engines supported above, mod-PATH3DU also supports MODFLOW-USG.

A more thorough explanation of the method is documented by Rhamadhan (2016).

 

 

NOTE: mod-PATH3DU is developed/maintained by SSPA (a third party) and is not directly included in Visual MODFLOW Flex.  It is available for free via registered download at the SSPA website for mod-PATH3DU.

 

Visual MODFLOW Flex currently only supports mod-PATH3DU versions 2.12 and later.

 

 

Please Note: MOD-PATH3DU version 2.12 currently only supports fully structured grids or unstructured grids with continuous layers. In other words, the lateral discretization must be the same for all layers in the model.  The Visual MODFLOW Flex grid generator for Q-Grids does not generate such grids where pinchouts are specified, therefore the use of V-Grids is recommended for conceptual models with pinchouts when using MODFLOW-USG.  Version 3, likely to be released in spring 2019, will support discontinuous layers in unstructured grids.

 

Working with Particles in Visual MODFLOW Flex

At the Define Particles step, you can create, modify and view groups of Forward and Backward Particles, including those that have been imported from a Visual MODFLOW Classic model.

 

 

Assign

You can assign particles using one of the following methods:

 

 

 

 

Points...

First, select the model layer, row, or column where you want the circle to be assigned.

Select the option [Assign > Circle] from the toolbox.

Using the mouse, left-click on the Layer, Row, or Column view at the desired locations for the particles.

Once finished, right-click and select "Finish", or click on the Finish button from the toolbox, and the following window will appear:

 

Assign Points in Layer View

Assign Points in Row or Column Views

Assign End Time

 

The following data inputs will be available:

Particle group name: A unique name for the particle group

Select Object: not applicable as you have defined the location of the particles

Particle Propagation: provides the option to select whether the particles will propagate forwards or backwards in time

Assign Elevation: (layer view only)

oSpecified Layers: the Assign to layer method will be used to assign particles to specified layers as described below

oSpecified Elevation: the Custom elevations method will be used to assign particles by elevation as described below

Release Time Tab: provides the option to select when the particle group is released (i.e. tracking is initiated)

oBeginning of Simulation: particles in the group are released at relative model time = 0; this is the default for forwards-in-time particles

oEnd of Simulation: particles in the group are released at the end of the model simulation period; this is the default for backwards-in-time particles

oCustom: particles are released at the specified (relative) time based on the time units specified in the project settings.

End Time Tab: provides the option to select when particle tracking is terminated (if the particle has not yet reached an endpoint such as a sink for forwards particles and a source for backwards particles). Note this applies only to mod-PATH3DU simulations

oBeginning of Simulation: remaining particles in the group are terminated at relative model time = 0; this is the default for backwards-in-time particles

oEnd of Simulation: remaining particles in the group are terminated at the end of the model simulation period; this is the default for forwards-in-time particles

oCustom: remaining particles in the group are terminated at the specified (relative) time based on the time units specified in the project settings.

Assign to Layer: if the Assign Elevation option is specified layers, this option provides you with a table with entries for each layer where you can assign the particles to the specified layer(s) and whether the particles will be assigned to the layer Top, Center (middle), or Bottom.

Custom elevations: if the Assign Elevation option is specified elevation, this option provides you with following inputs

oMethod: Choose from:

Constant: a specified constant elevation based on the entered Value

Surface: a surface data object from the project explorer (use the blue arrow to choose an item selected in the Data Explorer)

Horizon: a horizon object from the model explorer window (use the blue arrow to choose an item selected in the Model Explorer)

Water Table: a water table object from the model explorer window (use the blue arrow to choose an item selected in the Model Explorer)

 

Click OK when you have finished.

 

Circle...

A circle of backward tracking particles is typically assigned around a pumping well in order to determine the capture zone for that well.

 

First, select the model layer where you want the circle to be assigned.

Select the option [Assign > Circle] from the toolbox

Move your mouse cursor to the location where you want the center of the circle to be located (typically on top of a well cell, at the center of the cell). This option is available in Layer View only

Left-click once to create the circle

The "Define Particles Window" will appear

 

 

 

Define the Radius for the Circle

Define the # Particles to be inserted

Define the Particle Propagation Direction, Assign elevation options, and Release time, as explained above.

Click OK when you have finished.

 

Polyline...

First, select the model layer, row, or column where you want the polyline to be assigned.

Select the option [Assign > Polyline...] from the toolbox.

Using the mouse, left-click on the Layer, Row, or Column views to draw the polyline in the desired view.

Once finished, right-click and select "Finish", or click on the Finish button from the toolbox, and the following window will appear:

 

Assign Polyline in Layer View

Assign Polyline in Row or Column Views

Assign End Time

 

The following data inputs will be available in addition to those described above:

Particles on polyline:

oEvenly spaced: the number specified particles will be placed using even spacing along the pathline

oDrop particle at every: particles will be placed along the polyline using the spacing provided, in project length units

 

Click OK when you have finished.

 

Polygon...

First, select the model layer, row, or column where you want the polygon to be assigned.

Select the option [Assign > Polyline...] from the toolbox.

Using the mouse, left-click on the Layer, Row, or Column views to draw the polygon in the desired view.

Once finished, right-click and select "Finish", or click on the Finish button from the toolbox, and the following window will appear:

 

Assign Polygon in Layer View

Assign Polygon in Row or Column Views

Assign End Time

 

The following data inputs will be available in addition to those described above:

Particles on polygon:  particles will be placed on a rectangular grid within the polygon using the specified number of columns and rows

Click OK when you have finished.

 

Using Data Object

Select the option [Assign > Using Data Object] from the toolbox and the following window will appear:

 

 

From the Data Explorer, select a Points, Polyline, or Polygon data object that represents the features/locations where particles are desired.  If you do not have a file containing these features, then you can create and digitize new points, polylines, or polygons using the drawing tools. Refer to Creating New Data Objects for more details.

Click the button to insert the data object into the input field.

The window will update and provide the same options as assigning by points..., polyline..., or polygon... as described above based on the input type.
 
Please Note: In addition specifying the particle elevations by layers or specified elevations, you may also use the elevations of the feature objects
 

Click OK when you have finished.

 

Delete Particles

[Delete >] By Drawing a Box, allows you to delete just a set of particles. Note that this option will only delete the particle type that is active at that time (either Forward or Backward).  Before proceeding with the Delete, be sure you have the desired particle type layer as active (and visible in the 2D Viewer).  To do this, click on , and move the desired layer type (Forward or Backward particles) to the top of the layer list. Once complete, select Delete > Draw in a Box, and left click with your mouse to anchor the corner of a box around the selected particles you wish to delete, then draw the outline of the box, and release the mouse button to complete the box. The particles that lie within that box will be deleted, though only for the current model layer.
 

[Delete >] Current Layer deletes all particles (Forward and Backward) in the current layer

[Delete >] All Layers, deletes all particles in all layers

 

MODPATH Options

There are several settings for the MODPATH run, which can be define at the "Translate" step. For more details, see MODPATH

 

 


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