1

Visual MODFLOW Flex: Conversion Guide

Introduction The purpose of this guide is to help you understand how to build a MODFLOW groundwater model using the conceptual modeling workflow in

Visual MODFLOW Flex. Each section will provide a brief summary of how the inputs are defined in Visual MODFLOW Classic (VMOD Classic),

and how the inputs are defined in Visual MODFLOW Flex (VMOD Flex).

Objectives

After reading this guide, you should achieve the following:

Gain a better understanding of how to build a conceptual model using Visual MODFLOW Flex, with your data

Understand the similarities and differences in numerical modeling (in Visual MODFLOW Classic) and conceptual modeling (in Visual

MODFLOW Flex)

The benefits of building conceptual models over numerical models

For more information Visual MODFLOW Flex tutorials

Comparison between Conceptual and Numerical Modeling

Web-based help

2

Step 1

VMOD Classic VMOD Flex

Define the Grid Import your data

Define the grid (number of rows and columns) over the area of your groundwater model, and reference this in space (typically to a known XY location). Import site maps (DXF, shapefiles) or background maps (georeference air/satellite photos). Digitize the cells that should be inactive.

It all starts with your data! Import your shapefiles, raster grids, images, points, and well locations. Import and interpolate points data to create surfaces; import Surfer or ESRI Grids (Raster, DEM). Analyze this in 2D/3D, and determine the area for your conceptual model. Defining the grid is the last step in the conceptual model workflow (see Step 6). Inactive cells are defined automatically during the conversion to the numerical model (Step 7).

3

Step 2

VMOD Classic VMOD Flex Define the Grid Layer Elevations Define Conceptual Model Structure

Assign grid layer elevations by:

importing XYZ points and interpolating;

importing Surfaces: Surfer or ESRI Grids (Raster, DEM); or

specifying defined thickness or elevation, or other methods

Define the model area with a polygon. Define geological surfaces by:

importing XYZ points and interpolating;

importing Surfer or ESRI Grids (Raster, DEM) Convert Surfaces to Horizons, by specifying a horizon type for each surface (Erosional, Conformable, or Discontinuous). Horizons are 3D surfaces that form the tops and bottoms of the geological formations.

4

Step 3

VMOD Classic VMOD Flex Assign Property Parameters to Grid Cells Define Property Zones

Assign hydraulic parameters to selected grid cells by:

drawing polygon/polylines,

importing XYZ points and interpolating, or

importing Surfaces: Surfer or ESRI Grids (Raster, DEM)

Assign hydraulic parameter to 3D geological formations by:

defining a constant value

linking to a Surface (imported from file; interpolating XYZ pts)

using polygon shapefiles with attributes

5

Step 4

VMOD Classic VMOD Flex Select Grid Cells and Define Boundary Condition Parameters Select Shapes and Define Boundary Condition Parameters

Assign boundary condition parameters to selected grid cells by:

drawing polygon/polylines

importing shapefiles with attributes,

importing gridded data: Raster Grids

importing MODFLOW packages or IJK files

importing transient data from external files

Assign boundary condition parameters to selected conceptual features defined by shapes. Define the geometry by:

using polygon/polylines you have drawn in VMOD Flex

importing DXF files or shapefiles with attributes Define the parameters using:

constant value, shapefile attributes, Raster grids

time schedule data (for transient data)

6

Step 6

VMOD Classic VMOD Flex Translate and Run Generate Numerical Grid

Translate the numerical inputs into MODFLOW packages and run MODFLOW.

Define the grid size (#rows and #columns) and the layering type (Deformed or Non-conforming). Refine the grid where needed or add local (child) grids around areas of interest.

7

Step 7 (VMOD Flex)

VMOD Flex

Convert Conceptual Model to Numerical Model With VMOD Flex, you’re almost there! Once you have the grid, you can generate multiple numerical models from the conceptual model objects (property zones and boundary conditions). This is done automatically through a conversion routine that automatically:

Assigns inactive cells to the regions outside the conceptual model area

Adjusts for complex geological horizons

Calculates the correct cell locations for properties and boundary conditions (including wells) The results are shown below.

Property Zones (viewed by Column (top) and by Layer (bottom) Boundary Condition Cells (red: constant heads; blue: rivers)

8

Step 8 (VMOD Flex)

VMOD Flex Translate and Run

Translate the numerical inputs into MODFLOW packages and run MODFLOW.

9

Step 9 Adjusting the Model Grid After the model has been run you may need to adjust the grid to account for new features or to improve model accuracy/stability. This involves

refining or coarsening the horizontal grid or the vertical layering.

VMOD Classic VMOD Flex Adjust the grid and manually correct

the properties and boundary conditions. Adjust the grid and automatically re-generate

the properties and boundary conditions.

With numerical modeling, changing the grid is easy, but the difficult part comes afterwards when you need to adjust the property zones and boundary conditions that are assigned to those grid cells. This is a time-consuming task, and often it is simpler to just delete the previous properties and boundaries and re-assign to the new grid cells.

With conceptual modeling, the property and boundary condition objects are assigned to shapes, not grid cells. This gives you the freedom to make any change to the numerical grid and automatically re-generate the numerical model with the appropriate properties and boundary conditions (as explained in Step 7).

Example of the impact of refining the grid on previously defined

river boundary conditions cells. After refining the grid and re-generating the numerical model, the river

boundaries are automatically assigned to appropriate grid cells.