Using the SAMG Solver for MODFLOW

Are you needing to use multiple processors to run your MODFLOW or MODFLOW-USG model? Using multiple processors can speed up the model processing time. In order to use multiple processors for MODFLOW in GMS, the SAMG solver provides the means to do this.

The SAMG solver is developed by the Fraunhofer Institute for Algorithms and Scientific Computing (SCAI). It contains a library of subroutines for the highly efficient solution of large linear systems of equations with sparse matrices. SAMG has an advantage in being almost unconditionally numerically scalable. This means that the computational cost using SAMG depends linearly on the number of unknowns.

Before using the SAMG solver, check to see if you have it added to your license. Do this by going to the Register command in the Help menu. Look through the list of components to make certain the SAMG parallel interface / solver has been enabled.

The SAMG solver works with the LMG solver package in MODFLOW. The LMG solver needs to be selected to access the SAMG solver. The LMG solver links the MODFLOW project to the SAMG solver.

SAMG warning message

To activate the SAMG solver do the following:

  1. Open the MODLOW Global/Basic Package dialog.
  2. Then open the MODFLOW Packages dialog.
  3. Turn on the LMG – Link-AMG option.
  4. A warning may appear discussing the SAMG solver. This warning can be turned off if desired.
  5. Once the LMG solver has been turned on, the Link-AMG (LMG) Package dialog can be accessed through the MODFLOW menu in order to set the solver parameters.

Using the SAMG solver is not ideal for all MODFLOW projects. It is generally meant for large linear problems and simulations which use several time steps. For the right project, SAMG can help significantly accelerate the model run time.

Contact Aquaveo’s licensing team to see about using the SAMG solver with your GMS projects.

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The HDF5 File Format and MODFLOW

The HDF5 file format is a cross platform binary format for storing scientific data. HDF5 allows you to reduce the size of the file data by compressing repeated values. This allows your data to be read and written much faster than if you stored the data as ASCII (plain text) files. GMS can read and write to HDF5 files, and stores its own HDF5 files with the other MODFLOW data files.

Exporting an HDF5 file

There are no hard file size limits. The number of objects within an HDF5 file is not limited, and the format supports complex relationships through grouping and linking. In addition to support for common metadata types, you can create user-defined metadata to accommodate whatever needs you have in your project. The only limits with these are the capabilities of your computer.

HDF5 was designed to be extensible and to allow for future changes to the platform. GMS takes advantage of this by checking HDF5 data for key values in order to substitute them into your arrays or lists. This allows you to better use transient parameters and very large pilot point sets. GMS handles the pilot point interpolation through an external routine that helps speed up the processing.

There are external HDF5 file viewers and editors, such as HDFView. However, it is very easy for you to modify the data in such a way that it makes the HDF5 unreadable by GMS. Because of this, we recommend only viewing the files using these tools. You should only manually edit the file if given explicit directions by a developer. If modifications need to be made to the HDF5 file, we recommend you make the changes to your project in GMS which will re-export the HDF5 file.

Learn more about the HDF5 file format from the HDF Group, a consortium of scientists worldwide that works on the HDF5 format. You can also read the article on the XMSWiki about using the HDF5 format with MODFLOW. This article goes into more details about multiple specific uses for the file format.

Make use of HDF5 files with MODFLOW in GMS today!

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Using the CLN Package to Model Faults and Fractures

Needing to model features or faults in your MODFLOW-USG model? The CLN Package is used in conjunction with MODFLOW-USG to model features that are significantly smaller than the individual cells in a 3D groundwater flow model. This tool allows you to model one-dimensional linear features within a three-dimensional simulation domain when the flow connections are separate from those of the aquifer. This makes it ideal for modeling fractures and faults.

CLN network map

There are a number of requirements to use the CLN package:

  1. The MODFLOW version must be set to MODFLOW-USG in the MODFLOW Global/Basic Package dialog.
  2. There must be a coverage with Wells and Wells (CLN) turned on in the Coverage Setup dialog.
  3. The coverage must have a default layer range of at least two layers, and it must be active on the layers where the CLN features will be.

The CLN package works with unstructured and 3D grids. It allows connections to be set up between the CLN cells to regular groundwater cells. You can connect a CLN cell to one or more groundwater flow cell, and CLN cells do not have to be ordered upstream to downstream. These features allow you to scale the conceptualization of the flow as needed.

For example, a single cylindrical CLN cell (representing a long fracture or fault within the ground) connected to multiple groundwater flow cells may be pumped to simulate multi-node well connections. The CLN cell extracts water from the groundwater flow cells as part of the solution to the coupled CLN and groundwater flow equations.

You can treat a CLN cell as a well by using the WEL Package to assign a source or sink to the cell. This can also be done using multiple CLN cells. Multiple fractures can be connected together to form one larger network. To make things easier, you can also use shapefiles to map the features of the CLN cells and connections.

CLN network map examples

By using this method, you can create a more complete picture of the domain you are modeling. Try using the CLN package to model faults and features in GMS today!

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Using MODFLOW-NWT for Mine Dewatering

Have you tried dewatering a mine model using MODFLOW-NWT? MODFLOW-NWT is a version of MODFLOW provided by the USGS that uses a method to solve MODFLOW models that are non-linear due to unconfined cells or non-linear boundary conditions, creating an asymmetric results matrix. Because mines that need to be dewatered sit below the water table, cells within the simulation will become dry at times due to the removal of the water from the mine. MODFLOW-NWT is great for such situations as it handles dry cells well.

Modeled mine example

Mine dewatering models are often used when estimating the required flow capacity for the pumps used in the field. You can create a regional model to establish a baseline, then create one or more models with various pit depths to establish the best approach for the mine in question. While elevations cannot be set to transition during a MODFLOW run, multiple conceptual models can be created within the same project in order to keep all of the data together.

It’s important to remember the difference between wells and drains when creating the dewatering model. Drains only subtract water when the head is above the elevation of the drain. Therefore, drains can be used to represent the seepage face around the edges of the pit, and the drain elevations should be set to equal the ground surface elevations. The amount of water removed by a drain is related to the conductance of the material of the seepage face. Drains are generally used at the bottom and side seepage faces of the mine.

Wells remove a specific amount of flow, and can therefore dewater the cell they are in prior to dewatering the rest of the mine. MODFLOW-NWT, as mentioned previously, handles these dry cells very well, keeping them active in case they become wet again.

Because MODFLOW-NWT uses 3D grids, you should make sure the cells inside the pits are inactive. You do this by creating the boundary polygon, and then removing a "donut hole" of cells within the pit area. Use the Activate Cell(s) in Coverages tool to inactivate the cells. You’ll have to do this for each layer, and each layer should be on its own coverage.

Finally, always use the Model Check to make sure there aren't any obvious errors or missing values in any of the inputs. After MODFLOW-NWT finishes running, you should review the water table by looking at the drawdown output contours. You can also do this by switching to Ortho view and reviewing the head in the appropriate rows or columns.

Try out making a mine dewatering model using MODFLOW-NWT in GMS today!

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