Groundwater Modelling Advice Draft

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Common Questions Answered (FAQ)

What are the benefits of integrating groundwater modelling into TUFLOW for flood simulations?

Adding groundwater modelling to TUFLOW HPC improves flood simulations by showing how water moves both on the surface and underground. This helps in long-term simulations by considering how water soaks into the ground and flows below the surface. As a result, it better predicts how water levels change over time, especially during and after rainfall, leading to more accurate flood forecasts.

Why is water not leaving the system post-peak in an interflow model?

If water remains in the system after peak flow, review the following parameters:

• Soil thickness: Increasing thickness can enhance storage, potentially slowing discharge.
• Horizontal hydraulic conductivity: Low values may restrict lateral groundwater movement, limiting drainage.
• Baseflow contributions: Excess baseflow can sustain higher water levels artificially.
• Downstream boundary conditions: An HT boundary may hinder efficient water exit. Consider testing with a QH boundary.

Running sensitivity tests by adjusting horizontal conductivity, initial moisture, and soil thickness can help identify key factors affecting outflow rates.

Why does groundwater rise immediately before rainfall occurs?

This behaviour suggests groundwater is already at or near saturation, causing rapid surface expression. To slow groundwater emergence:

• Reduce initial groundwater depth (Set IGW Depth Layer 1).
• Adjust soil porosity to control storage capacity.
• Increase horizontal hydraulic conductivity to improve lateral flow.

Testing the model with no rainfall and a small initial groundwater depth can confirm if groundwater mechanisms are functioning as expected.

How can interflow behaviour be better aligned with observed conditions?

• Ensuring groundwater inputs and outputs are balanced is key.
• Comparing groundwater accumulation areas with surface water flooding can help verify results.
• Using the groundwater XDMF output can assist in visualising flow behaviour and refining parameter selection.

Fine-tuning soil properties, hydraulic conductivity, and boundary conditions will improve interflow simulation accuracy.

Why does setting up a second soil layer for a railway embankment require using the ‘CO’ type?

When defining Soil Layer 2, the convective (CO) layer type must be used to ensure correct infiltration behaviour. If another type is assigned, TUFLOW may return an error such as error 2314 (invalid soil ID) or error 3031 (internal error).

To resolve this:

• Define a new soil ID for Layer 2 using the CO type.
• Update the GIS soil layer under the embankment to use this new CO type.
• Set vertical conductivity to zero if needed to prevent downward infiltration.

Can soil layers be assigned only along a railway embankment, or must they cover the entire model?

Soil Layers 1 and 2 do not need to be applied across the entire model. However, areas without a defined soil layer may cause issues, so the following options are recommended:

• Define Soil Layer 1 across the model and limit Soil Layer 2 to the embankment.
• Assign a default soil type where no groundwater flow is needed.
• Ensure that areas without groundwater flow have a zero or non-transmissive soil depth to prevent unintended behaviour.

If further refinements are needed, enabling groundwater XDMF output can help visualise groundwater levels and flow paths.

Why does the TUFLOW sub surface flow equation include porosity, and what changes are planned?

Benchmarking tests have identified that the current sub surface flow equation in TUFLOW underestimates steady state discharge rates due to the inclusion of porosity. While the steady state water level gradient is correct, transient state simulations show discrepancies.

To address this, future updates are planned to provide a switch to turn the porosity term on or off in the equation.

• OFF for correct modelling.

• ON for backward compatibility with previous versions.

Why is hydraulic conductivity in TUFLOW measured in mm/hr instead of m/d?

TUFLOW currently uses mm/hr for hydraulic conductivity to align with the Green Ampt infiltration rate, which is commonly used in surface water modelling. Some literature also adopts this unit. However, m/d is more widely used in groundwater science, and discussions are underway to support both units for consistency.

Will terminology in the documentation be updated for consistency with groundwater sciences?

Terminology changes are planned in the future to align with standard groundwater terminology. Feedback is welcomed if any inconsistencies remain after the update.

Why is the groundwater model affecting areas beyond the expected flow path?

Groundwater movement in TUFLOW is influenced by topography, soil properties, and groundwater parameters. If groundwater is introduced, infiltrated water may reappear on the surface depending on these factors.

Key considerations:

• Without horizontal groundwater movement, infiltrated water is lost from the system. When groundwater movement is enabled, water can accumulate in low-lying areas or flow towards the downstream boundary.
• Unexpected groundwater presence in areas without assigned soil depths may be due to groundwater behaviour at boundaries or model-wide settings.
• The 2d_po regional outputs introduced in version AF can assist with analysing groundwater movement and verifying model behaviour.

Should soil layers be assigned across the entire model?

If groundwater is being simulated, defining soil layers across the full model domain can provide more control over groundwater behaviour.

Options include:

• Assigning a depth of zero in areas where groundwater should not be present.
• Setting a large depth in areas where groundwater storage is needed but horizontal transmission is not desired.

Model validation using observed data is recommended to confirm that groundwater interactions align with real-world conditions.

How can a lined filtration trench be modelled in TUFLOW?

A lined filtration trench can be represented in TUFLOW using one of the following methods:

1D Channel Approach

• Model the trench as a 1d_nwk “Q” type structure, which uses a depth-discharge relationship.
• Connect the trench to multiple 2D cells using a 2d_bc SX line.
• If the trench is long, divide it into multiple sections to improve accuracy.

1D Pit and Culvert Approach

• Use multiple 1d_nwk Q pits connected via a 1d_nwk culvert.
• The connected 2D cells are automatically selected using the sag or on-grade method (see section 5.12.3.3 of the TUFLOW manual).
• To manually control cell connections, set the 1d_nwk Conn_No attribute to a negative value. For example, Conn_No = -1 ensures each pit connects to only one 2D cell.
• This method allows the trench to be represented as multiple pit points, each selecting one 2D cell and linking to the 1D node using X connectors.

Additional Considerations

• Pit Inlet Discharge Curve: If using a 1D network, pre-compute the discharge for various depths to define a suitable pit inlet discharge curve.
• Interflow Functionality: The use of interflow depends on the cell size relative to the trench feature.
• Two-Stage Modelling Approach: One method involves running a 2D infiltration simulation first to determine infiltration rates. The infiltration can then be applied as a 1D boundary condition in a second simulation to represent flow into the drainage system.

TUFLOW is currently developing functionality to dynamically link interflow to a 1D network, which will allow infiltrated subsurface flows to connect directly to 1D nodes or pipes in future releases.

What methods and result outputs can be used to quantify infiltration losses over a given area in TUFLOW?

To assess infiltration losses, the following outputs can be used:

Map Output Data Types

• CI (Cumulative Infiltration): Displays the total infiltration over time.

• IR (Infiltration Rate): Shows the infiltration rate at each timestep.

Point Output (2d_po) Approach

• If infiltration data is needed for a specific area, a 2d_po region can be set up.

Both methods help understand how infiltration happens across different areas.

Why does changing the initial soil moisture in the Green-Ampt (GA) infiltration method not affect infiltration rates as expected?

In the 2023 release of TUFLOW, the treatment of initial soil moisture in the GA method was updated to accommodate horizontal soil water movement. This change allows soil moisture to flow between sub-surface cells and be removed via evapotranspiration, making it more dynamic over time. Instead of remaining static in the infiltration equation, the initial soil moisture is now used as the cumulative infiltration (F) at the first timestep of the simulation. This means:

• The infiltration rate at the start of the simulation is influenced by the initial soil moisture, but subsequent infiltration behaviour depends on the balance of infiltration, drainage, and evapotranspiration.

• If no soil thickness is defined, the model assumes dZ = 0, meaning the initial moisture value is ignored altogether.

Suggested Workarounds

1. Reverting to the pre-2023 method.

Use the backward compatibility switch:

Defaults == Pre 2023

Note: This changes multiple default settings, not just the GA method.

2. Adjusting the soil porosity in the .tsoil file.

Instead of defining initial soil moisture separately set: Adjusted Porosity = Porosity - Initial Moisture

Example: If soil porosity is 0.385 and initial soil moisture is 0.200, set the porosity to 0.185.

TUFLOW is updating documentation to clarify this change and is working on a dedicated command to allow users to revert to the original GA method without affecting other default settings. Further refinements are also being considered for long-term simulations involving multiple wet/dry cycles.

If you have feedback or specific cases where this affects your modelling, please let contact TUFLOW Support via support@tuflow.com.

Can TUFLOW trap groundwater beneath impervious areas to prevent unrealistic exfiltration?

Currently, TUFLOW does not directly simulate pressurised groundwater flow beneath impervious surfaces. The existing groundwater model calculates exfiltration based on mass balance, meaning that groundwater can still migrate upward even in areas where the surface is defined as impervious.

Recognising the importance of this feature for improving groundwater representation in urban modelling, consideration is being given to incorporating a method to account for trapped groundwater as part of future TUFLOW release.

In the meantime, users may mitigate this issue by:

• Removing impervious locations from the soil layer entirely.

• Setting horizontal hydraulic conductivity to zero for soil polygons beneath impervious areas.

• Adjusting the soil thickness and properties to reduce unrealistic seepage effects.

If this issue is recurring or if there are specific use cases where this feature would be beneficial, feedback is encouraged via support@tuflow.com.

What is the recommended method for representing a railway ballast area in TUFLOW?

A railway ballast area in TUFLOW can be represented as a soil layer (specific soil ID) with high infiltration, suitable porosity, and high hydraulic conductivity in both horizontal and vertical directions.

A global soil thickness value can define the depth of this layer relative to the surface elevation. Similarly, a global soil base elevation value beneath the railway ballast area can set the absolute elevation of its bottom. Both parameters can also be varied spatially with GIS and/or grid layers.