8.8 Initial Conditions

Two options are available for setting initial conditions in the model. Initial Water Levels (IWLs) can be defined in the 1D and 2D domains to set a global constant or spatially-varying water level. Alternatively, the use of restart files sets initial water levels, flow velocities and flow regime based on a previous simulation of the model.

8.8.1 Initial Water Levels (IWL)

8.8.1.1 1D Domains

For 1D domains, initial water levels (IWL) can be set globally as a constant using the .ecf command Set IWL. IWLs can also be set to vary spatially using one or more GIS layers. The default initial water level at 1D nodes is zero metres above datum (0 mAD).

To set up a GIS IWL layer for the 1D domains:

Create a 1d_iwl layer using an empty layer created by Write Empty GIS Files.
Digitise points snapped to nodes and assign each point an initial water level value, alternatively points can be copied from the _nwk_N_check files and assigned an IWL.
Save the GIS layer.
Use the Read GIS IWL command to read in the IWL values.

Any number of IWL layers may be used, noting that if a node’s IWL occurs more than once, the last occurrence prevails (i.e. TUFLOW or ESTRY overwrites any previous IWL already set).

Differences in initial water levels for related features in the 1D and 2D domain can cause model instabilities at the start of a model simulation. If your model becomes unstable in the first few timesteps, review the initial water level values to ensure they are consistent in both domains and also match the water level of any head boundaries that they are be connected to.

Table 8.16: **1D Initial Water Level (1d_iwl) Attribute Descriptions**
No	Default GIS Attribute Name	Description	Type
1	IWL	Initial water level of object relative to model datum (m).	Float

8.8.1.2 2D Domains

2D IWLs can be set globally as a constant using the Set IWL (.tcf file) or Set IWL (.tgc file) commands. The default initial water level is zero metres above the model datum (or 0ft if Units == US Customary). IWLs can also vary spatially using one or more GIS and/or grid layers. The (Read GIS IWL can be read into both the .tcf or the .tgc, shown in Example Model EG06_001. The Read Grid IWL) can be read into the .tgc file, shown in Example Model EG06_002. This is particularly useful for setting initial water levels in lakes, dams, etc.

The easiest way to set up a GIS IWL layer is to:

Create a 2d_iwl layer using an empty layer created by Write Empty GIS Files.
Digitise regions or points and assign each object an initial water level value.
Save the GIS layer.
Use the .tcf Read GIS IWL command or tgc Read GIS IWL command to read in the IWL values.

The Read Grid IWL .tgc command is used to read the IWL from a grid (.tif, .flt or .asc). This can be used to set the IWL based on the water level in a previous simulation, if TIF, FLT or ASC is specified in the Map Output Format. The results from one simulation can be directly read by another.

Any number of IWL layers may be used, noting that if a cell’s IWL occurs more than once, the last occurrence prevails (i.e. TUFLOW overwrites any previous IWL already set).

Models using 2D initial water level (2d_iwl) are provided in the Boundary Condition Options Example Model Dataset on the TUFLOW Wiki.

Table 8.17: **2D Initial Water Level (2d_iwl) Attribute Descriptions**
No	Default GIS Attribute Name	Description	Type
1	IWL	Initial water level of object relative to model datum (m).	Float

8.8.1.3 Automatic Initial Water Level

It is possible to automatically set the model’s initial water level to the downstream boundary water level using Set IWL == AUTO. This command is very useful when performing a Monte Carlo assessment that requires a large number of simulations with each simulation having a unique initial water level!

8.8.2 Initial Groundwater Levels

This section discusses available commands to set initial conditions in groundwater layers. As of the 2023-03 release, when using TUFLOW HPC, it is possible to have numerous groundwater layers in the vertical (see Section 7.3.5.2). If using this functionality the initial water levels can also be set per layer by using ‘Layer N’. If using only one layer, the syntax may be either ‘Set IGW Depth’ or Set IGW Depth Layer 1’.

The initial water level in the groundwater layer(s) can be set globally in the .tgc using the following commands:

They can also be set on a spatial basis using:

“IGW Depth” is assumed to be the depth of water in the soil (water content divided by porosity). “IGW Level” is assumed to be the level of the water table in each layer. If both methods are specified for a given grid cell, the highest initial condition will be adopted. The input units should be in meters or feet (if using Units == US Customary). Note, the initial conditions do not automatically cascade into layers below (i.e. setting the initial water level in the top layer will not automatically set the layers below to be 100% saturated).

Setting the initial conditions in the .tgc for any given grid cell will override the “Initial Moisture” parameter set in the .tsoilf. The difference between the methods is that the .tsoilf sets the initial moisture by soil type, whereas setting the initial conditions in the .tgc allows spatial distribution. If no initial conditions are set in the .tgc for a given grid cell, the initial conditions will be determined by the “Initial Moisture” defined in the .tsoilf.

In previous releases, the groundwater initial conditions could be set using the following commands. For backward compatibility, it is still possible to use these commands, though only if using one groundwater (soil) layer in the vertical:

If the above commands are used with the multiple vertical layers (Convective layers), ERROR 2588 will be produced.

The groundwater feature may only be used in conjunction with at least one of the soils infiltration methods described in Section 7.2.7. Consequently, the .tgc commands used to define the groundwater depth / level must be read in following (below) at least one of the commands used to define the soil type: Set Soil, Read GIS Soil or Read Grid Soil.

Table 8.18: **2D Groundwater (2d_gw) GIS Attribute Descriptions**
No.	Default GIS Attribute Name	Description	Type
1	Groundwater	The ground water level in metres above datum (if using the command Read GIS GWL) or depth below ground surface in metres (if using the command Read GIS GWD) of the groundwater table at the start of the model simulation. The units are feet if using Units == US Customary.	Float

8.8.3 Restart Files

Gradually varying water surface initial water level conditions are best defined using a restart file. This is achieved by running the model for a warm-up period prior to the main event to create a restart file which establishes the initial water levels and flow velocities. This is achieved using the following commands:

Write Restart File at Time: Sets when to write the restart file in hours.
Write Restart File Interval: Sets the interval in hours between writing the restart file.
Read Restart File: Reads a restart file written from a previous simulation.
Write Restart Filename: Controls whether restart files are overwritten or are time-stamped.

The 1D and 2D restart files use a .erf and .trf extension respectively and are written to the results folder.

The restart file contains information on the water levels, velocities and flow regimes for the 1D and 2D parts of the model. The number of 2D cells and 1D channels must be the same between the original model and the model using the restart file. Permitted changes between runs could include different boundary conditions, cell elevation increases (cell elevation decreases may cause issue) and roughness values.

Models using write and read restart files are provided in the Output Options and Boundary Condition Options Example Model Datasets on the TUFLOW Wiki.

The 2023-03-AD build added functionality to allow restart data fields to be ignored. To specify which fields to ignore the command “Restart File Ignore Fields ==” can be set in the .tcf where the fields are any combination of:

Geometry (elevations and Manning’s n values)
Groundwater (groundwater depths and groundwater tracer concentrations)
Maximum (scalar and vector maximums; both values and times, minimum timestep)
Rainfall (cumulative rainfall, cumulative rainfall material losses and cumulative cell wet time)
TimeOutputCutoff (Time Output Cutoff; time of first inundation and cumulative time inundated)
Tracer (surface and groundwater tracer concentrations)
Velocity (velocity and turbulence values)
WaterLevel (water level)

For example: “Restart File Ignore Fields == Geometry Groundwater” will ignore the geometry and groundwater information in the restart file (for these, the values set in the control files are used).

By default (without the command above) the geometry information in the restart file is ignored and the .tgc information is used instead. See also HPC Restart Geometry == Restart File | {TGC} command. If the “Restart File Ignore Fields ==” command is included, but does not include geometry then message “WARNING 2902 -‘Restart File Ignore Field’ does not include geometry, overwriting geometry in .tgc with restart file” is issued.

Note: Only one of “Restart File Ignore Fields” or “HPC Restart Geometry” should be used, an error is given if both commands are present in a control file.