How to define new initial conditions for a 1D river network
  • 03 Aug 2022
  • 3 Minutes to read
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How to define new initial conditions for a 1D river network

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Article Summary

Initial values of flow and stage are required at every node before commencing an unsteady or steady time stepping simulation. Failure to supply adequate values of these may cause the model to fail or lead to initial instabilities in the simulation. Additionally, it may be required to specify Unit Mode and Unit State for some structures, for example sluices. The latter (for example when setting the initial gate opening for a Vertical Sluice) is particularly important when starting a model with the gate operating in LOGICAL mode. Initial values of other 1D model variables, for example velocity, Froude Number and thalweg (lowest bed elevation) may be supplied, although these have no impact on the model solution.

This section explains how to define initial conditions to a 1D model network. The initial conditions of a network can be defined within the Network file (DAT) or in a separate file (ZZS or IIC). When a Network file has initial conditions included these are displayed in the Network panel, in the window located below the network chart, separated by a splitter bar

You have several alternatives to set of modify initial conditions for a 1D model:

  1. Define manually the initial conditions to your 1D network – You can directly type values in the Initial conditions window for each of the units in your 1D network. Usually stage and flow values are sufficient to start the 1D model simulation.

  2. Perform a steady simulation - Steady run is used to provide the initial conditions for an unsteady run (this may not be possible for Muskingum/Muskingum-Cunge nodes). The new values of flows and stages are written to the ZZS file. These steady results can then be used as the initial conditions for an unsteady run if desired.

  3. Import an external file – you could make a copy of steady results file (ZZS) and then edit this outside of Flood Modeller (in any text editor). The revised file can then be added to your project by right-clicking on the Simulations section in the Project panel and selecting Add Item from the displayed menu. You will be prompted to browse to your initial conditions file.

  4. Create snapshots during a simulation – the advanced options in the 1D simulation interface allow you to specify one or multiple snapshots of current model conditions to be exported to separate files at user defined times within the simulation. These data are written to separate IIC files, which are the same text format as a ZZS initial conditions file. These can then be used as initial conditions for subsequent simulations, e.g. the conditions at the end of a forecasting simulation become the start point for the next forecast simulation. You can also utilise the TabularCSV tool to generate initial conditions data from 1D models.

Useful tip: The initial conditions from the ZZS file can be imported to the 1D network file to become the new default initial conditions within the DAT file.

Specify the source of the initial conditions file in the 1D simulation file – In the Files tab you must select the initial conditions to be used in your simulation. The options available are to use initial conditions that have been manually typed into the network file ( for this the ‘Network File’ option must be selected) or to use any external file such as .zzc file which will contain the results of the steady run or .iic file which will contain a snapshot created during a simulation (for this you should select ‘Initial Conditions File'). Then either drag a file from your Project panel into the box provided in the 1D simulation interface or browse to the location of the file to select it


Useful tip: When you have a complex channel, you can improve the initial conditions of your model running an unsteady simulation where the flows varies toward a constant model base flow. Once the simulation is complete you can use the Tabular CSV tool to extract initial conditions from the model results, and then use the generated IIC or ZZS file in your 1D simulation file.

More about Initial Conditions

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