HPC Impact Assessment Guidance: Difference between revisions
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When completing an impact assessment, the modeller should endeavour to compare “apples with apples”. This means attempting to keep everything in the model exactly the same, apart from the development features. Because the HPC solver implements adaptive timestepping, the implementation of new features into the TUFLOW model for a development design case
Though this has not been an issue to date and while it is not expected to produce any artificial impacts, there are some steps that can be taken to ensure that observed impacts are a result of the development features and not a result of change in the model timestep.<br>
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If the modeller suspects the model is producing artificial impacts, the base case and design case models should be run with a reduced Control Number Factor. To reduce the likelihood of varying timesteps in the base case and design case model runs, applying a CN factor of 0.8 is recommended for all model runs in a Flood Impact Assessment. It would not be unreasonable to reduce this factor as low as 0.5 if the modelled results presented signs of artificially modelled impacts. To define the CN Factor the following command can be implemented in the TUFLOW Control File (TCF).<br>
<font color="blue"><tt>Control Number Factor</tt></font><font color="red"><tt> ==</tt></font><tt> 0.8 </tt><br>
If there is no difference observed in the modelled impacts, as shown below, when running the models at a CN factor of 0.8 and 0.5, then it is unlikely that the timestep is the cause of any artificial impacts.
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[[File: CN Flood Impacts.png |600px]]
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