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mark@mrgwater.com.au

Surge of Rainfall at Start of Simulation

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Hi all,

I am running a direct rainfall model and I am getting an instability right at the start of the run, before the rainfall actually starts. It only appears when I add some Z_SH layers to my TGC file.

Can anyone help me figure out how to stop this as it really affects my results?

I have a 2m grid and normally run on a 1 sec timestep but I have to drop it to 0.25 sec to get it running then the surge is introduced.

Please Help!

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Mark,

Where is the instability occurring? I assume that it is occuring near the areas modified by your Z_sh layer? If so, how is the Z_Sh layer modifying the default terrain in this area?

And you say that this instability occurs before any rainfall is introduced - so is there any water in your model at the time of the instability?

David

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Just some clarification here. We have had a look at the model and there are a number of things to note:

For a rainfall boundary, the default method for rainfall application is as described in the manual:

The STEPPED (default) option holds the rainfall constant for the time interval (ie. the rainfall has a histogram stair-step shape). This means, for example, the second rainfall value in the time-series is applied as a constant rainfall from the first time value to the second time value. As with all rainfall boundaries, the first and last rainfall entries should be set to zero (otherwise these rainfall values are applied as a constant rainfall if the simulation starts before or extends beyond the first and last time values in the rainfall time-series).

So for the rainfall boundary below:

Time, Rainfall

0.00 , 0

0.25 , 5

0.50 , 10

1.00 , 0

The rainfall will fall at a constant rate between time 0 and time 0.25 hours, the total depth in this period is 5mm. Between 0.25 and 0.5 hours a further 10mm will fall and after this no rainfall occurs. The method for applying rainfall can be changed see the .tcf command "Rainfall Boundaries == [ {STEPPED} | SMOOTHED | SMOOTHED TIME CENTRED ]" in the TUFLOW manual.

Therefore, the model was crashing as soon as the rainfall started, in the vicinity of the Z Shapes. There were two issues with the Z shapes:

  • A polygon to lower the cells by a specified amount was specified with a "no merge" option, this was lowering the cells to ~30m below the surrounding elevations. Changing this to "Add No Merge" fixed the issue;
  • Areas were raised to a set high level out of the flood depths. However these were in the direct rainfall area, and had rainfall applied to the top of these buildings, and a large drop at the edge of the buildings. A better approach would be to use a SA RF approach or external hydrology model, so that the flows are not directed to the top of the buildings.


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