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Initial Loss on Direct Rainfall boundaries

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In the tmf file the Tuflow manual says that: The third and fourth numbers are optional and set the initial and continuing loss rates in mm and mm/h if using Read GIS RF.
My understanding was that the 10mm in 3rd column of tmf will impact only the first non-zero value of the hyetograph. And if this is less than 10mm then only that first step will be removed (changed to 0) and that’s the end of initial loss story – the rest of the hyetograph will not be impacted by this 3rd column.

Does Tuflow keep removing the rainfall until the 10mm loss is accumulated or does it just take 10mm from the first step and leave the rest alone?



Good question!

Tuflow takes the initial loss off the cumulative rainfall applied in your 2d_RF layer. So, it’s not just that first time step

Firstly, Tuflow coverts the hyetograph to a flow to be applied per cell. You can see the gross values of this conversion in your .tlf (and below figure). This is before any removal of losses.

The losses are then removed from the cumulative total on the cell, so that a total of (10mm* Cell area) of volume will be removed, using up those initial timesteps until this volume per cell is reached.
Only after the losses are removed, the flow is applied to the cells (ie: the rainfall isn’t applied then losses are deducted after that.)

Using your data I’ve made a test case below.
In the second figure, the dark red dashes are the m3/s per cell that tuflow is applying as a part of the 2d_RF layer without losses (as reported in your .tlf) and the dark blue dashes are the m3/s per cell with losses. I’ve back calculated this from your information.
You can see the loss volume in the difference at the start. The tail ends of both lines are the same as the loss has been removed.

This is also confirmed in looking at the depth in the model at a cell, refer to the third figure.

Thus, you can see that more than your initial timestep is used by the initial loss.

At the moment, it is not clear to see what is exactly being applied after losses.
However, in the forthcoming 2015 release there are new outputs for checking the rate and cumulative rainfall applied to each cell.
So in the future, this kind of question will be more easily checked and visualised.

I hope this helps, please let me know if you require more information.










Edited by rachel.jensen

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