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Found 2 results

  1. Q: 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? A: 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. Thanks, Rachel
  2. Q: What is the direct rainfall approach and what is the difference between the 2d_rf and 2d_sa_rf layers? A: For both the direct rainfall and the SA RF approach the specified boundary is a rainfall hyetograph. The direct rainfall approach applies the rainfall hyetograph to active cells with the region. For the SA RF the rainfall hyetograph is converted to a flow (in m3/s or ft3/s using the area attributers in the GIS layer, this is described further below. For both types the input hyetograph must be in mm (or inches) versus hours. 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. Each value represent the rainfall that falls per increment, for example in the table below between 0.0833 (5 minutes) and 0.1667 (10 minutes) a rainfall depth of 2.5mm is applied, as opposed to a cumulative depth or rainfall rate. A total of 16.5mm of rainfall is applied over a 30min period in the hyetograph below. Time Rainfall 0.0000 0.0 0.0833 0.0 0.1667 2.5 0.2500 5.4 0.3333 3.5 0.4167 5.1 0.5000 0.0 The rainfall (2d_rf) layer applies the rainfall hyetograph to all the active 2D cells, the attributes are: · Rainfall name · factor 1 · factor 2 Both of the factors are multipliers and can be used to modify the boundary. E.g. an f1 might be an area reduction factor and f2 a climate change factor. These are multiplication factors and should be set to 1 if no change is desired. If these are set to 0 no rainfall will be applied and you will get a warning to tell you that this has occurred. For the direct rainfall approach the losses can be specified in either the materials database (the can be varied based on the land-use or 2d_mat layer) or via a soils / infiltration layer. Note: using the double precision build of TUFLOW is recommended for direct rainfall models to ensure accuracy in small flows. The sa rf layer (2d_sa_rf) also uses a rainfall hyetograph, but the required attributes are: · Name · Catchment_Area · Rain_Gauge_Factor · IL · CL The catchment area, factor, IL and CL are used to convert the rainfall, to an inflow boundary (e.g. time - flow). This is then applied to the 2D cells using the source area inflow type (2d_sa). For the SA inflow the default approach (for the 2013 version) is to direct the flow initially to the lowest cell within the polygon and when multiple cells are wet the flow is then spread between wet cells with the flow being proportioned by the cell depth. To turn off the proportioning to depth use the .tcf file command: SA Proportion to Depth == OFF A minimum depth can also be set, below this depth the cell will receive no inflow: SA Minimum Depth == <minimum depth> Regards TUFLOW Support team
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