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Direct Rainfall Approach, 2d_rf versus 2d_sa_rf

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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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Hi Tuflow/Members,

 

I am using the 2d_sa_rf method to model the private lots within an urban catchment.  I want to push the flows from the lots straight out into the road reserve because I am only interested in flooding on the private lots if the flow comes from the road (ie. road/drainage system under capacity).  My only concern with this method is the timing of the flows.  Do you think it is OK to push the flows out from the lots without considering the time of concentration?  I'm running 0.5 - 2hr storm durations. 

 

Thanks,

Sophia

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