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IL/CL losses - rainfall and infiltration

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Q:

I see that IL/CL losses can be specified in either the materials layer or the soil layer.  Are you able to explain the differences between these two approaches?

 

A:

The infiltration losses (specified in the soil file) and rainfall losses (specified in the materials file) are applied separately.  The infiltration losses (using either a Green-Ampt or IL/CL approach) will infiltrate ponded water into the ground, rainfall losses will remove the loss depth from the rainfall before it is applied as a boundary on the 2D cells. 

 

Note that the infiltration IL/CL is totally separate to the rainfall IL/CL losses (used to generate excess rainfall for direct rainfall simulations).

It is possible to use both methods in the same simulation – for example, rainfall that doesn’t reach the ground, such as interception by trees would be modelled as a material IL (applied as a loss to the rainfall) and infiltration into the ground as IL/CL via soil types.

 

Specifying the “fraction impervious” on the material allows the materials and the soils to be independent, i.e. the same soil can be present under both road and forest.  However, this fraction impervious only applies to the infiltration into the soil and not to the rainfall losses.

 

In the log file you will see the material and soil properties reported separately:

Example Material Properties

#4 - Material 4:

Fixed Manning's n = 0.030

IL = 1.0mm,  CL = 0.0mm/h

Landuse Hazard ID not set.

SRF (Storage Reduction Factor) = 0.

Fraction Impervious = 0.

 

Example Soil Properties

#1 - Soil 1 [based on pre-defined soil type SAND]:

Suction = 49.5 mm

HydCond = 117.8 mm/hr

Porosity = 0.417

Initial Moisture = 0.2

Soil Capacity = 0.217

 

 

If specifying infiltration losses these can be checked with the infiltration outputs (CI and IR).  The _grd_check.mif will also contain the spatial distribution of soil type as read by TUFLOW.

 

Hopefully this clarifies the way the losses are applied.

 

Regards

TUFLOW Support Team

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Hi,

 

I am doing some direct rainfall models for study flood from surface water. I'm using the new infiltration feature with Green&Ampt method. The models are highly sensitive to the way of representing the infiltration (fraction impervious value of the overlying material layer, soil type, initial moisture...).

 

Would you have some recommendation/advice about these parameters?

 

More precisely, what "fraction impervious value" would you recommend for grass (short/long)? 100% pervious doesn't seem realistic to me.

 

Thank you very much in advance for your help.

 

Regards

Gael

 

 

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Hi,

I am undertaking modelling for a catchment with majority of the area being sandy soil. Would it be more appropriate to model with continuing loss (subtract from rainfall) as well as a soils layer with high infiltration rate (remove water in the hydraulic model)? Or do i just use one or the other?

Thanks.

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Hi,

The soil infiltration and rainfall losses can be used in the same simulation.

Rainfall that doesn’t reach the ground, such as through the interception by trees, is best represented as a rainfall loss. This would remove the loss depth from the rainfall before it is applied in the whole model.

The soil infiltration will then be applied only to the wet cells based on the soil type.

Regards,

Pavlina

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Hi everyone,

I am dealing with a similar situation. I am currently carrying out a surface water hydraulic modelling, but due to the huge size of the upstream catchment, I am forced to use two inflow boundary conditions (i.e. 2d_bc QT and 2d_rf).

In order to avoid modelling the entire catchment, I established the 2d_bc (QT) associated with the upstream catchment hydrographs based on the FEH method (from Flood Modeller) and catchment descriptors from FEH website (https://fehweb.ceh.ac.uk/).

On the other hand, a second boundary condition (2d_rf) has been established for rainfall hyetographs for my ‘active’ catchment (2d_code). These hyetographs were created using the ReFH method (also from Flood Modeller), which calculates the ‘loss model’ and then extracts the ‘net rainfall’.

However, in order to consider infiltration within the model, the ‘Standard Percentage of Runoff’ (SPR) of the FEH catchment descriptors would consider the infiltration for both the upstream catchment hydrographs and the hyetographs within the 2d_code area, is that correct?

My questions are:

Would the ‘Standard Percentage of Runoff’ (SPR) of the FEH catchment descriptors would be enough to consider the infiltration in the upstream catchment hydrographs?

Regarding 2d_rf, does the ‘loss model’ from ReFH method already consider the infiltration? Or should I apply an additional method?

Many thanks

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Hi,  

The FEH rainfall-runoff model does consider infiltration losses both in the development of runoff (direct/total runoff and baseflow), as in the case of the upstream 2d_bc boundary, and the development of net rainfall, as in the case of the 2d_rf boundary.  Therefore, if you're using runoff or net rainfall from FEH/ReFH and assuming your FEH/ReFH model is calibrated appropriately, you shouldn't have to also apply additional infiltration.  To do risks double-counting the losses.

Duncan

 

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