Direct Rainfall (Rain on Grid) Modelling Guidance

Direct Rainfall

Please see Direct Rainfall (Rain on Grid) Webinar.

Rainfall Losses and Soil Infiltration

TUFLOW supports a range of rainfall loss and soil infiltration options. These are split into two broad categories, defined by their respective calculation methods:

• Rainfall Excess Losses
• Soil infiltration Losses

The fundamental difference in calculation approach is described in the below Common Questions Answered (FAQ) section.

Rainfall Excess losses can be implement in a variety of ways, including

1. Loss values specified within the TCF Read Materials File command, linking to TGC Read GIS Mat inputs;
2. Loss values specified within the GIS layers associated with the TBC Read GIS SA RF command; or
3. Global Rainfall Initial Loss and Global Rainfall Continuing Loss TBC commands.

Similarly, a variety of soil infiltration options are supported. The available options include, Initial / Continuing Infiltration, the Horton Infiltration method and the Green-Ampt Infiltration method. The following link provides some future discussion on the Green-Ampt method.

• Green-Ampt Infiltration Parameters

Common Questions Answered (FAQ)

What is the difference between rainfall excess and soil infiltration?

Rainfall Excess Approach - Read Materials File and Read GIS SA RF continuing loss specification:

• This initial and continuing loss approach is a simplistic calculation method comparable to the loss methods included in traditional hydrology models (e.g. RORB, URBS, WBNM etc).
• The calculation approach is as follows:

• The user defines the rainfall hyetograph (time vs depth (mm)) boundary condition inputs.
• The rainfall value is reduced by the loss value (i.e. rainfall excess) before the boundary condition input is applied to the 2D cells.
• The GIS SA RF takes the rainfall excess (units = m) and multiples the value by the Area attribute (units = m2) in the GIS object to convert rainfall depth to a volume (m3).
• TUFLOW applies the calculated rainfall excess flow to the lowest cell in each GIS polygon during the first timestep that wetting occurs. Every timestep thereafter the inflow is distributed over the wet cells within the polygon.

Soil Infiltration Approach - TUFLOW Soils File (.tsoilf):

• This approach is a more realistic representation of the actual physics associated with water infiltration into the soil.
• The calculation approach is as follows:

• The user defines the rainfall hyetograph (time vs depth (mm)) boundary condition inputs.
• The total rainfall value is applied directly to every 2D cell within the 2d_rf polygon.
• When a 2D cell is wet the soil infiltration function subtracts the appropriate loss volume of water from it. Computationally this is referred to as a “Sink” term.

What is the best approach for modelling buildings in rain on grid model?

There are number of common ways to model buildings in direct rainfall model and the choice depends on the flood modelling task. All have their advantages and disadvantages:

• Represent buildings as high Manning's n:
  • This is a very common and quick to setup approach that significantly slows down water passing through the buildings.
• Raise elevation with topographic updates:
  • Raising elevation eliminates water passing through the buildings, however as rainfall is assigned to all active cells, it will be falling on top of the buildings and from there to the ground.
  • With bigger rainfall, this might create a waterfall like output around the buildings.
• Exclude buildings from rainfall polygon:
  • This removes the rainfall from the model that supposed to fall on the buildings and underestimates the amount of rainfall entering the model.
  • If the area of the buildings is negligible in comparison to the full model, it might be acceptable to leave the rainfall out for simplicity.
• Exclude buildings from rainfall polygon and include it with 2d_sa_rf:
  • The cut out rainfall from the buildings can be included in the model again with Read GIS SA RF command. Digitise a small 2d_sa_rf polygon for each building on the ground where runoff from the building is expected and assign attributes that convert rainfall hyetograph into flow and input it into the model as if it was 2d_sa. Check TUFLOW example model EG03_005.tcf on how to setup 2d_sa_rf.
  • Most rainfall that falls on buildings collects on the roof and through gutters and downpipes finds its way directly into the sub-surface drainage system. In such case, Read GIS SA RF PITS command can be used to feed the water directly into the pits. Every 2d_sa_rf polygon needs to have at least one 1D pit within. If multiple pits are present, the flow from the polygon splits equally.

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