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 numerous different industry standard ways to represent buildings in a direct rainfall model. Australian Rainfall and Runoff Guideline, Project 15 (Representation of buildings in 2D Numerical Flood Models) discusses some of the available options. Common TUFLOW modelling approaches are summarised below:

1. Using depth varying Manning's n, apply a low value (n = 0.02) at shallow depth ( d < 0.03m) and a high value (>0.3) to the building footprint:
   • This is a very common and easy to implement option. The low Manning's n aims to mimic the quick runoff response associated with drainage from the roof. The higher Manning's n aims to represent the losses associated with deeper floodwater impacting the side of the building.
2. Raise the building footprint elevation using TUFLOW's topographic update features (eg. Read GIS Z Shape):
   • Raising the model topography creates an obstruction to flow. It prevents floodwater from passing through buildings (as is the case with the Manning's n approach)
   • The application of rainfall on top of the building can however produce some undesirable results that require further consideration. Those being, water falling from the building roof to the ground can require a reduced model timestep to maintain model stability. This can slow the simulation speed. Also, depending on the Map Cutoff Depth assumptionsn, water may be present in the results on the building rooftops. This may not be desired for mapping purposes. There are a variety of options available to resolve these issue:

• Retain the model design described above, though post process the results before reporting. Either delete the model result where there is overlap with the building footprint, or overlay building footprint polygon objects over/above the result dataset in the GIS Map Layout (hiding the flood model result within the building footprints).
• Exclude buildings from the rainfall polygon: This removes the rainfall from the model that would otherwise fall on the buildings. This approach will underestimate the amount of rainfall entering the model. If the collective building footprint area is negligible in comparison to the entire model, this approach may be acceptable.
• Exclude buildings from the rainfall polygon and suppliment the exclusion using font color="blue">Read GIS SA RF inflow boundaries. Digitise a 2d_sa_rf polygon for each building (with a buffer of one of more cell length) where the building footprint has been excluded from the direct rainfall region. The sa_rf input will convert the input rainfall hyetograph into flow volume, depositing the water to the lowest 2D cell withni the 2d_sa_rf regions (ie. on the ground surrounding the house). Refer to TUFLOW example model EG03_005.tcf for a demonstration of this inflow boundary condition option.

1. • 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 to automatically deposit the flow. If multiple pits are present, the flow from the polygon splits equally.

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