DRAFT Direct Rainfall (Rain on Grid) Modelling Guidance

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Direct Rainfall

Rainfall depth can be applied directly to 2D cells across the entire catchment to simulate catchment runoff. This direct rainfall approach enables the integration of catchment hydrology and hydraulic modelling within a single framework.

Comprehensive detail is provided in the Rainfall Boundaries section of the TUFLOW Manual, including input data requirements, available modelling approaches, methods for rainfall losses and adjustments, and other key considerations for producing a fit-for-purpose model.

Direct rainfall can also be applied as a Source-Area Boundaries with the 'RF' rainfall option. See the FAQs below for a guide to the how Source-Area RF boundaries differ from Rainfall Boundaries.

Common Considerations for Model Configuration

The following list is not exhaustive, but provides common considerations for a basic direct rainfall model. See the TUFLOW Manual for further detail.

  • Rainfall Data:
    • Inputs are specified in simulation time (hours) versus depth (mm) (or time versus inches if using Units == US Customary).
    • 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.
  • Solver:
    • It is highly recommended to use the TULFOW HPC Solver with sub-grid sampling (SGS).
  • Cell Wet/Dry Depth:
    • It is recommended to consider reducing Cell Wet/Dry Depth from the default 0.002m (or 0.007ft) to less than a mm (e.g. 0.0002m or 0.0007ft), due to substantial sheet flow. This applies for both TUFLOW Classic and HPC solution schemes, unless using sub-grid sampling (SGS). sensitivity test.
    • If SGS is turned on, a reduced cell wet/dry depth is not necessary because the cell wet/dry calculation conducted on the SGS storage curve captures a greater change in depth. In this case, the default of 0.002m (0.007ft) is likely sufficient, however sensitivity testing and review is recommended.
  • TUFLOW Executable (Model Precision):
    • For TUFLOW HPC, the single precision version of TUFLOW (iSP) is likely sufficient for direct rainfall models. The double precision version of TUFLOW (iDP) may still be required particularly for simulations with very small rainfall or evaporation boundary rates (particularly where those boundaries may be applied over very deep water), long term simulations, fine scale processes such as groundwater, or 1D-2D linked models at high elevations.
    • For TUFLOW Classic, the double precision version of TUFLOW (iDP) is likely required for direct rainfall models.
    • It is recommended to sensitivity test running the model with both single and double precision. If the results are unacceptably different or if the mass error is significantly lower with double precision, then double precision should be used.
    • Note that the choice of single or double precision also impacts simulation times and memory allocation. Compared to single precision (iSP), double precision (iDP) versions:
  • Hydraulic Roughness:
    • It is highly recommended to adjust the Manning’s n values to calibrate the hydraulic response of catchments. Application of depth varying roughness or “Law of the Wall” approach can be also considered to take into account of the high roughness at shallow depth and/or to counter the overestimation of the pressure gradient in a direct rainfall model.
  • Map outputs:
    • Activate the following Mao Output Data Types, as applicable: RFR, RFC, RFML, CI and IR.
    • Apply Map Cutoff Depth and Map Cutoff Vector to facilitate fit-for-purpose mapping that limits the display of user defined shallow depths (does not affect hydraulic computations).

Useful Resources

Tutorial Models

TUFLOW Example Models

Other Guidance

The following links provide specific guidance on a number of features related to direct rainfall modelling:

Webinars

Publications and media

Common Questions Answered (FAQ)

How does applying direct rainfall differ between using 2d_rf and 2d_sa_rf GIS layers?

2d_sa_rf (Source-Area Boundary with 'RF' option) 2d_rf (Rainfall Boundary)
Paired command Read GIS SA RF (Source-Area Boundary with 'RF' option) Read GIS RF (Rainfall Polygons approach)

or Read GIS RF Polygons (TUFLOW Rainfall Control File .trfc with Polygons approach)

Linked boundary type Rainfall hyetograph Rainfall hyetograph
Catchment area User specified in the “Catchment_Area” attribute Utilises digitised polygon extent (i.e. no direct user specification of catchment area)
Application of rainfall to 2D cells Rainfall depths are converted to flows based on catchment area, initial loss and continuing loss information specified in the GIS layer’s attributes before application to the 2D cells via the default Source-Area methodology (lowest cell first followed by all currently wet cells).

Utilising the additional ALL option with the command (Read GIS SA RF ALL) will more closely match the application methodology of 2d_rf layers (i.e. applies to all cells within polygon).

Applies a spatially uniform rainfall rate to every active cell within the digitised polygon, after first removing any rainfall loss depths and converting to flow.
Losses Applies attribute-specified losses only.

Losses specified in the Materials Definition file are not applied. Losses are not applied to negative rainfall values (e.g. if modelling evapotranspiration).

Applies losses defined via the Materials Definition file.

Losses are not applied to negative rainfall values (e.g. if modelling evapotranspiration).

Overlapping polygons Source terms are applied cumulatively on a cell-by-cell basis, regardless of if they have the same or unique boundary "Name" attributes. For Read GIS RF command, source terms are applied cumulatively if they have unique Name attributes, whereas only the first is used if they have the same "Name" attribute

For Read GIS RF Polygons command, only the last overlapping polygon is used regardless of if they have the same or unique boundary "Name" attributes

Rainfall Boundary Factor Applicable Applicable

What is the difference between rainfall excess and soil infiltration?

Rainfall Excess Approach:

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). It applies to:

  • Rainfall Boundaries:
    • Global Losses (defined (along with Global Rainfall) in the TUFLOW Boundary Control File .tbc);
    • Materials-based losses (defined in the materials file); and
    • SCS Curve Number soils-based losses (defined in the TUFLOW Soils File .tsoilf).
  • Source-Area Boundaries:
    • attribute-specified losses with the 'RF' rainfall option (defined in the 2d_sa_rf layer).

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.
  • For Rainfall Boundaries, the rainfall excess depth (units = m) is taken and converted to a rainfall volume (m3) on a cell-by-cell basis. @Shuang- check?
  • For Source-Area Boundaries with the 'RF' option, 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. Also specifying the 'ALL' option will instead distribute the rainfall excess flow across all 2D cells within the GIS polygon.


Soil Infiltration Approach:

This approach is a more realistic representation of the actual physics associated with water infiltration into the soil. It applies to soil infiltration configured with the TUFLOW Soils File (.tsoilf).

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 digitised rainfall boundary area.
  • 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 over the area of the building footprint, with the application of a lower value (n = 0.02) at shallow depths (d < 0.03m) and a higher value (n > 0.3) at a more significant depths (d > 0.1m):
    • This is a very common and easy to implement option. The low Manning's n value aims to mimic the quick runoff response associated with drainage from the roof. The higher Manning's n value 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 (e.g. 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 which can slow the simulation speed. Also, depending on the Map Cutoff Depth assumptions, 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 issues:
      • Retain the model design described above, with subsequent post-processing of the mapped results before reporting. Either, delete the model result where there is overlap with the building footprint, or overlay the 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 under-estimate 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 represent the rainfall that would be falling on the building using Read GIS SA RF inflow boundaries. To do this, digitise a 2d_sa_rf polygon for each building (with a buffer of one of more 2D cells) where the building footprint has been excluded from the direct rainfall region. The 2d_sa_rf input will convert the input rainfall hyetograph to flow, deposited initially on the lowest 2D cell, then for subsequent timesteps distributed over all wet cells, within the 2d_sa_rf regions (i.e. on the ground surrounding the building). Refer to TUFLOW Example model EG03_014.tcf for a demonstration of this inflow boundary condition configuration.
      • Exclude buildings from the rainfall polygon and represent the rainfall that would fall on the building using Read GIS SA RF PITS inflow boundaries. This approach is similar to the previous method, although instead of directing the inflow to the ground surrounding the building, it is directed into the sub-surface drainage (underground pipe) system. To implement this approach every 2d_sa_rf polygon must encompass at least one 1D pit. If multiple 1D pits are within a single 2d_sa_rf region, the flow from the polygon is split equally between the 1D pits. The pits can be snapped to a 1D node, connected via the pit-search distance or via an x-connector, to allow captured by the pit to enter a 1D representation of the sub-surface drainage system. Refer to TUFLOW Example model EG15_000.tcf for a demonstration of this inflow boundary condition option.
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