Direct Rainfall (Rain on Grid) Modelling Guidance: Difference between revisions
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= Direct Rainfall =
Please see <u>[https://www.tuflow.com/library/webinars/#feb2021_direct_rainfall Direct Rainfall (Rain on Grid) Webinar]</u>.<br>
= TUFLOW Executable =
* The single precision version of TUFLOW is recommended for direct rainfall models using HPC.
* The double precision version of TUFLOW is recommended for direct rainfall models using TUFLOW Classic.
= Rainfall Boundary Conditions=
The following links provide boundary condition guidance for applying direct rainfall:
* [[TUFLOW Rainfall Control File Examples]]
* [[TUFLOW NetCDF Rainfall Format]]
= Rainfall Losses and Soil Infiltration=
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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
* <u>[[Green-Ampt Infiltration Parameters | Green-Ampt Infiltration Parameters]]</u>
= Common Questions Answered (FAQ)=
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#* 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
:::* 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 <font color="blue"><tt>Read GIS SA RF</tt></font> 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 (ie. on the ground surrounding the building). Refer to TUFLOW <u>[[TUFLOW_Example_Models#Boundary_Condition_Options | Example model
:::* Exclude buildings from the rainfall polygon and represent the rainfall that would fall on the building using <font color="blue"><tt>Read GIS SA RF PITS</tt></font> 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
== What is the recommended cell wet/dry depth for direct rainfall models? ==
For models with SGS turned off, a reduced cell wet/dry depth of 0.0002m (0.0007ft) is recommended due to the substantial amount of shallow sheet flow. <br>
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 the case, the default of 0.002m (0.007ft) may be sufficient; however, it should be reviewed and selected according to the magnitude of flooding depths.
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