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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>
 
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= 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 some futurefurther discussion on the Green-Ampt method.
* <u>[[Green-Ampt Infiltration Parameters | Green-Ampt Infiltration Parameters]]</u>
 
 
= Common Questions Answered (FAQ)=
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== 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. <u>[https://downloads.tuflow.com/_archive/Australian_Rainfall_Runoff_Project_15_Subproject_report_buildings_final.pdf Australian Rainfall and Runoff Guideline, Project 15 (Representation of buildingsBuildings in 2D Numerical Flood Models)]</u> discusses some of the available options. Common TUFLOW modelling approaches are summarised below:
# Using depth -varying Manning's ''n'' over the area of the building footprint, applywith the application of a lowlower value (''n'' = 0.02) at shallow depthdepths ( d < 0.03m) and a highhigher value (''n'' > 0.3) toat thea buildingmore footprintsignificant 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.
# Raise the building footprint elevation using TUFLOW's topographic update features (eg. <font color="blue"><tt>Read GIS Z Shape</tt></font>):
#* 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 result in the need forrequire a reduced model timestep to maintain model stability. Thiswhich can slow the simulation speed. Also, depending on the Map cutoffCutoff Depth specificationassumptions, 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 mitigateresolve these issueissues:
::::* Retain the model design described above, thoughwith subsequent post-processing processof the mapped results before reporting. Either, by either deletingdelete the model result where there is overlap with the building foot printfootprint, or overlayingoverlay 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. andThis underestimatesapproach 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 EG03_014.tcf]]</u> for a demonstration of this inflow boundary condition configuration.
::::* 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 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 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 <u>[[TUFLOW_Example_Models#Multiple_Domain_Model_Design | Example model EG15_000.tcf]]</u> for a demonstration of this inflow boundary condition option.
# 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 <font color="blue"><tt>Read GIS SA RF</tt></font> 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 <u>[[TUFLOW_Example_Models#Boundary_Condition_Options | example model EG03_005.tcf]]</u> on how to setup 2d_sa_rf.
== What is the recommended cell wet/dry depth for direct rainfall models? ==
#* 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, <font color="blue"><tt>Read GIS SA RF PITS</tt></font> 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.
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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