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Direct Rainfall (Rain on Grid) Modelling Guidance: Difference between revisions

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# 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.
# 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 undesirable results. Those being, water falling from the building roof to the ground can result in the need for a reduced model timestep to maintain model stability. This can slow the simulation speed. Also, depending on the Map cutoff Depth specification, 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 mitigate these issue:
::::* Retain the model design described above, though post process the results before reporting, by either deleting the model result where there is overlap with the building foot print, or overlaying building footprint polygon objects over/above the result dataset in the GIS Map Layout (hiding the 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 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.
#:::* TheExclude cut out rainfallbuildings from the buildingsrainfall canpolygon be includedand insuppliment the modelexclusion againusing with <font color="blue"><tt>Read GIS SA RF</tt></font> command.inflow boundaries. Digitise a small 2d_sa_rf polygon for each building on(with thea groundbuffer of one of more cell length) where runoffthe building footprint has been excluded from the buildingdirect israinfall expectedregion. andThe assignsa_rf attributesinput thatwill convert the input rainfall hyetograph into flow andvolume, inputdepositing itthe intowater to the modellowest as2D ifcell itwithni wasthe 2d_sa2d_sa_rf regions (ie. Checkon the ground surrounding the house). Refer to TUFLOW <u>[[TUFLOW_Example_Models#Boundary_Condition_Options | example model EG03_005.tcf]]</u> onfor howa todemonstration setupof 2d_sa_rfthis inflow boundary condition option.▼
::::* 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 <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.
#* 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.
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