Direct Rainfall (Rain on Grid) Modelling Guidance: Difference between revisions
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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 buildings in 2D Numerical Flood Models)]</u> discusses some of the available options. Common
# 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
# 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 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
## Exclude buildings from rainfall polygon:
# 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.
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