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

# 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.

#* 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: