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

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# 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 require a reduced model timestep to maintain model stability. This can slow the simulation speed. Also, depending on the Map Cutoff Depth assumptionsnassumptions, 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 issueissues:
:::* Retain the model design described above, though post process the results before reporting. Either delete the model result where there is overlap with the building footprint, or overlay 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. This approach 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.
:::* Exclude buildings from the rainfall polygon and supplimentsupplement the exclusion using <font color="blue"><tt>Read GIS SA RF</tt></font> inflow boundaries. 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 sa_rf2d_sa_rf input will convert the input rainfall hyetograph into flow, deposited initially on the lowest 2D cell, then distributed over all wet cells, within the 2d_sa_rf regions (ie. on the ground surrounding the house). Refer to TUFLOW <u>[[TUFLOW_Example_Models#Boundary_Condition_Options | example model EG03_005.tcf]]</u> for a demonstration of this inflow boundary condition option.
:::* Exclude buildings from the rainfall polygon and supplimentsupplement the exclusion using <font color="blue"><tt>Read GIS SA RF PITS</tt></font> inflow boundaries. This approach is similar to the previous method, though 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_rf_sa region, the flow from the polygon is split equally between the 1D pits.
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