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

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.

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) at a more significant depths (d > 0.1m) to the building footprint:

#* 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. Thiswhich 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:

:::* Retain the model design described above, thoughwith subsequent post-processing processof the mapped results before reporting. Either, delete the model result where there is overlap with the building footprint, or overlay 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. This approach will underestimateunder-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 supplementrepresent the exclusionrainfall 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 intoto 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 housebuilding). Refer to TUFLOW <u>[[TUFLOW_Example_Models#Boundary_Condition_Options | exampleExample model EG03_005EG03_014.tcf]]</u> for a demonstration of this inflow boundary condition optionconfiguration.

:::* Exclude buildings from the rainfall polygon and supplementrepresent the exclusionrainfall 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, thoughalthough 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_sa2d_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.