How to use Flood Calculator when working with a static flood extent

The simplest form of a flood extent is generated by overlaying a fixed water level across the entire ground grid. Areas where the ground is below this water level will then show up wet. The 1D Flood Calculator provides the option to introduce a Static Flood Extent to represent a fixed water level instead of using a combination of a TIN and 1D model results. The selected ground grid data will then be subtracted from this plane to yield a flood extent.

The advantage of this method is that it enables a basic flood extent to be created quickly, without performing any modelling. However, the results produced should only be used as a crude initial assessment for the following reasons:

• There is no accounting of pathways that would enable water to flow to the flooded areas. Therefore the result would likely over predict flood extent.

• There is no accounting of water surface slopes that would be present on actual flood flows. Thus flood depths would not be as accurate on study areas with pronounced slopes.

To access the horizontal plane feature in Flood Calculator click on the 'Static Flood Extent from Grid' option located above the list of tasks. The list of available model time step results (in the lower left corner of the window) is replaced by an input box for you to set a horizontal plane water level. When the Input Water Level box appears, it already contains a value which is an average value between the highest and the lowest values in the ground grid highlighted in the Select Ground Grid box. You can overwrite the value by clicking on the Input Water Level box and typing a new value.

After selecting the ‘Static Flood Extent From Grid’ option and specifying an Input Water Level, you can then either add to the task list by pressing the Add button or run the flood calculation immediately by clicking the Run button. In either case you will be prompted to select an output format, output filename and location. By using the Add button and adding the task into the list of tasks, it is also possible to include Static Flood Extent from Grid calculations in a batch run.