How to create an empty grid to represent my lower limit of flooding

Damage Calculator can calculate Annualised Average Damage (AAD) values. The required input data for this analysis are flood model results from multiple simulations representing different AEP events (within the same scenario). The AAD calculation process will plot these data on a chart of damage verses probability (i.e. return period). It will then calculate the area under this chart as the AAD value.

The calculation assumes a linear extrapolation of the chart from the highest return period dataset to obtain the zero-probability damage value. This enables the calculation of the additional area contributing to AAD beyond the highest return period, as shown in the diagram below:

As the above diagram shows, the AAD calculation does not consider any contribution to AAD total from low level flooding (i.e. low damages from return periods smaller than lowest modelled RP).

Depending on your available model data, this omission is likely to have a negligible effect on total AAD. However, if you wish to include a contribution from this area you will need to make an assumption for the highest return period that leads to zero damage (i.e. the onset of flooding or flood damage). You will then need to define a dummy grid consisting of all missing data entries (i.e. “-9999”) that, when added to your Damage Calculator analysis, will result in zero damage and when assigned your onset of flooding return period will provide the AAD calculation with the required data to include the “initial triangle” area (on the above damage curve).

This section explains how to set up a dummy grid to represent zero damage for your study area:

1. Load the flood depth grids that your modelling has already produced into the Flood Modeller map view (using Add GIS Data button).

2. On the Map Tools tab of the main toolbar, click on the Grid icon to reveal a sub-menu of grid related tools. Select the Grid Calculator tool.

   

3. The Grid Calculator will be displayed in a new pop-up window. It allows you to build a mathematical expression that includes one (or more) of the compatible grids currently loaded in the map view. When the Grid Calculator starts up, your currently loaded grids are listed on the left side. The expression you will build to create a zero-damage grid can use any of your existing flood depth grids, assuming they are all the same in terms of resolution, numbers of rows and columns and lower left coordinate (which they likely will be if they are all derived from different scenarios run using the same model). The selected grid will be used only to define the size and location of the grid, while the expression will define the cell values (setting all to the missing data flag = “-9999”).

   Start the expression definition with an open bracket - click the appropriate button to add bracket symbol in expression textbox (at top of window). Then highlight one flood depth grid in the left-hand list and click “Add Layer to Expression”. The selected grid should appear in the Expression textbox.

   

4. Build an expression to set all existing grid values to zero and then subtract 9999 to set all cell values to “-9999”. This expression will be:

   Click on the calculator keypad to build the expression. The final version should look like this:

   

5. Use the browse button (next to the Output file textbox) to specify a filename and location for the new grid the tool will produce. Ensure the Load to View checkbox is ticked and then click Run.

   

6. The new grid will be added to your current project, however it will initially not be visible in the map. This is because Flood Modeller by default will not show cells set to the missing data value. You should see the new grid added to the top of the Layers Panel. To check it has been created correctly:

   1. Double-click on the new layer in the Layers Panel (or select Properties in the right-click menu). The Properties window will be displayed.

   2. Go to the Symbology tab and select the Colour Ramp sub-tab.

   3. Below the colour ramp tick the Visible checkbox for “NoData” values.

   4. Click OK. The new grid should appear as a solid rectangle, coloured to whatever the Displayed Colour setting was set to for NoData (this is black by default).

   

7. You can now add this grid to your Damage Calculator analysis with the associated return period / AEP set to whatever you want to represent the onset of flood damages. This will ensure the low return period, low level damages are included in your calculation of AAD.