How to use the flood event duration tool

This section provides a step by step guide on how to produce an event duration grid:

1. Prior to accessing the event duration tool you have the option of loading 2D results into the Flood Modeller interface. This can be done using the Add GIS Data function (Home tab of main toolbar) or by opening the Flood Modeller project in which the model was run.

   Loading these data into the map view is optional as the event duration tool also allows you to browse to files to use in the analysis. However, selection of data for the tool is faster and easier if you pre-load it into your map view.

2. The event duration tool is accessed from the Flood Modeller toolbox, which is located in one of the tabs in the right hand panel of the interface. It can also be accessed by clicking the Modelling Toolbox button in the Map Tools tab of the main toolbar. The tool is in the Flood Mapping > 2D Models section. Double click on Event Duration Grid to open the tool. A new window is displayed as shown by the example below:

   

3. Define event duration settings. The analysis performed by the event duration tool is defined by the following user settings:

   • Select a 2D results file (.2dm). A dropdown list is populated with available results within the current project. In addition a browse button is provided to enable you to select a file external from the current project.

   • When a file is selected the associated raster datasets (depth and water level) are displayed. Select the parameter to be analysed and the time steps of the associated simulation period are listed.

   • A time bar shows the full duration of the model run. You can adjust these to a subset of the total runtime by using the mouse to drag the start and end bars. Alternatively you can type in a start and end time step in the spaces provided.

   • Select a calculation method. Three options are provided for the calculation of threshold exceedance:

     • Some 2D model outputs contain a cell 'status' value which signifies if a cell is wet or dry. This can be utilised for event duration calculations and should be the equivalent of applying a zero threshold

     • If the 2D model water level dataset is selected then this must be combined with the underlying ground elevation to calculate flood depth, i.e. depth = water level - ground level. The period this value is greater than zero is calculated and written to the output event duration grid. This analysis utilises the ground grid embedded within the specified 2D model results file.

     • Threshold comparison – the selected 2D model dataset is compared directly with a user specified threshold to calculate the period over which the threshold is exceeded.

   • The vertical time scale can be calculated as hours, minutes or as a user-defined increment equal to 'time step x user-specified factor'

   • A text box is provided to enter an event threshold value in metres. The default setting for this is zero depth, i.e. the tool will just count the periods when each cell is wet

   • A filename (and save location) must be specified for the event duration output grid together with a grid format. Available format options are binary (.flt) and ASCII (.asc) grids. Editing this field is optional as the tool will provide a default filename (“2D results name_EventDuration.asc”) and specify the 2D results folder as the save location.

   • A tick box is provided to specify whether to automatically load the newly generated event duration grid back into your map view. This box is ticked by default (i.e. the grid generated by the analysis will be loaded into the map).

4. After specifying all event duration threshold criteria, click Run. The tool will then generate a grid file. Some typical examples of event duration grids are shown below. These are event duration grids for Thames Embayment Region with thresholds set to 0.0m (left) and 0.5m (right):

   

   It should be noted that the severity of event durations output from this analysis is totally dependent on the input data. If the model has not schematised any drainage then flooded areas will remain wet longer than they would in reality. Also model run time will influence maximum durations in some cases, e.g. number of tidal cycles within simulation.

5. To close the event duration tool without running any analysis click the Close button.