How to create a flood event duration grid from 2D results

Introduction

2D models can be configured to produce a variety of useful outputs. However, currently one potentially useful omission from these options is a data output that relays information relating to event duration. A clear way to display event duration data is as a grid where the vertical scale, i.e. colour scale, represents total time when a threshold water level was exceeded.

Although these data are not available from TUFLOW or Flood Modeller Pro 2D it is a relatively simple post-processing operation to produce it. Therefore, Flood Modeller provides an option to calculate event duration grids from any 2D model result grid.

The raw output data required to produce an event duration grid is simply the flood depths through the duration of the model run (or a user specified sub-period of this). These data can be taken directly from a water depth 2D model output or calculated from the combination of water level 2D model output and an underlying ground grid. The tool utilised the ground grid contained within the 2D results file, hence there is no need to specify a separate DTM for the analysis.

Using the 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:
  1. 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.
  2. To close the event duration tool without running any analysis click the Close button.