This animation layer shows the flash flood impact levels over the river network forecasted for the upcoming five days. Forecasts are hourly out to a leadtime of approximately six hours, and six-hourly after that.

Methodology Overview

This forecasted impact level is estimated by intersecting a flash flood hazard forecast with static exposure data on a risk matrix. Both the flash flood hazard and the exposure data are split into three categories to create the risk matrix. The low, medium, and high values for flash flood hazard indicate where there is a 5%-50%, 50%-80%, and >80% probability of exceeding the 2-year return period threshold, this is shown on the y-aixs of the impact matrix (Fig. 1). This flash flood hazard is estimated by comparing blended forecasts of precipitation (from the OPERA radar mosaic and ECMWF NWP forecasts) accumulated on the river network, with the reference values derived from climatology. This climatology is from:

  • 8-year gauge-adjusted OPERA data
  • A dataset of 20-year reforecasts obtained with ECMWF Integrated Forecasting System (IFS).

The exposure data accounts for population and critical infrastructure in the form of health, education, transport, and energy generation facilities. Data for each of these five categories was harmonised and combined with equal weighting to create a combined exposure layer. The categories for low, medium, and high exposure are 1.0-1.3, 1.3-1.6, and 1.6-2.0 respectively, this represent the three exposure categories on the x-axis of the impact matrix (Fig. 1). These categories were chosen based on the statistical distribution of exposure values across the EFAS domain, and consequently mean 81.0% the exposure values are classified as low exposure, 8.0% as medium exposure and 1.2% as high exposure. This reflects the reality that most grid points in Europe have a low population density and with few exposed critical infrastructures.

This combination of the flash flood hazard level with exposure gives impact levels for the river network cells for all rivers with an upstream area <=2000 km2 and a time of concentration <=24 hours.

Figure 1. The impact matrix upon which flash flood hazard probability (y-axis) is combined with exposure (x-axis) to decide the impact category as shown by the colour of each cell. The definition of each impact category is shown on the right hand side.

Visualising within the EFAS Webviewer

The layer can be viewed after selecting it from the Flash Flood layers tab within the EFAS webviewer. An animation slider box will appear in the bottom left of the screen (Fig. 2) and by default the data from the most recent forecast will be loaded. A drop down menu just above the animation slider box can be used to select a different forecast date and time within the past 5 days.

The buttons on the left of the animation slider can be used to step forwards and backwards in time for each timestep of the forecast. The play button will play the animation in a continuous loop. When animating each timestep, it can take a few seconds to load the next timestep, this is shown by a loading icon which appears within the animation slider which will disappear once the next timestep has been loaded. 

The animation slider box is shaded in white for the first 6 hours of the forecast horizon, this represents the horizon during which radar nowcasts are blended with the ECMWF ensemble NWP. For the remaining timesteps the animation slider box is shaded red, this reflects that the forecast is derived from ECMWF ensemble NWP only.

For the first few hours of the forecast horizon the timestep is of the forecast is 1 hour, after this the timestep becomes 6 hourly. Typically the first 6 hours of the forecast will have a 1 hour timestep, but this could be longer depending on the time of day when the forecast was produced. A box is displayed above the animation slider which shows the size of the current timestep which is being shown in the animatino slider.

Figure 2. Flash flood impact forecast for the Dublin area on 4th September 2022 at 04:00 UTC

Suggested Workflow for Analysing Animated Flash Flood Impact Layer

When viewing the animated TAMIR - impact forecasts layer it can be difficult to identify specific locations and timesteps which may be affected by flash flooding. This is partly because when zoomed out it is very difficult to see the shaded 1 km grid cells. To solve this, the following workflow is recommended:

  1.  Load in the TAMIR Catchment Summary layer for the forecast leadtime horizon window of interest e.g. load in the TAMIR - Impact Max 0-6 Catchment Summary if you are interested in the first 6 hours of the forecast leadtime horizon 
  2. Use this layer to identify the specific sub-catchments which are highlighted as being at risk of flash flooding during this forecast horizon
  3. Zoom in closer to the highlighted subcatchments of interest
  4. Load in the TAMIR - impact forecasts layer, use the animation slider to identify the specific 1 km grid cells and timesteps where flash flood impacts are possible

The visibility of the TAMIR - impact forecasts layer can be improved by changing the map background to show country borders only, this can be toggled from the top right of the webviewer, it is the icon to the right of the WMS icon