Please note that this product is experimental and is not operationally supported, meaning that there may be some gaps in the availability on the EFAS web interface due to outages.
The radar-based river flash flood impact layer forecasts 1-arcminute (~1.4 km) river grid cells where there is a risk of impacts due to river flash flooding, which refers to the rapid rise of small rivers (with a drainage area <=1000 km2) due to intense rainfall. The forecasts are generated every hour using a blending of ensemble precipitation forecast inputs derived from the latest radar observations and NWP forecasts from the ECMWF ensemble (the blending procedure is described in the methodology section of Radar-based Accumulated Precipitation 80th Percentile). The precipitation forecasts are used to predict grid cells where there is a probability of flash flooding exceeding the 2 year return period threshold. The flash flood probability is overlaid with a static dataset which summarises the exposure of population and critical infrastructure. The result of this overlay is used to categorise flash flood impacts on an impact matrix as being either low (yellow), medium (orange), high (red) or severe (purple), river grid cells affected by flash flooding will be highlighted in one of these four colours (Figure 1).
The forecasts have a 1-hour time step for the first 6-hours of the forecast, and a 6-hour time step thereafter up to a maximum lead time of 120-hours (5-days). On the EFAS-IS mapviewer, an animation slider (bottom left corner) allows to visualise each time step of the forecast. Please note, this layer is only available for the current and previous 5 days.
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Figure 1. The Radar-based river flash flood impact layer shown on the EFAS web interface for the Dublin area on 4th September 2022 at 04:00 UTC (left), and the impact matrix (right) which is used to assign the colour of each river grid cell affected by flash flooding.
Methodology
There are two main steps in the generation of the river flash flood impact layer: 1) river flash flood hazard forecasting using blended radar and NWP precipitation, 2) the prediction of river flash flood impact (Figure 2).
Figure 2. 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.
Step 1: River Flash Flood Hazard Forecasts
A probabilistic forecast of river flash flood hazard is generated firstly by blending hourly nowcasts of precipitation derived from radar observations with NWP precipitation forecast from the ECMWF ensemble. More details about how the two datasets are blended can be found in the methodology section of: Radar-based Accumulated Precipitation 80th Percentile. Radar data are not available in all areas covered in the EFAS spatial domain, a map of radar coverage can be found in the Radar Coverage layer. The resulting blended 51-member ensemble precipitation forecasts have a spatial resolution of 2 km, and 1-hour time steps for the first 12 hours lead time which increases to 6-hours thereafter for a maximum lead time of 120-hours (5 days).
Next, at each time step of the forecast, at each grid cell with an upstream drainage area <=1000 km2, the rainfall is summarised as the total amount of rain which was forecasted to fall across the area upstream of the grid cell over the preceding time period equal to the river's time of concentration, Tc. The time of concentration refers to the time between rain falling and the river reaching its peak, it was computed at each grid cell using the method of Giandotti, 1934 which was derived for small rural catchments:
Tc = (4√A + 1.5L) / 0.8 √H
where A = upstream drainage area (km2), L = the slope length (km) and H = difference between the mean elevation in the area upstream of the grid cell and the elevation of the grid cell (m)
The summarised rainfall is compared against a climatological threshold, which is the summarised rainfall associated with the 2-year return period. This climatological threshold is derived from the following datasets:
- 8-year gauge-adjusted OPERA radar rainfall data
- A dataset of 20-year reforecasts obtained with ECMWF Integrated Forecasting System (IFS)
At each location on the river network and each forecast time step, the total number of ensemble members which exceed the summarised rainfall associated with the 2-year return period is computed and then divided by the total number of ensemble members (51) to give a probability of exceedance (%).
Step 2: River Flash Flood Impact Prediction
The next step is to compute the exposure, at each river location where the exceedance probability of the 2-year return period of accumulated rainfall (step 1).
The exposure data accounts for population and critical infrastructure in the form of health, education, transport, and energy generation facilities. Population data were obtained from the GHSL (Global Human Settlement Layer), the critical infrastructure data were from HARCI-EU within EU member states and OpenStreetMap for non-EU member states. Data for each of these five categories was harmonised and combined with equal weighting to create a combined exposure layer whose values ranged from 1.0-2.0.
The river flash flood impact level is computed at each time step by intersecting the river flash flood hazard forecast with the combined exposure dataset. The low, medium, and high values for flash flood hazard probability on the y-axis of the impact matrix (Figure 2) indicate where there is a 5%-50%, 50%-80%, and >80% probability of exceeding the 2-year return period threshold.
The categories for low, medium, and high exposure, on the x-axis of the impact matrix (Figure 2), relate to combined exposure values of 1.0-1.3, 1.3-1.6 and 1.6-2.0 respectively. 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 flash flood hazard level with exposure gives impact levels for each grid cell on the river network at each time step of the 120 hour forecast period.
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 (Figure 3) 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 8 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 animation slider.
Figure 3. 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 Radar-based river flash flood impact 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-arcminute grid cells. To solve this, the following workflow is recommended:
- Load in the Radar-based river flash flood impact catchment summary layer for the forecast leadtime horizon window of interest e.g. load in the Radar-based River Flash Flood Impact Catchment summary 0-6h if you are interested in the first 6 hours of the forecast leadtime horizon
- Use this layer to identify the specific sub-catchments which are highlighted as being at risk of flash flooding during this forecast horizon
- Zoom in closer to the highlighted subcatchments of interest
- Load in the Radar-based river flash flood impact 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 Radar-based river flash flood impact 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