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 catchment summaries are four layers which summarise the Radar-based River Flash Flood Impact product for the following four lead time windows at the catchment level:
- 0-6 hours (based on blending radar precipitation nowcasts and the ECMWF ensemble weather forecast)
- 7-24 hours (based only on the ECMWF ensemble weather forecast)
- 25-48 hours (based only on the ECMWF ensemble weather forecast)
- 49 -120 hours (based only on the ECMWF ensemble weather forecast)
The forecasted impact level in each river catchment is shown as one of four levels: low (yellow), moderate (orange), high (red) and severe (purple). The impact level is estimated using a risk matrix to intersect a flash flood hazard forecast with static exposure data of population and critical infrastructure. Both the flash flood hazard and the exposure data are split into three categories to create the risk matrix (see an example in the left hand side of Figure 1).
By clicking on specific catchments, users are able to see a pop-out window showing the forecasted impact level on the impact matrix (Figure 1). There is also some additional information on the number of people, education, health and energy generation facilities forecasted to be affected.
Please note, when the layer is loaded on the EFAS web interface, it is not possible to select different 'Forecast steps' in the box at the bottom left of the screen (Figure 1). This is because
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Figure 1: Example of the Radar-based river flash flood impact catchment summary layer for 0-6 hours on the 10th February 2025 at 22:00 UTC (left). River catchments where flash flood impacts are possible are shaded according to their classification on an impact matrix (shown at the bottom of the figure). When a catchment is clicked a pop out window appears (shown on the right) showing information about the affected catchment.
Suggested Use of the Catchment Summary Layers
The radar-based river flash flood impact catchment summary layers can be very useful for identifying the general areas and time periods where flash flood impacts are possible. It is recommended that these layers should be consulted first before looking at the animated TAMIR - impact forecasts layer which can be difficult to visualise when zoomed further out and if you don't know the specific timesteps when flash flooding is possible. After using the catchment summary layers to identify the catchments and time windows where flash flood impacts are possible, the animation from the Radar-based river flash flood impact layer can be used to identify the specific 1-arcminute grid cells and time steps where impacts are possible.
Methodology
There are three main steps in the generation of these four river flash flood impact catchments summary layers: 1) river flash flood hazard forecasting using blended radar and NWP precipitation, 2) river flash flood impact prediction, 3) summarising river flash flood impacts at the catchment scale (Figure 2).
Figure 2: Workflow used to generate the radar-based river flash flood impact catchment summary layers.
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 (computed in the previous step). 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 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, 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.
Step 3: River Flash Flood Impact Catchment Summary
To create the catchment level summary, firstly for each of the 4 lead time aggregation windows (0-6h, 7-24h, 25-48h, 49-120h), the maximum impact level forecasted in each river grid cell during the aggregation window is calculated. Next, the catchments are shaded according to the 90th percentile of the impact level of all cells forecasted within each catchment. Use of the 90th percentile of impact (instead of the maximum) is to avoid communicating potentially misleadingly high forecast information to users based on single cell values.