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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 Fig. 1).
Figure 1: Example of the Radar-based river flash flood impact catchment summary layer for 0-6 hours on the 31st August 2022 at 00:00 UTC near Strasbourg, France. River catchments where flash flood impacts are possible are shaded according to their classification on an impact matrix (shown on the left side of the figure), which can be displayed when a catchment is clicked. Additional exposure information is given in a table at the bottom of the pop out window (shown in the bottom left of the figure).
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.
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 on this page: TAMIR Total Precipitation 80th Percentile - copy. 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).
The precipitation forecasts are then accumulated on a 1-arcminute (~1.4 km) river network for rivers with an upstream drainage area <1000 km2. Accumulating precipitation means that at each location on river network, at each forecast time step, the total amount of rain which fell over the previous number of hours equivalent to the river's time of concentration, referring to the time between rain falling and the river reaching its peak, is computed. The accumulated rainfall is compared against a climatological threshold which is accumulated rainfall associated with the 2-year return period. This climatological threshold is derived from the following datasetsThe low, medium, and high values for flash flood hazard 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. 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 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 accumulated 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 low, medium, and high values for flash flood hazard 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. 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.
The categories for low, medium, and high exposure are 1.0-1.3, 1.3-1.6 and 1.6-2.0 respectively and form the x-axis of the impact matrix. 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
Figure 1. The flash flood impact level forecast for a leadtime of 0-6h near Strasbourg, on the 31st August 2022 at 00 UTC
This combination of flash flood hazard level with exposure gives impact levels for the river network cells. 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 throughout the 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.
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