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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).
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.
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 (Figure 2).
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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. 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 , 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 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
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
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 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.By clicking on specific catchments, users are able to see a pop-out window showing where in the impact matrix the catchment lies (Fig. 1). There is also some additional information on the number of people, education, health and energy generation facilities forecasted to be affected.
Selecting Forecast Date
If one of the TAMIR Radar-based Catchment Summary layers has been chosen and no other TAMIR Radar-based layers are already loaded in the EFAS webviewer, then by default the data will be loaded for the date which matches the date in the top right above the legend window (Fig. 2Figure 3). Different dates can be selected using the date selection box in the top right above the legend window, but please note that it will only be possible to select data produced at 00 and 12 UTC on each day.
To select data produced at a specified hour within the past 5 days:
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- Load the
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- Radar-based catchment summary layer from the Flash Flood
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- product tab (Figure 3).
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- Use the date selector at the top of the box which appears in the bottom-left corner of the
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- web interface
- The day can be selected on the left hand side
- The time (in UTC) can be selected from the drop-down menu to the right of the day selector
- Note that the 6-hourly time options which appear underneath do not do anything to the visualisation of these layers
- Once a date and time has been selected, this is automatically applied to all the
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- Radar-based layers which are loaded in the EFAS web interface
Figure 3
Figure 2. Example of the TAMIR Radar-based river flash flood products catchment summary 0-6h loaded into the EFAS webviewer. The functionalities to load and manipulate the products are highlighted in red.
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These 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 TAMIR Radar-based river flash flood impact forecasts layer can be used to identify the specific 1 km -arcminute grid cells and timesteps time steps where impacts are possible.