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Next step is to compute the flood threshold exceedance values. CEMS-Flood primarily uses three main flood severity levels, the 2-year, 5-year and 20-year return periods. We determine for each of these
thresholds whether the predicted 30-day maximum river discharge in the ensemble forecast members (the control member and the 50 perturbed members) exceeds the threshold valueover time exceeds the threshold value for each individual forecast member (and deterministic forecasts for EFAS). We assign 1 if they exceed and 0 if they do not. This process is represented in Figure 2 by the coloured river cells, which show that the 30-day maximum river discharge over time exceeds the flood thresholds. Please note the triviality that if the 20-year threshold is exceeded for any cell, than both the 5-year and 2-year must be also exceeded for those cells, i.e. more and more cells are coloured (indicating exceedance), as we go from 20-year to 2-year level.
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Figure 3. Schematic representation of the exceedance probability computation based on the 51 ensemble members for the example 5-year return period (RP).
For EFAS, in which the system is composed of multiple forecasting system, the exceedance probabilities of the forecasts are combined into a total exceedance probability, using a weighted average over the forecasts.
Merged probabilities
In the following step we combine the probabilities of the three severity levels and create one probability map that hierarchically combines the three probabilities.
Firstly, we convert each probabilities into binary (yes/no) information by applying a minimum 30% threshold value, (typically 30%). If the exceedance probability exceeds 30%this minumum threshold value, we consider this flood category forecasted. This is represented by thicker black borders around the river cells in Figure 4a.
Then, the three probabilities are combined hierarchically (Figure 4b), using three probability categories of 30-50% (shown by light colour), 50-75% (shown by medium colour intensity) and 75-100% (shown by dark colour). If the 20-year probability exceeds 30%, then those cells get the 20-year severity level (purple) and one of the three subcategories (of of the purple colours) depending on the probability value. For cells which has less than 30% 20-year probability, if the 5-year probability exceeds 30%, then they will get the 5-year (red) severity level, again with one of the three red intensities depending on the 5-year probability value. Then if the 5-year probability is below 30%, then it is 2-year (yellow) severity, etc. Finally, if even the 2-year probability is below 30%, then the cell is characterised as 'no flood' and will be left uncoloured.
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The maximum exceedance probability cell is chosen as an extra point (in case it is different to the already selected cells), as often that will carry an important extra piece of information about the flood situation. If the maximum exceedance point is too close to the most downstream one, only the maximum exceedance point is kept. This process will deliver in our simplified example in Figure 6c the selection of two 5-year and 2-year cells (darker red and darker yellow) and three 20-year cells as the 20-year over-30% probability river section is long enough to have the every-10 rule giving another point.
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