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@@ -13,14 +13,27 @@ So far, we are working the first step of the project : help to convey messages a
 #### Tools : a real bag... and an app
 This can be achieved by drawing from a bag marbles colour-coded with respect to their probability. How can this colour be understood as this year's flood return period ? 
 
-This stage is the opportunity to discuss these notions, and understand that colors represent a class of return periods : on a total of 100 marbles:
-- 90 black marbles : their occurrence is on average 9 times out of 10 : 9 floods in ten years are "SMALLER" THAN the 10-yr return period flood. 
-Therefore, the remaining 10 marbles represent floods  "BIGGER" THAN the 10-yr return period flood. Among these : 
-- $`\textcolor{green}{ \text{the 8 green marbles}}`$ mean floods with a return period between the 10-yr and the 50-yr return period,
-- $`\textcolor{blue}{ \text{the only blue marble}}`$  represents floods with a return period between the 50-yr and the 100-yr return period, 
-- $`\textcolor{red}{ \text{the red marble}}`$ represents thus $`\textcolor{red}{ \text{all the other, higher floods : floods with a return period OVER 100-yr.}}`$ It means that the reference "100-yr flood" does not occur as such once in 100 years, but it is EXCEEDED once in 100 years.
-
-The probability of occurrence of floods "higher than the 50-yr return period" is the same as drawing one blue OR one red marble : ( 1 + 1 ) / 100,  so one in 50.
+This stage is the opportunity to discuss these notions, and understand that colors represent a class of return periods.
+
+- **90 black marbles** on a total of 100 marbles : they are drawn on average 9 times out of 10. If we think of floods, 9 floods in ten years are "SMALLER" THAN the 10-yr return period flood. 
+
+- the **remaining 10 marbles** represent therefore floods "BIGGER" THAN the 10-yr return period flood. 
+
+Among these 10 marbles : 
+- $`\textcolor{red}{ \text{a single red marble}}`$ represents  : $`\textcolor{red}{ \text{ floods with a return period OVER 100-yr.}}`$ 
+In other words, the red marble represents the 1% highest floods, so it does not represent a "100-yr flood" but all the floods above it. This means that the "100-yr flood" is not occurring once in 100 years, as such, but it represents a level EXCEEDED once in 100 years.
+- $`\textcolor{blue}{ \text{a single blue marble}}`$  represents floods with a return period between the 50-yr and the 100-yr return period.Explanation : the 50_yr return period flood has a probability to be exceeded of 1/50, which is 2/100, but the red marble already represents the class of floods above the 100-yr flood. It means that we need only one marble more, representing the floods above the 50-yr one but under the 100-yr one (the floods above are already represented by the red one)
+- $`\textcolor{green}{ \text{the 8 green marbles}}`$ mean floods with a return period between the 10-yr and the 50-yr return period. Explanation : The 10-yr return period flood has an exceedance probability of 1/10, which is 10/100, but the red and blue marbles already represents the floods above the 50-yr flood. It means that we need 8 marble more, representing the floods above the 10-yr one but under the 50-yr one.
+
+:bulb: probability can only be used for classes of floods. Keep in mind that :
+- the **exceedance probability** of the "N-yr return period flood" is **one in N**.
+- therefore, the probability to have a flood between the N-yr and the M-yr flood, with N < M,  is : $`(\frac 1 N - \frac 1 M)`$
+
+
+<details><summary>definition of exceedance</summary>
+Probability of exceedance is a statistical metric describing the probability that a particular value will be met or exceeded.
+https://www.sciencedirect.com/topics/engineering/exceedance
+</details>
 
 We can also begin to discuss how we can "order" floods (we can sort discharges, but not so easily flood events), and if a flood event really has a return period strictly speaking.