![]() ![]() In any given year it is 100% likely that it will or will not flood to a certain extent. The complement of this question would be: what is the probability that a flood of size X does not occur during the next n consecutive years? We want to know what the probability is of a certain sized flood happening at least once over nconsecutive years. in 30 years, the probability and size of a 100-year flood will be the same as today). Stationary means that the probability does not change over time (i.e. Independent means that the probability of occurrence of a certain size flood in one year does not change the probability of a flood in any other year. We must first assume that the probabilities of flooding each year are independent and stationary. ![]() Next, let us calculate the probability of an event occurring over multiple years. Of course, these probabilities describe the chance an event happens in a single year. "The city updated their flood maps to include our house in the 100-year floodplain."ĭo you know what is really meant by the term "100-year flood"? It is NOT the case that this event will only happen once every 100 years. "Bob, the heavy rains north of Waterville were responsible for the 100-year flood event downtown." You may have heard something like this before: It also shows that there is a fundamental misunderstanding of the vocabulary used to communicate the probability of a flood event. In my experience, the probability of flood exceedance is a great example of where calculating the probability of something not happening is more intuitive and easier to understand. Flood events, return periods, and probability We will do this using our understanding of probability and statistics. In this post, we will calculate the chance of such an event happening, or rather the chance your beautiful riverside property floods over the life of your 30-year mortgage. If at some point you ever want to buy property near water, a variation of this question will undoubtedly pass through your head: what are the chances that my floods? ![]()
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