Final answer:
The flood recurrence interval represents the statistical chance of a particular magnitude flood occurring in any given year, such as a '3-year flood' meaning a 33.3% chance each year. It is a measure of probability, not a prediction of exact timing.
Step-by-step explanation:
The length of time between flooding events is represented by the flood recurrence interval, which is a statistical measurement indicating the probability of a flood event occurring in any given year. It is calculated using historical data on flood events. For instance, if a flood recurrence interval is calculated to be 3 (a '3-year flood'), this means that there is a 1-in-3 chance, or approximately a 33.3% probability, that a flood of that magnitude could happen in any particular year. It's important to note that this does not predict the floods will happen exactly every three years but rather gives the statistical likelihood of such an event occurring each year.
Regarding the flood event from 9/23/2009 with a discharge of 21,200 cubic feet per second (cfs), plotting this event on a flood frequency graph would provide its recurrence interval. Based on this concept, it is possible for a similar flood event to happen again within the next 20 years. The historical data provides a statistical likelihood, not a prediction, which means the occurrence of floods does not follow a regular pattern like rolling dice doesn’t guarantee a certain number to come up in a set order.