If 100 year floods happen only once every 100 years, why have we had so many of them?
Good question. Keep in mind the definition-a 100 year flood has a one percent chance of occuring in any year at one specific location. But that's subject to a statistical issue as well as environmental issues. Watersheds can change due to landslides and development...which can also affect a river's ability to absorb rain water. Of course, there may also be changes in climate which could impact the frequency of major flooding. But statistics can also impact the frequency of major floods. There's not just one flood gauge in the Puget Sound area, there are almost 200. Because of that number, a flood reaching the magnitude of a 100 year flood can happen somewhere within the Puget Sound basin once every 4-1/2 years.