Hydrologists use a concept called a “100 year flood,” which describes an event with a recurrence interval of 1% per year, or an event with a probability of occuring once every 100 years. One could use other recurrence intervals, such as a “10 year flood,” with a ten percent probability of occurring each year. I first encountered the terms as a real estate lawyer looking at survey plans. Anytime a client bought or developed land anywhere near water the surveyors would note the location of the 100 year flood plain.
I’ve been thinking of these concepts because summer 2008 has brought more violent thunderstorms and more rain to the northeast than I can remember. I’m not suggesting the rainfall approaches 100 year storm levels, only that rainfall since summer’s advent is notably above normal. The Maine dock is my yardstick. Using it to measure water levels is tricky because I install it each spring and remove it each fall. Its elevation is always relative to the level of the lake. Other fixed points, like the end of the dock installation ramp, tell me that the lake is unusually high for July 31. The dock tells me that the lake is about nine inches higher than it was when I installed the dock in early June. I install it with the bottom edge just touching the water. In a normal summer by the end of July there would be 4-6″ of clearance between the bottom of the dock and lake–enough for the dogs to swim underneath.