A 100-year storm isn’t what it used to be
January 27th, 2008 by adminIn a story I wrote for today’s Kitsap Sun, I learned a lot about where the numbers are buried — numbers involving rainfall, streamflow and the mythical 100-year storm.
Some of the primary deposits for these natural numbers include the National Weather Service, U.S. Geological Survey, U.S. Army Corps of Engineers and Federal Emergency Management Agency, along with the state Department of Ecology and Kitsap Public Utility District. Each calculates and models the data in its own way.
I’ll be writing more stories and more blog entries about this subject in the future, but I’d like to point out that I was unable, though I seriously tried, to find anyone who could tell me how many 100-year storms we’ve had since 1990.
The reasons are varied. For one thing, each location has its own 100-year rainfall and each stream has its own 100-year flood level, but the measuring devices for rainfall and streamflow aren’t always in the right places. Also, the 100-year rain event is based on a continuous 24 hours of data, while most rain measurements are taken during a set 24 hour period; few take continuous or hourly readings.
Perhaps someone knows of an expert who has a guess exactly when we’ve had 100-year storms for a given area.
What most experts do acknowledge is that we’ve had at least several such storms in the past 20 years, which would suggest that the official maps need to be updated. Folks with the Corps of Engineers say they update their data more frequently than the state and counties, but I have yet to see actual data or a map based upon it.
So how big was the Dec. 3 storm? Kathleen Cahall, water resources manager for Bremerton, said she interviewed experts for some of the same agencies I did, but she was unable to find anyone who had done the calculation for Bremerton. So she acquired a copy of the theoretical rainfall curves and made her own extrapolation. Her best guess, based on current statistics, is that such as storm would not repeat for an average of more than 800 years.
As I pointed out in the story, planners and engineers are entering new territory with no guidance about how to proceed.
Scripps Interactive Newspapers Group
January 27th, 2008 at 3:51 pm
Based on her calculations though… is it possible we could have another record storm greater than 12/3 any time soon?
Exciting that planners and engineers have the opportunity to chart new territory and be challenged to write the “How to” book as they go along.
Sharon O’Hara
January 27th, 2008 at 8:10 pm
During this same time the Matson Kauai needed assistance from the Neah Bay tug when it stuck its bow into a 20 foot trough and had its pilot windows taken out by a 40′ wave according the Coast Guard.
January 28th, 2008 at 11:11 am
“calculating differently”.. sounds like one of the solutions that could be added to the “How to” book Sharon refers to! (get together on how to calculate it).. just a thought.