How Western Washington’s ‘100-year’ floods are changing

On Wednesday, a “GO NOW” evacuation notice was issued for people living in the “100-year flood” zone in Skagit Valley, a large area which includes Burlington, La Conner, Lyman and Hamilton, as well as parts of Mount Vernon and Sedro-Woolley.

But that term is a bit of a misnomer. It originates from flood recurrence intervals set in the 1960s when the floodplain maps used today by insurance companies and the Federal Emergency Management Agency were made, said Western Washington University geology professor Allison Pfeiffer. At the time, that standard was based on a 1% chance that that flow rate would be met or exceeded in a given year.

“That breaks down when you are in a world of a changing climate and now … the dice are weighted towards higher flows,” she said.