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

For communities in Skagit and Whatcom counties, flooding is a part of life.

This week’s flooding is likely to match or be worse than the flooding in 2021 that caused significant damage, especially along the U.S.-Canada border.

Nearly 5 trillion gallons of rain have soaked Washington in the past seven days, triggering evacuations and rescues.

Flooding is a natural part of how rivers function, but climate change is going to make things worse, threatening communities along rivers and in floodplains.

Climate change is playing a role for two main reasons: The snow line is creeping higher so more precipitation is falling as rain instead of snow. And warmer air moving over warming ocean water can hold more moisture than before.

These changes are also complicating how we understand terms traditionally used to describe flooding. 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.

As an example, there have been five and seven “100-year flood events” near Mount Vernon and Concrete, respectively, since 1990 — the most recent in November 2021, according to Western Washington University Communications. Prior to 1990, Mount Vernon and Concrete had experienced “100-year” flood events on average every 28 and 25 years, respectively.

How levees change flooding along the Nooksack River

While the ongoing flooding of the Skagit River is “scary” due to the forecast predicting “feet above anything we’ve seen before,” the Nooksack River is a different situation, Pfeiffer said.

The flood forecast for the Nooksack River has changed rapidly over the past few days, she said. On Sunday, the forecast predicted the river would reach flood level, but then throughout Wednesday, that prediction quickly changed from a moderate flood to near the record.

The Nooksack is a “very weird and wild river,” Pfeiffer said, and when it overflows its banks in Everson, the water exits into Sumas and into a topographic depression in British Columbia, which was a shallow lake before settlers drained it for farmland in 1920.

“This lake gets reborn. In other words, what is some of B.C.’s best agricultural land get[s] inundated with 6 feet of stagnant water,” she said. The area is known as Sumas Prairie, or by its First Nations name, Xhotsa, and is a point of international tension, Pfeiffer said.

Evacuation notices were issued in Sumas, Everson and Nooksack Wednesday and Thursday.

During the most recent major flood in 2021, which this event is expected to meet or exceed, an estimated 85% of residences in Sumas were damaged. One man died; hundreds of thousands of livestock also drowned; and there was more than $1 billion in damage, Pfeiffer said.

“The folks in Sumas are going to have to decide whether to rebuild their whole lives a second time in five years,” she said.

What levees do during floods

How rivers and communities flood has also been complicated by human-made levees or piles of raised rock and dirt along riverbanks, Pfeiffer said. Levees were originally built to prevent flooding, and now resemble a “patchwork” system of barriers of different heights and ages.

The issue with sticking to a strategy of just building levees ever higher is that it leads to an “arms race” downstream of the river, with flooding occurring in new places each time. For example, building taller levees on the Nooksack River near Everson would only worsen flooding on the Lummi reservation and in Ferndale, which narrowly missed major flooding in 2021, Pfeiffer said.

“The existing flood protection structures are going to become less and less adequate, and these overbank floods or levee-topping floods are just going to become more and more common in almost all the rivers in the region,” she said.

One solution, Pfeiffer said, is to set levees further away from a river, giving it more room to move during floods. However, this solution is legally and financially challenging since municipalities often have to buy out landowners along a river.

“Our rivers in this region are wild. A bunch of them are draining volcanoes that are crumbling and sending really large volumes of sediment downstream, she said.