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Guillaume Mauger, research scientist with the University of Washington's Climate Impacts Group.
Guillaume Mauger, research scientist with the University of Washington’s Climate Impacts Group. Photo: Lisa Stiffler.

Depending on where you live, communities in the Seattle region have already been hit by three to four destructive floods this winter — floods that usually come once a year, if at all — as local rivers swell over their banks and gush into neighborhoods, farmland and commercial areas.

And the flooding is expected to only get worse as global temperatures continue to rise, despite the newly announced Paris climate accord that aims to curb greenhouse gas emissions.

The app includes photos of various landmarks, including this image of the Spencer Bridge, under different flooding conditions.
The app includes photos of various landmarks, including this image of the Spencer Island Bridge, under different flooding conditions on the Snohomish River north of Seattle.

So scientists at the University of Washington’s Climate Impacts Group have developed detailed models to better understand what the flooding might look like in a warmer world — and it’s not a pretty picture. Their effort, which focused on the Snohomish River north of Seattle, found that for the so-called 10-year flood (an event that has a 10 percent chance of occurring annually), the area flooded by the river is expected to increase between 19 and 74 percent by later this century compared to levels seen at the end of the 20th Century.

To make their predictions, the researchers took models that incorporate the effects of climate change and warmer temperatures, river stream flow, historical flooding, landscape morphology and added predictions about rising sea levels to build a more complete image of what could happen in local rivers. They wanted to include the sea-level rise in the predictions because when the seas rise higher, it could increase the flooding near river deltas as the saltwater backs up the freshwater flow.

“In the past when we’ve talked about these projections, we’ve talked about sea level rise in isolation from river flow projections, but they both cause flooding,” said Guillaume Mauger, a research scientist and leader of the project. “So putting them both together is innovative.”

An app by The Nature Conservancy using analysis by UW scientists predicts how much flooding would be expected around the Snohomish River based on different conditions.
An app by The Nature Conservancy using analysis by UW scientists predicts how much flooding would be expected around the Snohomish River based on different conditions.

Perhaps the most interesting feature of the project, which was funded by The Nature Conservancy, is the development of a web app that displays the effects of climate change on an interactive map of the Snohomish River. The environmental organization used the UW research to build the app showing the river, which begins near Monroe and flows past the town of Snohomish, Everett and Marysville toward Puget Sound, under a variety of flood conditions.

Users can build maps based on historic flood depths and predicted average flood levels for mid century and the end of the century, select whether greenhouse gas emissions are low or high, and choose 10- and 100-year-flood scenarios.

The app also includes photos of three different landmarks — a stretch of Interstate 5, Spencer Island Bridge east of downtown Everett and Harvey Airfield in Snohomish — and how they would fare under the scenarios.


“We tried to tell the story in a few different ways — some that are dry and just the science, and some that are more narrative,” said Kris Johnson, senior scientist for the North America freshwater program for The Nature Conservancy.

“It definitely resonates with people in ways that it hasn’t before,” he said. “We’re not at all dogmatic or finger-wagging. We’re saying, ‘Here are the options and this is what it could look like.’ ”

The researchers found that the warmer temperatures will push the snow level higher and in effect widen the geographic area that drains into the Snohomish, as more precipitation falls as rain than snow. The climate models also predict heavier winter downpours for this region, all leading to bigger stream flows and higher risk of flooding.

One of the big worries with rising seas is something called “storm surge.” That’s when stormy weather periodically causes seawater to mound higher than normal, exacerbating sea-level rise caused by climate change. It turns out that in the Snohomish, the worst of high river flows are expected to occur before the storm surge hits, so the two won’t take place simultaneously and compound each other.

The Nature Conservancy funded the research as a local piece of a national campaign called “Floodplains by Design” that aims to better manage flood plains in a way that brings together different stakeholders concerned about rivers and surrounding lands.

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“This is really about trying to manage rivers and flood plains better,” Johnson said, “to put information in the hands of people who make decisions.”

WEST Consultants, a West Coast engineering firm, helped with the modeling. Sustainable Lands Strategy, a group that includes representatives from local governments, the Stillaguamish and Tulalip tribes, agriculture and environmental interests, are also part of the project.

“I would say we’ve begun to scratch the surface about how [the data] could work for us,” said Monte Marti, executive director of the Snohomish Conservation District.

The conservation district has applied for a federal grant to incorporate more farming information into the project in order “to really start to dive into the potential impacts on agriculture associated with climate change,” Marti said.

Many of the databases and models the researchers used are publicly available and free for anyone to use, including data produced by the Climate Impacts Group. Some of the information used for this project was generated by the UW scientists by taking global climate predictions and carefully fine-tuning or “downscaling” them to identify local effects. The UW’s Mauger and Johnson are eager for the tool to begin helping people like Marti make better-informed management decisions.

“What you want from these maps is to get really detailed about how you would plan things differently,” Mauger said. Instead of scientists giving vague predictions about higher stream flow, this project and app gives location-based specifics. “It’s that one extra step closer to reality for people.”

Editor’s note: This post was updated on Dec. 18 at 10:53 am to clarify the role of The Nature Conservancy in developing the web app.

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