Mapping the future of flood risk for the Stillaguamish & Snohomish Rivers

By Guillaume Mauger, Research Scientist, Climate Impacts Group, UW Seattle

 Sea level rise isn’t the only factor driving changes in flood risk for Washington State: many rivers and creeks in the region are projected to see higher peak flows in the future. This is especially a concern in estuaries, where the combined changes in river flow and sea level could dramatically change the outlook for flooding.

Rivers flood because of high flows, of course, and there are two principal ways that climate change could affect the size and frequency of flooding. The most well-known is by decreasing the amount of snowpack. Mountain snow acts as a reservoir, holding back water in winter and releasing it in summer. As temperatures rise, storm events bring more rain and less snow. Much of that excess rain goes directly into our rivers, leading to bigger floods. The other major effect is the intensity of our rain events – research has only recently begun to quantify how our big storms might change in the future, and that modeling generally shows that those storms will bring more rain. More rain means larger floods. This is all summarized in a nice infographic, developed by the Skagit Climate Science Consortium, or SC2.

This is all good to know, but it doesn’t address two key questions:

1. What is the combined effect of sea level rise and changes in river flooding?
2. Where will all of this extra water go?

To find out we could look to FEMA flood studies – as readers of this blog will know, flood studies have been conducted for many communities around Washington State. The problem is that these studies rarely include climate change. Even when they do, flood mapping is piecemeal in coverage and often produced using different survey data and models. This means that there’s no way to make apples-to-apples comparisons of risk across the region. This is a key need, since we know that impacts will be much greater in some places than in others, and we want to make the best use of our limited resources.
 
This was the motivation for our recent project with the Snohomish Conservation District. Recent advances in flood modeling mean that we can now model flooding over much larger areas – even the entire U.S. – and the Snohomish Conservation District wanted information to support their Agriculture Resilience Plan. We worked with consultants at Fathom, Ltd. to evaluate and optimize the same model over both the Snohomish and Stillaguamish watersheds, and produced a set of simulations evaluating future flood extent over the entirety of the floodplains in the two watersheds.
 
All of our results can be found on the CIG page for this project, including a link to The Nature Conservancy’s Coastal Resilience mapping tool – where you can browse the results (click on the flooding icon, then select Snohomish County, and Flooding) – and two technical memos summarizing our approach and findings. Overall, we find that flooding increases the most for the 2-year flood, with +17 to +22% more area flooded on average by the 2050s. For the big events, like the 100-year flood, the results are more mixed – we are currently working to better quantify changes in these really rare events.
 
This project was a pilot — intended to test the approach and its application. I would love to hear any thoughts or suggestions you have on ways to improve on the work, and in particular if you think it would be worth expanding to a larger area. Feel free to contact me at gmauger@uw.edu.
 
Acknowledgements: This project was managed by Cindy Dittbrenner at the Snohomish Conservation District, with funding from the NOAA Community Based Restoration Program, the Estuary and Salmon Restoration Program (Grant #17-1308P), and the Stillaguamish River Lead Entity Capacity Fund. Jamie Robertson of The Nature Conservancy did all of the GIS work for this project: visualizing the results, providing critical technical review of the findings, and calculating the changes in flooded area.