New tool can help plan for future flooding

Written by Kris Johnson, Senior Scientist, The Nature Conservancy

 Flooding is increasingly becoming a fact of life along the Snohomish River. In early December the severe flooding in the cities of Snohomish and Monroe inundated homes and farms and closed roads and a city park. For residents in the area it was déjà vu all over again, as just weeks before some of the same areas experienced a separate major flooding event. Clearly, managing flood risk along the Snohomish, and throughout western Washington, is challenging yet essential to the lives and livelihoods of millions of people.

And climate change is compounding this problem: warmer temperatures are leading to sea level rise and higher tides, and more winter precipitation is falling as rain rather than snow, causing flashier flows in the rivers. Understanding how flood risk is changing and incorporating best-available information into planning and decision-making is crucial for communities in Puget Sound.

With this in mind the Nature Conservancy led a recent pilot project to both evaluate how climate change might impact flooding and then provide this information to local decision-makers. In partnership with the University of Washington Climate Impacts Group and West Engineering we assessed how varying scenarios of climate change could alter flooding in the Lower Snohomish River and cause potentially greater impact to farms and buildings and infrastructure.

This analysis suggested that in just a few decades what is currently considered a “100-year” flood could be much more severe and could inundate part of I-5 near Everett and cause nearly 20% more costly property losses (see image).

To make this information useful and readily available we created new Floodplains by Design ‘app’ on the Coastal Resilience decision support tool so that local partners with the Sustainable Lands Strategy in Snohomish County could integrate maps and images of potential flooding into their project planning and decision-making process.

Managing increasing flood risk while sustaining high-value agriculture and also restoring salmon populations and protecting the environment will be a major challenge in Puget Sound in the 21st century. Rigorous science and sophisticated tools, like those developed by The Nature Conservancy for the Lower Snohomish River, can provide local communities with the best-available information that can help them plan accordingly.