Climate change is projected to lead to rising sea levels, declining snowpack and increasingly heavy rain events – all of which will contribute to more frequent and severe flooding. Most studies of future flood risk have focused on quantifying the amount of sea level rise or the increase in streamflow during flood events, but have stopped short of evaluating which areas are flooded and at what depth. The few studies that have modeled the extent and depth of future flooding (e.g., Hamman et al., 2016; Mauger and Lee, 2014) are limited in spatial coverage and are not easily comparable due to differences in the data and methods used. The purpose of this project was to pilot a new, inexpensive approach to developing a consistent set of flood projections over a much larger area. This allows for apples-to-apples comparisons of risk.

We quantified changes in the depth and extent of flooding for all but the uppermost reaches of the Snohomish and Stillaguamish watersheds. We developed the streamflow and sea level rise projections needed as input to the hydraulic model simulations, in both cases using the latest comprehensive set of projections for the region, and worked with consultants at Fathom, Ltd. to evaluate and optimize the flood simulations. In addition, we evaluated streamflow and sea level rise projections for use in a separate study of groundwater variations in the lower elevations of each basin. This project was conducted between June 2017 and June 2018.

Snohomish Flood Model 2018 GIS Map Package. Click and drag the file into another tab to start the download.

Snohomish Foot Bins 2018 TIFF Files. Click and drag the file into another tab to start the download.

The Sea Level Rise data visualization tool supports coastal decision making across Washington State.

The Washington Coastal Resilience Project has developed a suite of resources to improve risk projections, provide better guidance for land use planners and strengthen capital investment programs for coastal restoration and infrastructure.