Full Title

Effect of Climate Change on Flooding in King County Rivers

Project Overview

Climate projections indicate an increase in flooding in many Pacific Northwest watersheds over the course of the 21st century, in response to both declining snowpack and more intense heavy rain events. Research has shown that dynamical downscaling, using regional climate models, is needed to accurately project changes in heavy rain events (e.g.: Salathé et al., 2014). Recent work by the UW Climate Impacts Group and Department of Atmospheric Sciences has developed an ensemble of new regional climate model projections using the Weather Research and Forecasting (WRF) model.

A key feature of these projections is that they provide hourly estimates of future weather conditions (temperature, precipitation, humidity, wind, etc.) and account for changes in both the form of precipitation (i.e., rain vs snow) and storm event intensity. In this study, we used the new projections to model changes in future streamflow and evaluate potential changes in peak flows on the Snoqualmie, South Fork (SF) Skykomish, and Green rivers. For the Green River, we also accounted for the effect of reservoir operations by the U.S. Army Corps of Engineers (USACE) at Howard Hanson Dam.

The work included two phases: In Phase 1 we developed hydrologic models for both basins and produced initial results based on just two WRF projections. In Phase 2 we added 12 new WRF projections for a high greenhouse gas scenario (RCP 8.5), providing a large enough ensemble to assess the mean and range among future peak flow projections. In a parallel study we evaluated these same projections to quantify changes in heavy rainfall for use in stormwater management and planning.

Datasets and Products

All results are available online.

[Note as of October 15th, 2018: Correction to Table 6.3. In the previous version, the projections listed in the RCP 8.5 column for bcMACA were incorrect. These were replaced with the correct values.]

[Note as of November 24th, 2020: A mistake was identified in the climate data bias-correction procedure. All results have been corrected, leading to an update of the data linked above as well as Figures 1-5 and Tables 2-4 in the report]

Project Contact

Questions can be director to Guillaume Mauger (Principal Investigator) Climate Impacts Group, University of Washington.

Funders & Acknowledgements

This project was primarily funded by the King County Flood Control District, with additional support from the Critical Infrastructure Resilience Institute (CIRI), a U.S. Department of Homeland Security S&T Center of Excellence (award number 2015-ST-061-CIRC01).

We would particularly like to thank the King County staff members who provided invaluable feedback and suggestions over the course of the project: Curtis DeGasperi (Project Manager), Jim Simmonds, Lara Whitely Binder, Jeanne Stypula, Lorin Reinelt, Brian Murray, Kyle Comanor, and Fred Lott.

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