CIG scientists developed estimates of future flooding on the White, Green, Cedar, Snoqualmie, and South Fork Skykomish rivers. The aim of this study is to provide King County decision makers with relevant, specific data that will allow them to account for climate change in their flood risk management practices. For the Green, Cedar, and White Rivers, we also analyzed the potential effects of reservoir operations on future peak flows.

Phase Three: Final Data and Reports

Snohomish River Watershed

ReportDataTool

Cedar River Watershed

Report in progress.

DataTool

Green River Watershed

Report in progress.

DataTool

Puyallup River Watershed

Report, data and tool in progress.

 

Project Details

Project Background

Climate change is projected to result in less snow in the mountains and heavier rain events. These changes are expected to lead to an increase in flooding in many Pacific Northwest watersheds. To adjust their management practices to prepare for higher peak streamflows, decision makers need accurate, up-to-date projections of future streamflows.

This dataset differs from previous projections of flooding in King County because it draws on a new set of regional climate models, which provide more accurate estimates of changing heavy rain events. Previous projections of flooding in these watersheds relied on statistical downscaling, which looks at statistical relationships between global climate models and local observations to estimate future hydrologic changes at the regional level. The new projections rely on dynamical downscaling (see first link below), which models the physical processes, at local scales, associated with global climate model projections. 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). The dynamically downscaled regional climate models were developed in part with funding from King County.

A key feature of the projections produced as part of this project is that they provide hourly estimates of future weather conditions (temperature, precipitation, humidity, wind, and radiation), whereas previous projections only included daily variations in temperature, precipitation, and winds.

LEARN MORE: DYNAMICALLY DOWNSCALED HYDROCLIMATE DATALEARN MORE: ESTIMATES OF FLOOD RISK IN THE PNW (Salathe et al., 2014)

Approach

We modeled future streamflow using the Distributed Hydrology Soil Vegetation Model (DHSVM), Wigmosta et al. 2002). The model was calibrated to match observations on each river, and the calibrated model showed good performance for both monthly and extreme flow statistics. Changes in streamflow were estimated by using the new regional climate model projections, developed under a parallel study, as input to the DHSVM model simulations.

The work included three phases: In Phase 1 we developed hydrologic models for both basins and produced initial results based on just two regional climate model projections. In Phase 2 we added 11 new regional climate model 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. In Phase 3, we refined the approach, in particular to ensure better consistency between the historical and future datasets, and expanded the modeling to include the Cedar and Puyallup river basins. Additional detail is provided in the Phase 3 reports.

Phase One DataPhase One ReportPhase Two DataPhase Two Report

Related Studies

Additional studies and resources linked below.

Climate Impacts and Adaptation Fact SheetsMapping the Future of Flood Risk for Snohomish and Stillaguamish RiversRegional Model Projections of Heavy Precipitation for use in Stormwater PlanningRegional Climate Model Simulations of 21st-Century Flood RiskRegional Modeling of Windstorms and LightningAdditional Climate Impacts Group projects related to flooding

This project was primarily funded by the King County Flood Control District, with additional support from Pierce County, the Cascade Water Alliance. For the Phase 1 work, additional support came 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). The regional climate model simulations were funded by the Amazon Catalyst program with additional support from the King County Department of Natural Resources and Parks (DNRP) and Wastewater Treatment Division (WTD).

We would particularly like to thank our collaborators at King County, Seattle Public Utilities (SPU), and the U.S. Army Corps in this work. This includes current and former King County staff: Mark Beggs (Project Manager for Phase 3), Curtis DeGasperi (Project Manager for Phases 1 & 2), Lara Whitely Binder, Chase Barton, Chris Brummer, Jason Wilkinson, Aaron Kopp, Tracy Winjum, Judi Radloff, Jim Simmonds, Fred Lott, Jeanne Stypula (retired), Lorin Reinelt (retired), and Brian Murray (now at U.S. Army Corps); SPU staff: Elizabeth Garcia, Matthew Porter, John Edgerly; and U.S. Army Corps staff: Kyle Comanor, Joanna Curran, Sonja Michelsen, and Logan Osgood-Zimmerman (now at Bucknell University). The 12 new regional climate model simulations were produced by Rick Steed and Jeff Baars, under the direction of Cliff Mass. Raquel Lorente produced the 13th regional climate model simulation. Ken Brettman and Logan Osgood-Zimmerman at the U.S. Army Corps of Engineers (USACE) provided invaluable help on both the reservoir modeling and the key flow metrics of interest to reservoir operators.