Effect of Climate Change on Flooding in King County Rivers

Full Title

Effect of Climate Change on Flooding in King County Rivers: Using New Regional Climate Model Simulations to Quantify Changes in Flood Risk

Project Overview

Climate projections indicate an increase in flooding in many Pacific Northwest watersheds over the course of the 21st century, in response to an increasing proportion of mountain precipitation falling as rain instead of snow. Global climate models also project an increase in the intensity in the type of heavy rain events that cause most river-scale flood events. A parallel study, funded by the King County Department of Natural Resources and Parks (DNRP) and the Washington State Department of Ecology,1 has produced a new set of projections of 21st century climate, developed using a regional climate model. Specifically, the parallel study was funded by King County’s Stormwater Services Group (within DNRP’s Water and Land Resources Division), the Wastewater Treatment Division (also within DNRP), and a grant from the Washington State Department of Ecology.

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, 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.

Approach

We modeled future streamflow using the Distributed Hydrology Soil Vegetation Model (DHSVM, Wigmosta et al. 2002) to estimate changes in future streamflow. 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 the parallel King County study, as input to the DHSVM model simulation. The new regional model projections were selected to bracket the range from the low- to the high-end of precipitation changes among global models. For comparison, we also simulated future streamflow using an existing set of climate projections. Although developed using a simpler approach (“statistical downscaling”), these provide a larger set of estimates of future climate, which can be helpful for evaluating the range among projections. For the Green River, we then performed an additional set of simulations to evaluate the effect of reservoir management on peak flows downstream of Howard Hanson Dam.

This project builds on a parallel study, funded by the King County Department of Natural Resources and Parks (DNRP) and the Washington State Department of Ecology, in which we produced a new set of projections of 21st century climate, developed using a regional climate model: https://cig.uw.edu/our-work/applied-research/heavy-precip-and-stormwater/

Application/Intended End-users

King County and communities affected by flooding on the Green, Snoqualmie, and South Fork Skykomish rivers.

Datasets and Products

All results are available online here: http://cses.washington.edu/picea/mauger/2017_12_KingCounty_FloodModeling/DATA/pub/

The final report can be found here: UW-CIG_KingCounty_ClimateAndFlooding_FINAL-compressed_v10192018

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.

Key Personnel

  • Guillaume Mauger (Principal Investigator), Climate Impacts Group, University of Washington
  • Se-Yeun Lee, Climate Impacts Group, University of Washington
  • Jason Won, 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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