Land use change and projected climate change have raised serious concerns about the future vulnerability of ecosystems and infrastructure worldwide to extreme drought and flooding events. While in some locations precipitation may decrease overall, it is expected to increase in the Pacific Northwest. Previous studies have projected widespread increases in flooding during the 21st century due to the combined effects of increasing cool season precipitation and rising snow levels during storms.

Previous work modeled Pacific Northwest flooding using statistically downscaled monthly global climate model data and a physically based hydrologic model. Global and monthly data however, while useful for projecting general trends, cannot account for the variability seen on smaller spatial and temporal scales, likely resulting in underestimates of future flood risk. In this project we used a regional climate model to better evaluate the spatial distribution of changes, and at finer time scales.

The two main causes for increases in future flood risk are: more extreme storms and warming temperatures that shift precipitation from snow to rain. This research suggests that future changes in heavy rainfall will have serious implications for flood risk in the Pacific Northwest.

This paper presents climate projections from the ECHAM5/Max Planck Institute global climate model dynamically downscaled using the Weather Research and Forecasting (WRF) mesoscale model for the period 1970–2069. The resulting daily precipitation, temperature, and wind speed data were bias-corrected and used as input to a physically-based Variable Infiltration Capacity hydrologic model.

The global model has a grid spacing of 150-200 kilometers with 32 vertical levels, and an emissions assumption of “business as usual” during the first half of the 21st century followed by greater mitigation in the second half (A1B scenario). The regional model used nested grids at 36 and 12 kilometer spacing. Both featured six hourly outputs. The hydrologic model used one-sixteenth degree latitude–longitude grid spacing (about 5x7 km), and was run for 297 individual watersheds in the Pacific Northwest region with daily streamflow outputs. We then estimated the 100-year event magnitude by fitting an extreme distribution to the peak flows estimated for each water year.

Effect of Climate Change on Flooding in King County Rivers. CIG scientists developed estimates of future flooding on the Green, 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 River, we also accounted for the effect of reservoir operations by the U.S. Army Corps of Engineers (USACE) at Howard Hanson Dam.

Heavy Precipitation Projections For Use in Stormwater Planning. This project summarizes the projected changes in heavy rainfall events, from a new set of RCM projections, for use in stormwater and CSO planning. Results are designed to support both municipal and rural stormwater planning. The online tool allows users to evaluate projections as a function of precipitation intensity, duration, and frequency. The online repository includes summary statistics as well as time-series data for use in continuous simulation modeling.

Mapping the Future of Flood Risk for the Stillaguamish and Snohomish Rivers. We developed a data visualization for flooding along the Snohomish and Stillaguamish watersheds, and wrote two technical reports describing our approach and methods for this work. We hope planners and resource managers in these areas will find the visualization useful in informing climate-savvy decisions. The technical reports are intended for decision makers and scientists who want to better understand, and possibly replicate, our methods.

Climate Impacts and Adaptation Fact Sheets for Flood Risks Managers. We partnered with The Nature Conservancy and the Floodplains by Design initiative to produce a set of fact sheets to serve as a quick reference on climate change impacts for six Puget Sound rivers. We also produced a fact sheet highlighting a variety of flood risk adaptation actions being taken in Washington State. These seven facts sheets are intended to make climate change information accessible and relevant to resource managers, city planners, engineers and others who need to plan around changing flood risks in their work.

This research was supported through funding from the U.S. Army Corps of Engineers. Dr. Hamlet also received partial support for this project from a grant from the National Science Foundation (Grant EAR-0838166). We thank Robert Norheim of the UW Climate Impacts Group for producing Figures. 1, 3, and 4.

Photo by Sounder Bruce, CC-by-SA-4.0