Hamlet Flooding Risk Estimates

Estimates of changing flood risk in the 21st century Pacific Northwest based on regional– scale climate model simulations

US Army Corps of Engineers Grant: $175,000 total funding from 2010 to 2011
Additional funds needed for completion: $300,000

Background and motivation: Previous assessment of changing hydrologic extremes using statistically downscaled monthly time-step global climate model data and physically based hydrologic models has demonstrated that many areas in the Pacific Northwest are likely to experience substantial increases in flooding in response to regional climate change. In particular, many mountain watersheds in Washington and Oregon west of the Cascades are projected to experience increases in the 100-year flood (Q100) of 20–30% by the 2040s. These initial estimates of changing flood risk are very conservative in that they are fundamentally based on monthly global climate model data, and do not include potential changes in precipitation extremes at daily time scales. Furthermore, initial studies using regional climate models have shown robust increases in precipitation intensity on the west slopes of the Cascades, suggesting increased flood impacts in these areas.

To further evaluate these potential changes, we downscale daily precipitation, temperature, and wind simulations from the Weather Research and Forecasting regional climate model implemented at 12 km resolution, and use the resulting data to drive a physically based hydrologic model (The Variable Infiltration Capacity (VIC) model at 1/16th degree resolution).

Extreme storm (Nov 6, 2006) simulated by the WRF climate model (right) compared to gridded observations (left)

Objective: The overall objective of the study is to use the more detailed projections of future winter storms to evaluate changes in natural flood risk associated with climate change across the Pacific Northwest.

Project Team: The project team consists of Alan F. Hamlet, Eric Salathé, Matt Stumbaugh and Se-Yeun Lee, all at the University of Washington in Seattle.

Expected outcome (2011): Preliminary results from the study for the ECHAM5 A1B scenario for the 2050s show substantial increases in future flood risk in many Pacific Northwest river basins in the early fall due both to more extreme (and earlier) storms, and coincident shifts from snow– to rain-dominant systems due to warming. Unlike statistical downscaling techniques, the regional climate model provides an explicit, physically based simulation of the size, location, and intensity of historical and future extreme storms, including atmospheric rivers. These daily time-step extreme weather scenarios for the 2050s also provide valuable information for other planning purposes related to hydrometeorological extremes.