The observed and anticipated increasing trends in extreme storm magnitude and frequency,
as well as the associated flooding risk in the Pacific Northwest highlighted the need for revising and updating the local intensity-duration-frequency (IDF) curves, which are commonly used for designing critical water infrastructure. In Washington State, much of the drainage system installed in the last several decades use IDF curves that are outdated by as much as half a century, making the system inadequate and vulnerable for flooding as seen more frequently in recent years. In this study, we have developed new and forward looking rainfall and runoff IDF curves for each county in Washington State using recently observed and projected precipitation and watershed data.

Regional frequency analysis coupled with Bayesian uncertainty quantification and model
averaging methods were used to developed and update the rainfall IDF curves, which were then used in hydrologic model to develop the runoff IDF curves that explicitly account for effects of snow and drainage characteristic into the IDF curves and related designs. The resulted rainfall and runoff IDF curves provide more reliable, forward looking, and spatially resolved characteristics of storm events in Washington State that can assist local decision makers and engineers to thoroughly review and/or update the current design standards for urban and rural stormwater management infrastructure.

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