Climate change is expected to affect flooding via rising sea levels, heavier precipitation events and receding snowpack. Other changes — particularly wildfire and sediment processes — could also exacerbate flood risk in the Puyallup watershed. In addition to the consequences for flooding, climate change may have other impacts relevant to managing floodplains, including lower summer streamflows, warmer water temperatures, saltwater intrusion in the lower basin and hotter and more frequent heat waves.

In order to properly design and prioritize floodplain efforts, the Floodplains for the Future Partnership needs better information about the risks posed by climate change. Existing studies have begun to quantify climate change impacts as well as the specific vulnerabilities of the different interests (floods, farms and fish) in the Puyallup River basin. However, past studies have not been centralized or coordinated in a way that supports climate-resilient floodplain planning, nor have the studies addressed the variety of climate change impacts that are anticipated.

These gaps do not simply concern the projected changes in specific impacts, but also the relative benefits of available solutions and the barriers to implementing needed changes. The purpose of this work is to produce a central document detailing climate change impacts of concern, existing science that quantifies these impacts and gaps in the science that may limit informed decision-making. This will facilitate a consistent approach to assessing and addressing climate change impacts among Floodplains partners.

All of our work was conducted in close collaboration with Floodplains for the Future partners and stakeholders. Based on initial discussions and responses to a presentation on climate change impacts, we developed a series of “climate briefs” that summarize climate adaptation concepts, climate change sensitivities, impacts and gaps in the science related to climate change in the watershed. The first three were developed as a quick reference for Floodplains partners to use as they work to integrate climate change in floodplain planning. The fourth brief, on science gaps, involved more in-depth conversations with Floodplains stakeholders about the needs for planning, and extended beyond quantifying physical climate impacts by including: (1) knowledge gaps related to sensitivity and adaptive capacity, and (2) base-level system understanding that is needed before evaluating climate change impacts (e.g., current sediment processes).

We then used the gaps document to generate a set of “mini-scopes” — short one to two paragraph descriptions of the work needed to address each gap, including the types of skills needed, approximate timeline and level of effort. These were presented to the Floodplains Partnership, which then facilitated a discussion to identify key priorities among the list. We then worked to support implementation of the priority projects through more detailed scoping, identification of potential funding sources and consultants or other contractors who could do the work.

Increasing Capacity for Floodplain Management in Whatcom and Snohomish Counties. This project will increase the capacity for climate-resilient floodplain management by working with integrated management groups to raise awareness about climate change, identify key impacts, synthesize the relevant science and coordinate priorities for new science. The work will focus on Whatcom County and Snohomish County.

Integrating Climate Resilience in Flood Risk Management. We worked with the Washington State Silver Jackets – an interagency group that includes FEMA, the Army Corps, and several state and federal agencies charged with managing flood risk – to explore how they and their member agencies can better integrate climate change impacts into flood risk management in Washington State.

Effect of Climate Change on Flooding in King County Rivers. 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.