Climate Change and Culverts: Supporting Climate-Resilient Design

Numerous studies have shown that climate change can have a large effect on peak streamflow for many Pacific Northwest streams, primarily as a consequence of declining snowpack and more intense heavy rain events. Higher peak flows are expected to result in wider stream channels. This is important because new culverts are typically sized based on channel width. Since the expected service life of a culvert typically ranges from 50 to 100 years, culverts designed based on today's channel width are unlikely to be sized appropriately by the end of their service life in many parts of the region. This would have significant adverse impacts on fish populations, including many threatened and endangered salmonid populations. Washington State is already subject to a federal injunction requiring the state to replace about one thousand undersized culverts that are an impediment to upstream migration, at an estimated cost of $4 billion. This does not include the cost to replace culverts that are not covered by the injunction, which could result in a 10-fold increase in costs.

This presentation will describe the most recent phase of an ongoing collaboration with the Washington Department of Fish and Wildlife (WDFW) - the state agency in Washington that has regulatory authority over culvert construction - to develop new modeling and interactive tools to support climate-resilient culvert and habitat restoration design. The work includes important scientific advances, using new dynamically-downscaled projections that better capture changes in heavy rain intensity, as well as substantial improvements in the representation and delivery of the results. In particular, the team has developed a new online tool that provides engineers with site-specific projected changes in future streamflow and future channel width for culvert and in-stream restoration design. Ongoing engagement with civil engineers has helped to ensure that the tool is usable and includes the information they need in their work. The presentation will highlight the new tool while also synthesizing results of the modeling and key feedback from prospective users.