By Katy Serafin, Postdoctoral Research Fellow, Stanford University
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When a storm event is barreling down on the Washington coast, elevated sea levels often cause low-lying areas to flood, while coastal dunes may erode as they are pummeled by waves. A question many of us may have is what is actually driving the flooding and erosion we experience? Is it large waves? Storm surge?  A high tide? A heavy rainfall event with subsequent peak streamflow? A combination of a few of these variables, or dominantly driven by one? Coastal and estuarine flooding and erosion events are complex problems, often driven by many different processes like waves, storm surge, tides, and streamflow. Thus, understanding the variability of specific processes and how they combine to drive extreme sea levels will help to identify vulnerable locations, information crucial for appropriately responding to risk.
​Photo of the 1/18/2018 storm on the outer coast of Washington. Photo: Rick Mraz (Washington Department of Ecology)
​Photo of the 1/18/2018 storm on the outer coast of Washington. Photo: Rick Mraz (Washington Department of Ecology)
​In order to better understand how different processes combine to drive extreme sea levels along the US West coast, our research uses total water levels (TWL), the superposition of high tides, non-tidal residuals, and wave runup, a wave-induced elevation of the water level and a function of beach slope, wave height, and wave length. We discovered that a latitudinal gradient exists in the magnitude of the annual and 100-yr TWL return level event (e.g., a 100% and 1% chance of occurring any given year, respectively) across the entire West coast, where TWLs are the highest magnitude in Washington and decrease moving towards California. This is largely because Washington State has the largest tidal range and some of the largest waves compared to Oregon and California. Along the high energy, open-coast US West coast environment, waves are the overall dominant driver of extreme TWLs during the annual and 100-yr event. There is, however, also a latitudinal gradient in the relative contribution to extreme TWLs, where TWLs in Washington are slightly more composed of high tides and non-tidal residuals than Oregon or California. This coast-wide perspective examines the regional variability of TWLs in the present-day to lead to a better understanding of how sea level rise, changes in storminess, and possible changes in the frequency of major El Niño events may impact future coastal flooding and erosion.
Aerial photo of Ocean Shores, WA from 8/12/2016. Photo: Shoreline Photo Viewer, Washington Department of Ecology: https://fortress.wa.gov/ecy/gisresources/shoreline_photos/yr2016/fullsize/160812_08945.jpg​
​While characterizing the drivers of extreme sea levels is important for understanding the evolution of the hazard in a changing climate, the magnitude of extreme TWLs is especially relevant when communities experience impacts from flooding events. For example, at both a regional and a site-specific scale (in this case, the North Beach Subcell, from Ocean Shores, WA to approximately Copalis Beach, WA) that takes into account nearshore-transformed waves and beach morphology, Washington has the highest TWLs compared to study sites in Oregon and California. Washington, however, experiences the least impacts (defined as the amount of times the TWL exceeds the toe of the dune contour) compared to our two other study sites in Oregon and California. This is largely because the North beach subcell is prograding, or growing (meters per year in some places!) and has high, wide sandy dunes. This isn’t to say that there aren’t locations that are impacted often (e.g., the erosion at Ocean Shores, WA), but compared to the study sites in Oregon and California, our Washington study site experienced less impacts overall.
Aerial photo of Copalis Beach, WA from 8/12/2016. Photo: Shoreline Photo Viewer, Washington  Department of Ecology: https://fortress.wa.gov/ecy/gisresources/shoreline_photos/yr2016/fullsize/160812_09007.jpg
​Open coast, sandy beaches are only one area of concern when planning for the impacts of flooding and erosion. Communities and infrastructure along estuarine environments may also have to include river-driven flooding in their estimates of extreme water levels. One such community is La Push, WA located at the mouth of the Quillayute River on the Quileute Indian Reservation. Much of the community’s infrastructure is adjacent to the river and/or the coastline and experiences flooding during high streamflow and/or ocean events. Statistical simulations of thousands of coincident streamflow and ocean events merged with a numerical model allowed for a robust characterization of the drivers of flooding along the ocean-to-river gradient. We discovered that high water levels along the river could be driven by either ocean, river, or a combination of both ocean and river processes. Thus “design'” events (e.g., the 100-yr event) could be underpredicted at specific locations if compound forcings (e.g., waves, rivers, tides, etc.) are not considered, misrepresenting the risk of flooding events the community faces.
Aerial photo of La Push, WA from 8/11/2016. Photo: Shoreline Photo Viewer, Washington Department of Ecology:
https://fortress.wa.gov/ecy/gisresources/shoreline_photos/yr2016/fullsize/160811_07581.jpg
​Our work shows that extreme water levels are driven by a range of forcings, many of which have changed in the past and will continue to change into the future. Small changes to the elevation of TWLs can drive large, nonlinear changes in impacts due to the threshold nature of coastal flooding and erosion events. For example, the same future change to the offshore wave climate may drive site-specific differences in TWLs and their related impacts due to variations in the geologic setting (beach topography and shelf bathymetry) and the relative contribution of different processes to TWLs.  As sea levels continue to rise and the frequency and intensity of storms change, appropriately characterizing compounding hazards producing extreme TWLs will strengthen many coastal communities’ ability to absorb impacts or adapt to extreme TWL events in the future.
 
See a more in-depth description of Katy’s PhD research at Oregon State University from her CHRN webinar on the WA Sea Grant YouTube channel:
​https://www.youtube.com/watch?v=skBwO33Fntg&t=16s