Flooding in Westport, Washington on December 14, 2024
This past December has been notably stormy, including a few events, especially on the coast (photo above) and along the Strait of Juan de Fuca (check out these great aerials from Three Crabs Rd near the mouth of the Dungeness River by John Gussman at DC Productions), that led to some flooding and other damage. The highest water levels of the month occurred on December 14th and were associated with a storm coinciding with one of this year's winter "king tides", But overall, the month has been characterized by a lot of positive "non-tidal residual" (referred to hereafter as NTRs), which is the part of coastal water level controlled by weather and not by the astronomical tides. NTRs are recorded by tide gauges, and for our purposes here are simply defined as the difference between the predictable astronomical tide, and the actual measured water level at a tide gauge.
The non-tidal residuals recorded by the Friday Harbor tide gauge in December 2012 and December 2024
Back in 2012 I wrote here about a similar December, during which NTRs were generally high for a good bit of the month. To illustrate this the plot above compares the NTRs from December 2012 and December 2024 as recorded at the Friday Harbor tide gauge. The things to note here are 1) that those NTRs were positive for long stretches of both months - in other words the measured water level at the tide gauge was higher than predicted most of the time and 2) that the maximum NTRs exceeded 0.50 m, or roughly 1.5 feet, on multiple occasions in each month. For our region those are relatively large NTRs.
The manual staff gauge used by coast watchers on Puget Sound to measure water level
But what I want to focus on a bit here is a bit of a rabbit hole I went down because of an email discussion I had the good fortune to be part of with a group of property owners on Puget Sound. They reached out to discuss some of their water level forecasts and observations, and I absolutely love these kinds of notes, and the thought that there are people out there that really dig into this kind of stuff. This particular group has devised a really savvy system for both measuring (using a manual staff gauge, photo above) and predicting extreme water levels on the coastline so they can prepare for flooding along their waterfronts. Amongst other things they were looking to discuss a few days in November during which their observations and predictions didn't really line up because of the difficulty in predicting NTRs.
NTRs (top) and air pressure (bottom) measured at three tide gauges in Washington over December 2024. The weird measure early in the month from Friday Harbor is a low-quality measurement that will eventually be removed from these preliminary data, downloaded from NOAA.
The thing about NTRs in Washington is that they generally are well correlated with atmospheric pressure - so high pressure generally leads to low, or negative NTRs (i.e. measured water level is near, or slightly below predicted), and low pressure generally leads to large NTRs, such that coastal water level is higher than predicted. Above and below, for example, are the NTRs and pressure measurements from three tide gauges in Washington, that show this relationship, referred to as an "inverse barometer" effect. The group that I was emailing with used this relationship in their forecasts, to predict the water elevation for their area by combining the predicted tide with atmospheric pressure forecasts.
NTR vs. Pressure for data collected at three gauges in Washington during December of 2024
But what this group pointed out in our email exchange were time periods when this relationship sort of broke down. In particular they pointed to a day in November...November 21st, 2024, when this general relationship between pressure and the NTR didn't hold, and diving into the data I think I'm starting to see what they are talking about. Here, for example, are the pressure vs NTR data for three tide gauges in Washington for November 2024, with the measurements from November 21 highlighted in red:
What is interesting here is that on this day there really is no relationship between pressure and the NTR...the pressure more or less stays the same whereas the NTR at each of these tide gauges is varying by over 0.3 m, or about a foot, over the course of the day. I don't have a great, clear way to explain this, except that it does point to two important lessons. First, the ocean is messy, and clean relationships almost never hold all the time. While there clearly is some relationship between atmospheric pressure and NTRs when looking at the data in aggregate, when we focus in on this particular day (and there are others like it), the relationship isn't so clear. Second, there are all sorts of other processes that can lead to variations in NTRs. My friend and colleague Eric Grossman at the USGS, for instance, has explored this problem in a paper, describing what they call remote sea level anomalies that are generated in the Pacific Ocean and can propagate into the Salish Sea. While it is not completely clear if the processes that are alluded to in this paper explain these observations, at least its a place to start.
As the winter storm season closes in on us on the coast of Washington, and on the eve of what is shaping up to be a pretty significant storm, I wanted to spend a few minutes looking back at the anatomy of a pretty interesting coastal storm that hit the coast and western and central Strait of Juan de Fuca last winter...on January 9th 2024. This one was notable to me because it was pretty damaging across the Port Angeles waterfront, and ended up shutting down Ediz Hook, a long, sand spit that forms Port Angeles Harbor that I spend a lot of time studying, surveying, and generally thinking about.
A view along Ediz Hook NOT taken during a storm...April 2012
While Ediz Hook used to be a relatively low sandy spit one of its distinctive features now is a huge rip rap defensive breakwater on its seaward side, part of an Army Corps of Engineers Erosion Control project built in the 1970s. This rip rap raises the elevation of the seaward crest of Ediz Hook to elevations of around 9 to 11 feet above MHHW...pretty darn high...and up until this January 2024 storm I had never observed or heard of anything close to over-topping of this feature. But this storm did it, sending water and debris over the breakwater, flooding the road that runs along Ediz Hook, and ultimately closing Ediz Hook until the storm subsided and crews could clear the road.
Flooding of Ediz Hook on 9 January 2024
So what were the ingredients? Like most of these stories it starts with a winter high tide, predicted at about 8.5 feet MLLW, just a shade shy of highest astronomical tide at this site (coming in at 9.2 feet). Also like most of these stories, that high tide was bolstered by a storm surge of about 1 foot, bringing the observed still water level up t 9.5 feet MLLW. This is a high tide, but well clear of the record of 10.5 feet MLLW reached in 2003, and not even close to making the top 10 highest water levels recorded at this station since observations started in the 1970s.
Driven by the high tide and storm surge alone, this storm was a nothing. But layered on top of this high water were waves...but interestingly the wave story has a similar narrative to that for tides and surge: The waves were big, but nowhere near record-breaking. A wave buoy just to the west of Port Angeles, for example, recorded significant wave heights of just about 10 feet, whereas the New Dungeness buoy in the Strait of Juan de Fuca recorded significant wave heights of about 8.5 feet. These are big waves, no doubt about it...but nowhere near maximums. The New Dungeness buoy, for example recorded a maximum significant wave height of over 12 feet during a storm in December of 2006.
So, like many of the impactful storms on the coast, this one was about a variety of factors coming together and co-occurring...in this case the tide, storm surge and waves. None of those factors was necessarily extreme in isolation but taken together they created something powerful and noteworthy. This kind of interaction amongst various factors, none of which are extreme or present risks in isolation, poses real challenges in understanding these types of storms and their impacts, accurately forecasting them, and even building a sense for how likely they are to occur in the future.
and it does still look very different than it did before dam removal started (check it out in 2011 here). But this year the site was also obviously different from last in that much of the deep mantle of sand deposited during dam removal had been stripped off. So that is interesting in and of itself...we've been curious about how the Elwha River delta would evolve in the long-term now that dam removal is long over, and this gives us some indication. But what really struck me was what was UNDER the remaining thin layer of sand on the seafloor...a few inch thick layer of silt and mud, which you can see in this photo below, with my slate for scale:
And get a sense for the composition in this video. So what is the big deal? This is a high current site, where mud and silt generally doesn't stick around. I think this layer of mud and silt tells us something about how rapid and how massive the delivery of sediment was during the dam removals. I envision a pulse of very fine material blanketing the sea floor, presumably followed almost immediately (perhaps within a tidal cycle?) by a pulse of sand, both pulses massive enough to blanket the seafloor with many inches of material...
Some of the cabins at Kalaloch are (or were) quite close to the edge of the coastal bluff. Photo from 6 July 2023
Eroding coastal bluffs are hard to manage, especially when valuable assets are perched on their edges. Coastal bluffs are by definition tall, and their erosion can be rapid and forced by a variety of processes, so engineering solutions aren't always easy to devise, and are invariably expensive and difficult to maintain. At Kalaloch in Olympic National Park, two factors recently converged to create particularly dangerous problems. First, rates of erosion of coastal bluffs in a small area just south of Kalaloch Creek accelerated, and second, the locations where those erosion rates accelerated also happened to be developed. Buildings, in this case valuable rentable cabins at Kalaloch Lodge, were put at increased risk and, as a consequence, in March 2024, five cabins were deemed unsafe and removed.
A similar perspective as in the photo above, with cabins removed. The former cabin sites are marked with the brown burlap. Photo from 10 April 2024
I was part of efforts that looked for every opportunity to try to understand that erosion, in the hopes that some cause of erosion might be identified that could illuminate a potential solution. As is often the case, though, the best we can do is use our monitoring data to characterize rates of erosion, and how those rates vary along the shoreline, as I did in this recent recorded talk.
Camano Island on December 27th, 2022. Photo courtesy of Joan Schrammeck
The coastal flooding event in Puget Sound on December 27th, 2022 was extreme, and notable for a whole variety of reasons that I explored in this post shortly after the event. I've also finally, after playing with the data for almost a year, convinced myself that this event had a near zero chance of occurring without the sea level rise observed in Puget Sound over the past 100 years or so. This conclusion, if it holds up to scrutiny, is also a big deal, as it could be the first documented evidence of an actual impact of observed sea level rise in Washington.
The way that I went about thinking about this was to first generate what is called a return frequency curve, using annual maximum water level observed in Seattle and recorded by the tide gauge in Seattle since its inception in 1898. I focused on Seattle first because it was impacted by flooding on 27 December 2022, and it has a nice long tide gauge record to support an analysis like this. It is worth noting that I haven't yet repeated this analysis for any of our other tide gauges in Washington, and suspect that the results wouldn't be as clear because of their shorter records. NOAA generates return frequencies too, but here is what mine looks like:
These curves are useful, as they allow us to use the history of extreme events recorded at tide gauges to analyze the expected frequency and magnitude of extreme events . On the y-axis is the maximum water level elevation, in feet relative to Seattle's Mean Higher High Water, and on the x-axis is years, from 1 to 10,000 years (the scale is logarithmic). We can use statistical models to fit a curve to these data, in this case I used a Generalized Extreme Value (GEV) model:
The GEV model fit to the observed annual extreme water levels from Seattle suggests that that the event on December 27th, 2022, during which coastal water level in Seattle reached an elevation of almost 4 feet above Mean Higher High Water, has an annual chance of occurrence of about 0.2%, or in other words should be expected to occur about once every 500 years on average. It was an extreme event.
But sea level is also rising in Seattle, and we know that even small amounts of sea level rise can significantly change the likelihood of these types of extreme events. How did Seattle's history of sea level rise influence the likelihood of the extreme flooding observed on December 27, 2022? another way of putting this is, what would the likelihood of the peak water level from 27 December 2022 have been if there was no sea level rise occurring in Seattle? To try to answer this question I simply removed sea level rise from the record in Seattle (assuming it is a linear rate, which is what NOAA calculates, but may not be the case), and then re-calculated a return frequency curve (shown below in red):
and then repeated the process of fitting this set of hypothetical extreme events from Seattle with a new GEV model:
So here the impact of sea level rise on the return frequency curves is readily visible. But if we follow the water level of the December 27, 2022 event (the highest black dot in the plot) over to the left to find where it intersects the red curve...we see that it never intersects the red curve. This suggests that, based on the best fit to the data, the extreme water level reached during the December 27th event, at least as recorded in Seattle, would have been improbable in the absence of sea level rise.
Now these statistical models are uncertain, because they are built around a very small number of events (they are, after all, extreme), and above I show the 95% confidence intervals (the red dashed lines) around the best fit to the observations (the solid red line). Here the upper confidence bound DOES intersect with the water level elevation reached on December 27, 2022. But even if we assume that the "real" return frequency curve follows that upper confidence limit we find that the event observed on December 27th 2022, with sea level rise removed, has a return frequency of, at best, once in 700 years on average. At the very least it seems safe to conclude that sea level rise that has already occurred in Seattle made the extreme water level reached on December 27th 2022 more likely (a 1-in-700 year event becomes a 1-in-500 year event) than it would have been.
**I'm not extraordinarily well-versed with extreme value analyses, so I reached out to a colleague for a review (which I don't typically do with these blogs) and wanted to thank that reviewer for their time and contributions. I'm going to keep that person anonymous only to emphasize that any errors of interpretation are mine alone. I also want to emphasize that here I've assumed two things: 1) that sea level rise is the ONLY component of coastal flooding that is changing, which I think is likely the case but I didn't test, and that 2) the rate of sea level rise in Seattle is linear, which I think is a good assumption but could be incorrect.**
Another large gap in my blog, but I've got an excuse this time - I've been doing a good bit of publishing, and that has taken up whatever bits of time I can find for writing. And I have to say that I'm pretty pleased with what I've been able to be a part of this year, both in terms of the number of manuscripts, but also because of the variety, and a because most are open access. So here is the list thus far:
1) The first paper of the year came out in March, after I wrapped up a >2-year long project coming up with ways to try to measure sea level rise vulnerability in Puget Sound along with colleagues from Coastal Geologic Services. Typically, these kinds of projects end with something like a technical report but I was so interested in the results that I really wanted to carry it forward to publication.
2) The second of the lot, and the only one that is not open access, is the conference paper above (for the Coastal Sediments conference), that I wrote along with George Kaminsky from the Washington Department of Ecology, and Adrianne Akmajian from the Makah Tribe. I was pretty excited to get this one out, just because it allowed me to get a bit back into the sort of shoreline dynamics world that I love. This one is NOT open access, but available if you send an email to immiller@uw.edu
3) The next three are part of a special issue of Frontiers in Ecology and Evolution focused on large dam removals, and span a range. The big one for me was the second paper I've written with my dive team collaborators about our long-term investigation of the marine ecosystem in the Strait of Juan de Fuca. This one was definitely my big push for the year, and I am absolutely thrilled that it is out. It is open access, so find it here.
My first paper that I consider "terrestrial" was super fun to be a part of, though my contributions were pretty minor, was about how the vegetation community near the Elwha River mouth changed through dam removal. This one is accepted but not published as of yet, but should be shortly.
And the final paper in the special issue was entirely different for me, and also one that I made pretty small contributions to, but I'm proud of none-the-less. It is focused on documenting how we, across the entire Elwha research community, partnered with the community in various investigations. Find it here.
4) Will there be one more? I am deep into drafting another paper for a special issue of Oceanography, that will likely come out in 2024...but who knows...maybe we will get it out sooner....
Photo by David Barker, submitted to the King Tides program via MyCoast, on the morning of December 27th. Gig Harbor area
Well somehow I went a whole year without a post...not exactly sure what happened to be honest, except that I was very focused this year on a few projects that are now wrapping up. But an event that hit the Washington shoreline in the last week of December was more than enough to snap me out of it. The long and the short of it is that this event broke high water records at all but one of the tide gauges in the Puget Sound basin, some of them set 40 years ago. It was a doozy.
Photo by Joan Schrammeck, submitted to the King Tides program via MyCoast, on the morning of December 27th. Camano Island.
Most of the record-breaking for this event happened in the Salish Sea, and really in the Puget Sound basin, even though it was no walk in the park for Pacific Coast shorelines either. The photo below, for example, was taken at the trailhead on Rialto Beach in Olympic National Park on December 26th. The crest of the berm here is at an elevation of roughly 18 feet above MLLW, and if the water pushes much higher under the influence of tides, surge or wave run-up it spills down into the parking lot just behind.
Photo by Kim Sager-Fradkin, posted here with permission. Rialto Beach, Olympic National Park, December 26th.
But back to the Puget Sound basin, and here are the breakdowns of coastal water level records for the five tide gauges within the Puget Sound basin that set records during the event, specifically on the morning of December 27th, from north to south:
A few things of note here. First, the max water levels recorded in the table above are unverified as of yet, so may change. The Bremerton tide gauge is new, so not a surprise that a record was set there, but records were absolutely shattered in both Friday Harbor and Seattle, where tide gauges have been recording water level for 89 and 125 years, respectively. If the water levels are verified as they are reported in the table above, then Seattle's water level during this event was a full 7 inches above the record set just last year. To try to put that into context, that record was set last year on January 7th, 2022, by just barely exceeding the previous record (that one set in 1977) by less than half an inch.
Another surprise for me is just how wide-ranging this event was spatially...to have records set at tide stations in north and south Puget Sound on the same tide is notable to me. The various photos, and the astonishing video posted above, from the Point No Point area on the northern Kitsap Peninsula also illustrate this - they are coming in from all over Puget Sound.
What made it happen? The most obvious answer is that we rolled snake eyes in the annual game of chance that we play along the shoreline. The plot above is of water level (top panel), pressure (middle panel) and the "non-tidal residual" (storm surge, or the difference between the predicted and measured water level) for both coastal (left column) and Puget Sound (right panel) tide gauges. Essentially a high astronomical tide coincided with the peak of the low pressure passing through the area on December 27th, and that low pressure drove a very large (for our area) non-tidal residual that peaked at exactly the same time as the high tide. That same "perfect storm" didn't happen on the coast. Instead, the peak of the low pressure and non-tidal residual coincided with a falling tide, so while water levels were very high, they didn't shatter records.
But there are at least two other likely contributors that I'm not prepared to really rigorously attribute, but almost certainly played some role in driving water to record heights on December 27th. The first is sea level rise. At most of the locations in the table above there is a documented relative sea level rise trend. While the rate associated with that trend vary across those locations, in general they equate to something along the lines of a few inches over the four decades that have passed since many of our previous coastal water level records were set during the 1982-1983 El Nino winter. These few inches didn't "cause" this event, but they did contribute, and this on-going sea level rise makes it very likely that the records set in this December 27 event will be broken again. The second is the lunar nodal cycle, which is illustrated in the plot above this paragraph and was the subject of some media reporting last year. This astronomical cycle operates over a ~19 year period, and influences the range of the tides, or the difference between the average high and low tides. In other words, at the peak of this cycle, on average, high tides are a bit higher, and low tides are a bit lower. The data I show above are from the Friday Harbor tide gauge, and what I've done here is to average all of the highest tides for each year going back to the 1960's (to capture a few of these cycles)...and during the peak of the cycle the high tide is, on average, about 6-7 inches higher than it is during the bottom of the cycle. And we are, as you can see, heading towards a peak in this cycle. Again, this didn't cause this event, but may have contributed a bit to it.
