Flooding in Estuaries

Estuaries, where ocean tides meet river flows, are both beautiful and hazardous. Estuaries are known for their floods. But estuaries have another problem: the physical dynamics that lead to estuarine flooding are notoriously hard for scientists to track and model.

The multiple, compounding factors that lead to estuarine flooding are the subject of a paper published last summer in the journal Natural Hazards and Earth System Sciences.

The paper was led by CIRC alumna and Assistant Professor at the University of Florida Katy Serafin. Serafin was joined on the paper by CIRC researcher, Oregon State University Professor, and Interim Director of the Oregon Climate Change Research Institute (OCCRI) Peter Ruggiero. CIRC alumnus Kai Parker, and David Hill, an OSU researcher studying estuarine dynamics, also joined the effort.

(And while it is neither here nor there, the researchers’ study employed the puntastic title “What’s Streamflow Got To Do with It,” which explicitly references Tina Turner’s 1993 classic album, song, and biopic movie, “What’s Love Got to Do with It?” We would’ve been remiss in our blogging duties had we not mentioned this.)

Here’s the problem the team hoped to address. Due to their dual nature—part ocean, part river system—the flooding that occurs in estuaries is often driven by multiple factors. This includes the effects of the tide, how much water is flowing in the river or rivers that feed the estuaries, and storm surges from the ocean.

To study the dynamics of estuarine systems, Serafin and colleagues examined the estuary at La Push, Washington. The relatively small and narrow estuary marks the meetup spot of the Quillayute River and Pacific Ocean.

The low-lying community of La Push sits at the mouth of the Quillayute River, and has seen its fair share of flooding. Serafin, Ruggiero, and colleagues were uniquely qualified to study flooding along the river.

While at CIRC, Serafin and Ruggiero jointly developed a model of coastal flooding that calculated what’s called total water levels.

The Serafin-Ruggiero total water levels calculation, what they call SR 14 (not to be confused with the fabulous highway running through the equally fabulous Vancouver, Washington), models the combined effects of multiple factors, including the tides, storm surges, waves, sea level rise, and changes during El Niño events (which, via thermal expansion, can raise local sea levels by several inches). Combining these factors adds up, and gives you a total water level that is higher than you would get from any one factor working alone.

Serafin and Ruggiero’s initial total water levels work was done for two separate CIRC-related projects, the Tillamook Coastal Future Project and the Grays Harbor Coastal Futures Project, which tracked flooding hazards for two coastal communities along the Pacific Northwest coast. (More on that below.) River processes were not an important contributor to flooding in these two communities. However, understanding river processes would prove to be key to understanding flooding at La Push.

For the La Push effort, Serafin and Ruggiero essentially took the Serafin-Ruggiero (SR 14) analysis used to examine coastal flooding and applied it to the more complex environment of the estuary. As with their previous work, the researchers were interested in the compounding effects that could lead to total water levels that would result in flooding. With an eye on local river flows, the researchers used measurements from tide gauge data available for the years 1980 to 2016 that tracked water levels in the estuary where the Quillayute River meets the Pacific Ocean.

From this tide gauge data the researchers were able to determine the height of the water level near the ocean and the flow of the river, or the river discharge, giving them an understanding of how high the water level could get along the whole river. Having this big picture of the whole river gave the researchers the ability to analyze how the river’s flow interacted with the incoming tide and storm surges from the Pacific.

Using the tide gauge data, the researchers then employed several models, including their own total water level SR 14 model, to simulate the effects of the incoming tides and storms from the Pacific, streamflow along the river system, and the height of the water level along river.

We won’t go into the details of the modeling approach the researchers used here, but for simplicity sake you can think of Serafin and colleague’s approach in this paper as a kind of hybridization of a statistical-based modeling approach and a physics-based modeling approach.

This hybrid approach was taken, as the researchers explain, because they were interested not only in understanding and categorizing flooding that has occurred in the past (as represented in the tide gauge data)—which a statistical approach would have given them—but also flooding that was physically capable of occurring in the past along the full length of the Quillayute River (as opposed to just one point downstream)—which the researchers could only get if they considered what is physically possible using a physics-based modeling approach combined with a statistical-based modeling approach.

The reason the researchers wanted to know what could have occurred is because, well…being physically capable of occurring means that it might still occur in the future, even if it didn’t in fact occur in the past.

Using their hybrid approach, Serafin and colleagues were able to construct a probable synthetic record comprising both extreme flooding events that occurred from 1980 to 2016 and extreme flooding events that were physically capable of occurring during the same 36-year period but that did not actually occur.

In total, the researchers created seventy 500-year-long synthetic records for the period 1980 to 2016, meaning…yeah, they ran the models a lot, creating a sort of multi-verse of possibilities that dwarfs even what Marvel Comics has been able to achieve over the years.

Serafin and colleagues found that the hybrid approach paid dividends, producing results that challenged conventional research wisdom. Here’s how.

The researcher’s super-detailed synthetic record allowed them to also create seventy 100-year events for storms and river heights. A 100-year event is so-named because it has just a 1% chance of occurring within any given year. To put it another way, it’s super rare. But this rareness also implies that a 100-year event is well-beyond normal, and has likely not occurred in the observational record. Hence it’s hazardous and needs to be better understood. But, as it turns out, past research hasn’t been entirely up to the task. In fact, much previous research on 100-year events has made the assumption that multiple 100-year events—for instance, 100-year storms and 100-year river heights—all kind of nestle together with 100-year storms leading to 100-year river heights. This is not what Serafin and colleagues’ hybrid, super-long synthetic record of possibilities showed.

In a nutshell, Serafin and colleagues were able to determine that the 100-year storms they synthetically created for the Quillayute River did not in all possible cases lead to 100-year river heights on the river. This is significant because it shows that far more frequent events can lead to 100-year floods. In other words, extreme events can be driven by more average, non-extreme events.

The reason why this is so, Serafin and colleagues surmise, has to do with those compounding effects mentioned above and the complexity of estuarine systems. For instance, a large (but by means a rare) storm coupled with a large (but by no means rare) discharge of water can lead to a total water level that meets the criteria of a 100-year flood. This has direct relevance for communities concerned with flooding, like La Push, and points to the need to track multiple factors when predicting and planning for flooding.

Serafin and colleagues end their paper by noting that while much of their findings are specific to the estuarine dynamics that occur where the Quillayute River meets the Pacific Ocean—for instance, the estuary is fairly narrow, which means river flows play a larger role in flooding near La Push than they might in a wider estuary—the synthetic approach they took could nonetheless be replicated elsewhere.


The research reviewed in this post was funded by a grant administered by the Quinault Indian Nation on behalf of Quinault Indian Nation, the Hoh Tribe, and the Quileute Tribe. The research was performed by the Oregon Climate Change Research Institute (OCCRI). The Pacific Northwest Climate Impacts Research Consortium (CIRC), via NOAA’s Regional Integrated Sciences and Assessments (RISA) program, provided additional funding for the effort.

The initial total water levels work developed by Serafin and Ruggiero was created for two CIRC-related projects, the Tillamook Coastal Future Project and the Grays Harbor Coastal Futures Project, which have resulted in original research from Serafin, Ruggiero, Parker, and Hill.  Both projects were part of CIRC’s Community Adaptation effort.

OSU_icon_key_01Key Finding: Rare, extreme flooding events can result from a combination of more common, less extreme events.

OSU_icon_newspaper_01Related Stories:

OSU_icon_pencil_Black RotatedCitation: Serafin, Katherine A., Peter Ruggiero, Kai Parker, and David F. Hill. “What’s streamflow got to do with it? A probabilistic simulation of the competing oceanographic and fluvial processes driving extreme along-river water levels.” Natural Hazards and Earth System Sciences 19 (2019). https://ir.library.oregonstate.edu/concern/articles/5m60qz50n.


Feature Image: “La Push Washington,” May 11, 2016. (Photo Credit: Adam Singer, Some Rights Reserved.)


Nathan Gilles is the managing editor of The Climate Circulator, and oversees CIRC’s social media accounts and website. When he’s not writing for CIRC, Nathan works as a freelance science writerOther posts by this Author. 

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