Hawai'i land impacted by sea level rise may be double previous estimates

September 27, 2018, University of Hawaii at Manoa
Extensive shoreline erosion near homes at Mokuleia on Oahu's northshore. Credit: Brad Romine

By including models of dynamical physical processes such as erosion and wave run-up, a team of researchers from the University of Hawai'i at Mānoa (UH Mānoa) and the Hawai'i Department of Land and Natural Resources (DLNR) determined that land area in Hawai'i vulnerable to future sea level rise may be double previous estimates. The study was published today in the Nature journal, Scientific Reports.

A widely-used approach for assessing impacts of is the "bathtub" model wherein a static sea level surface is projected onto a terrain model.

"The bathtub method provides a good first look at low-lying flood-prone areas but underestimates the full extent of potential damage due to sea level rise, particularly on Hawaii's high-energy coasts," said lead researcher Tiffany Anderson, a faculty member in the Department of Earth Sciences at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).

As sea level rises, several processes are at work—coastal erosion results in permanent land loss but is also essential for preserving beaches; annual wave flooding rapidly escalates past a critical point; groundwater inundation and storm-drain back flow create new wetlands and cause urban flooding. These render coastal armoring insufficient as an adaptation strategy.

"Our more comprehensive assessment reveals important realities that can be overlooked with other methods. Critically, the "bathtub" approach, alone, ignores 35-54 percent of the total exposed to one or more of these hazards, depending on location and SLR scenario," said Chip Fletcher, co-author of the study and associate dean and earth sciences professor at SOEST.

Shoreline erosion near homes. Credit: Sam Lemmo

The team also found that typical elevations of Hawaii's low-lying coastal plains create thresholds of flood levels, above which rapid increases in flooding occur. As sea level rises, coastal lands are exposed to higher flood depths and water velocities. The prevalence of low-lying coastal plains leads to a rapid increase in land exposure to hazards when sea level exceeds a critical elevation of about 1 to 2 feet, depending on location. The team had identified this phenomenon in previous research and named it a "critical point."

"Additionally, a large portion of lands at risk of flooding are not in direct proximity to the shoreline," said Anderson. "Instead, they are low-lying areas where sea level rise causes the groundwater table to rise up to the surface. These areas can be located one to two miles inland from the coastline."

"It's important that we identify land areas vulnerable to sea level-related hazards because, if left unmanaged, flooding, wave inundation, and erosion will continue to encroach upon coastal lands that are typically heavily developed," said Fletcher. "Preparing for these effects will be very costly and take a long time to implement. With these results, stakeholders of all types are now able to establish empirically-based adaptation policies."

The modeling presented in this study was conducted to support the creation of the Hawai'i Sea Level Rise Vulnerability and Adaptation Report, which is the basis for further government planning initiatives.

"We can always count on these researchers at SOEST to provide the State with cutting edge research and modeling to improve our understanding of coastal hazards," said Sam Lemmo, study co-author and DLNR administrator. "Their work is immediately translatable to policy and regulatory guidance at the local level. Our goal was to create a scientifically rigorous visual tool to highlight the gravity of sea level rise in our islands to prepare our communities and government bodies for its impact, as well as to provide tools necessary to reduce the shock to our socio-economic system."

Anderson and team are currently incorporating rainfall into the computer model to determine how -related flooding might be exacerbated during rainfall events that occur during high tides. Hawai'i Sea Grant, also located at UH SOEST, and Tetra Tech, Inc. are helping guide State and county agencies in considering this new data in future planning.

Explore further: As sea level rises, much of Honolulu and Waikiki vulnerable to groundwater inundation

More information: Tiffany R. Anderson et al, Modeling multiple sea level rise stresses reveals up to twice the land at risk compared to strictly passive flooding methods, Scientific Reports (2018). DOI: 10.1038/s41598-018-32658-x

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9 comments

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howhot3
3.7 / 5 (6) Sep 27, 2018
Obviously sea-level rise due to global warming is going to be a very challenging problem for all future humans in coastal areas. It doesn't surprise me that Hawaii would be challenged by such a problem. What it really boils down to is how are we going to stop sea level rise due to man-made global warming?
Thorium Boy
2.6 / 5 (5) Sep 28, 2018
All islands are eroded and eventually eradicated, just like mountains. Right after he wrote "An Inconvenient Truth" in which he warned about sea-level increases, Al Gore bought a 12,000 square foot mansion, beachfront, on the east coast...
Old_C_Code
1 / 5 (3) Sep 28, 2018
So why isn't ALL of Hawaii's coasts eroding like this? Is the land area SINKING?

howhot you are a brainwashed fool, seeing your alarmist BS comments regularly is sickening.
rrwillsj
4 / 5 (4) Sep 28, 2018
old_coot, what is sickening is your agitproping for evil. You are like a dog, that after it vomits on the floor? Constantly returns to lick it up again!
wailuku1943
4.2 / 5 (5) Sep 28, 2018
Old_C_Code is typically failing to understand the larger picture. In this case, the pretty damn obvious observation that not all of the Hawaiian islands are the same. They differ markedly in age and geology. Much of my island (the Big Island) has rocky shores. Erosion isn't an issue. What could be more obvious than that sandy shores (such as as the places talking about in the article) have different vulnerabilities than rocky ones? But Old_C_Code just can't grasp that concept.
Old_C_Code
1 / 5 (2) Sep 28, 2018
The larger picture? Hawaiian islands are F**king islands.
Why aren't all of it's coastline impacted?
You dumbazz willsjizbag
wail: grow up chithead, you fools have no answers to valid obviously simple logical questions.

Why aren't all of it's coastlines impacted? Answer the question geniuses.
wailuku1943
5 / 5 (3) Sep 28, 2018
Whoa. You're even more clueless than I thought you were. Why don't we just let the other readers decide who understands the first thing about our island chain, and who doesn't. You're really shaming yourself, old dude. Your ignorance is showing -- well, probably more your unwillingness to pull your head out of that safe space where you've put it, and do a little research. I feel sorry for you, Mr. Stereotypical Engineer.
wailuku1943
5 / 5 (3) Sep 28, 2018
I'm kinda thinking your attention span is unusually short, Old_C_Code.

I refer you to what you wrote: "So why isn't ALL of Hawaii's coasts eroding like this?" The key word here is "eroding."

ALL our shores aren't eroding because shorelines differ from place to place.

Simple enought for you? Perhaps not, but try hard to comprehend this.

Sandy shores erode quickly and sometimes disastrously for structures built on or near them. Keep your mind fastened on "erode," although I suppose it's possible you don't know what it means.

Rocky shores are (maybe surprisingly, to you) formed from rock. Which is hard and durable in the face of the sea, no matter what its level. For the sea to erode (there's that word again) a rocky shoreline takes a very long time. The sea may overtop that shore, but it's not going to erode it.

OK now for the hard part. Some of our islands have rocky shores, some don't, and some have both. Get it?
TrollBane
not rated yet Oct 02, 2018
To add to wailuku1943's educational efforts, the island of the chain differ in age, with the big island being much younger. That's why most of the active volcanism is there. Movement of crustal plates gradually move an island away from the underlying hot spot in the mantle, after which the islands stop growing and begin to slowly collapse under their own weight, meanwhile undergoing erosion. It's the same as the Galapagos chain. Trace the islands northwest and you'll see them get older, smaller and more eroded until what's left are seamounts in a long chain extending for thousands of kilometers under the surface. Here's a handy primer on the subject: https://www.visio...orces/66

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