Many Hawai`i beaches are eroding and it should be no
surprise that the primary culprit is sea level rise.
Hawai`i researchers recently published a paper in the
journal Global and Planetary Change that concluded that the main cause of
coastal erosion in the Islands is rising ocean levels.
(Image: Maui beaches are eroding at an average of half a
foot a year. Shown here is a coastal building on Maui, threatened by chronic
coastal erosion. Credit: Zoe Norcross-Nuu.)
There are certainly other factors, like currents and the
relative rise and subsidence of the islands themselves, but sea level’s the big
one.
It means, in part, that people assuming those disappeared beaches will
return cyclically to their previous size will wait in vain, and that the state’s
decision-makers need to plan for continued coastal erosion.
“Shorelines find an equilibrium position that is a balance
between sediment availability and rising ocean levels. On an individual beach
with adequate sediment availability, beach processes may not reflect the impact
of SLR. With this research, we confirm the importance of SLR as a primary
driver of shoreline change on a regional to island-wide basis,” said Brad
Romine, a coastal geologist with the University of Hawai`i Sea Grant College
Program.
What he’s saying there is that if a beach still has a large
natural dune system behind it—like, for instance, Polihale on Kaua`i—the sand
will replenish the retreating shoreline and you’ll still have a beach. But of
course, most of the state’s dune systems are long gone.
Sea level has been going up at nearly a tenth of an inch a
year for most of the 1900s, but the level has increased recently to slightly
more than a tenth of an inch a year—from 2 millimeters to 3 millimeters. That
works out to sea level rise of three-quarters of an inch per decade
accelerating to more than an inch.
Doesn’t seem like much, but at the current rate, a kid born
today will see sea levels more than half a foot higher by retirement age.
Imagine an additional seven or so inches on top of today’s highest tides, and
the picture looks ominous for coastal roads, beach parks, coastal resorts, and
sandy beach oceanfront homes.
That’s because each inch in sea level rise translates to
several inches of coastal retreat. Maui beaches, of which 78 percent are
eroding, have lost on average half a foot a year. Most O`ahu beaches are also
eroding, but at a far lower rate, about an inch a year.
Some coastlines clearly get hurt more than others, and
calculating the different coastal responses has been a major piece of Charles “Chip”
Fletcher’s work.
“Improved understanding of the influence of SLR on
historical shoreline trends will aid in forecasting beach changes with
increasing SLR,” said Fletcher, Associate Dean and Professor of Geology and Geophysics
at the University of Hawai`i School of Ocean and Earth Science and Technology.
Citation: B M
Romine, C H Fletcher, M M Barbee, T R Anderson, L N Frazer (2013) Are beach
erosion rates and sea-level rise related in Hawaiʻi? Global and Planetary Change, doi: 10.1016/j.gloplacha.2013.06.009
A press release on the project adds: “The research described
in this paper was carried out by the University of Hawaiʻi Coastal Geology
Group with the support of the State of Hawaiʻi; Counties of Kauaʻi, Oʻahu and
Maui; U.S. Geological Survey; U.S. Army Corps of Engineers; NOAA; Hawaiʻi CZM;
Hawaiʻi Sea Grant; and the Harold K.L. Castle Foundation. This paper is funded in part by a
grant/cooperative agreement from the National Oceanic and Atmospheric
Administration, Project A/AS-1, which is sponsored by the University of Hawaiʻi
Sea Grant College Program, SOEST.”
© Jan TenBruggencate 2013
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