Sunday, September 8, 2013

Coastal erosion in Hawai`i? It's sea level rise, silly.

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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