Rising Sea Waters Threaten Hawaiian Shorelines
August 31, 2013

Rising Sea Waters Threaten Hawaiian Shorelines

April Flowers for redOrbit.com - Your Universe Online

In Hawaii, sea-level rise (SLR) has been isolated as a principal cause of coastal erosion.

After examining other influences on shoreline change including waves, sediment supply and littoral processes, and anthropogenic changes, the best explanation for the difference in island-wide shoreline trends, such as beach erosion or accretion, is the differing rates of relative sea-level rise on the islands of Oahu and Maui.

A new study, from the University of Hawaii-Manoa (UHM), School of Ocean and Earth Science and Technology (SOEST) and the State of Hawaii, Department of Land and Natural Resources, demonstrates how SLR is a primary factor driving the historical shoreline changes in Hawaii.

The findings, published in Global and Planetary Change, also show rates of historical shoreline change are about two orders of magnitude greater than SLR.

Understanding that SLR is a primary factor in shoreline change on a regional scale will allow managers and other coastal zone decision-makers to target SLR impacts in future research programs and long-term planning. For decision-makers charged with managing beaches, this study confirms future SLR is a major concern.

"It is common knowledge among coastal scientists that sea level rise leads to shoreline recession," stated Dr. Brad Romine, coastal geologist with the University of Hawaii Sea Grant College Program.

"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 Romine.

During the last century, globally-averaged sea-level rose at about 2mm (.08 in) per year. Prior research efforts have indicated the rate of rise is currently almost 3mm (.12 in) per year and may accelerate over the coming decades.

The new study reveals SLR is not only an important factor for the historical shoreline change in Hawaii, but will also become increasingly important with projected acceleration of SLR in this century. "Improved understanding of the influence of SLR on historical shoreline trends will aid in forecasting beach changes with increasing SLR," said Dr. Charles Fletcher, Associate Dean and Professor of Geology and Geophysics at the UHM SOEST.

"The research being conducted by SOEST provides us with an opportunity to anticipate SLR effects on coastal areas, including Hawaii's world famous beaches, coastal communities, and infrastructure. We hope this information will inform long range planning decisions and allow for the development of SLR adaptation plans," said Sam Lemmo, Administrator, Department of Land and natural Resources, Office of Conservation and Coastal Lands.

According to island-wide historical trends, the beaches of Maui are significantly more erosional than those on Oahu, with 78 percent of Maui’s beaches eroded over the past century. The island also saw an overall, island-wide average shoreline change of approximately 5 inches of erosion a year. Oahu beaches, on the other hand, have eroded 52 percent with an overall average shoreline change rate of just over one inch of erosion per year.

The team attributes the variation in long-term relative SLR rates along the Hawaiian archipelago, in large part, to variations in island subsidence with distance from actively growing Hawaiian Island and/or variations in upper ocean water masses. The significantly different rates of localized sea-level rise between the islands of Maui and Oahu over the last hundred years – SLR on Maui has been approximately 65 percent higher – provided a natural laboratory for the team to investigate possible relations between historical shoreline changes and SLR.

The researchers calculated island-wide and regional historical shoreline trends for the islands using shoreline positions measured from aerial photographs and survey charts.

The scientists manually digitized shoreline positions using photogrammetric and geographic information system (GIS) software from aerial photo mosaics and topographic and hydrographic survey charts provided by the National Ocean Service (NOS). GIS software also allowed them to measure shoreline movement through time. To reduce anthropogenic influences -- such as constructing seawalls or sand mining -- on shoreline change measurements, the team optimized historical shoreline data.

Other influences were controlled for besides SLR to determine if SLR is still the best explanation for observed changes. To eliminate other possible explanations, and to determine if the results are significant, the team used a series of consistency checks.