Differing rates of relative sea-level rise on the islands of Oʻahu and Maui remain as the best explanation for the difference in island-wide shoreline trends (that is, beach erosion or accretion) after examining other influences on shoreline change including waves, sediment supply and littoral processes, and anthropogenic changes.
Researchers from the University of Hawaiʻi at Mānoa’s School of Ocean and Earth Science and Technology (SOEST) and the state Department of Land and Natural Resources recently published a paper showing that sea-level rise is a primary factor driving historical shoreline changes in Hawaiʻi and that historical rates of shoreline change are about two orders of magnitude greater than sea-level rise.
As pointed out by authors of the work, knowing that sea-level rise is a primary cause of shoreline change on a regional scale allows managers and other coastal zone decision-makers to target SLR impacts in their research programs and long-term planning. This study is confirmation that future sea-level rise is a major concern for decision-makers charged with managing beaches.
“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 Hawaiʻi 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 sea-level rise. With this research, we confirm the importance of sea-level rise as a primary driver of shoreline change on a regional to island-wide basis.”
Globally averaged sea-level rose at about 2 mm per year over the past century. Previous studies indicate that the rate of rise is now approximately 3 mm per year and may accelerate over coming decades. The results of the recent publication show that sea-level rise is an important factor in historical shoreline change in Hawaiʻi and will be increasingly important with projected sea-level rise acceleration in this century. “Improved understanding of the influence of sea-level rise on historical shoreline trends will aid in forecasting beach changes with increasing sea-level rise,” said Dr. Charles Fletcher, Associate Dean and Professor of Geology and Geophysics at SOEST.
“The research being conducted by SOEST provides us with an opportunity to anticipate sea-level rise effects on coastal areas, including Hawaiʻi’s world famous beaches, coastal communities and infrastructure. We hope this information will inform long-range planning decisions and allow for the development of sea-level rise adaptation plans,” said Sam Lemmo, Administrator, Department of Land and Natural Resources, Office of Conservation and Coastal Lands.
Results of island-wide historical trends indicate that Maui beaches are significantly more erosional than beaches on Oʻahu. On Maui, 78% of beaches eroded over the past century with an overall (island-wide) average shoreline change rate of 13 cm of erosion per year, while 52% of Oʻahu beaches eroded with an overall average shoreline change rate of 3 cm of erosion per year.
The variation in long-term relative sea-level rise rates along the Hawaiʻi archipelago is due, in large part, to variations in island subsidence with distance from actively growing Hawaiʻi Island and/or variations in upper ocean water masses. The islands of Oʻahu and Maui, with significantly different rates of localized sea-level rise (sea-level rise has been approximately 65% higher rate on Maui) over the past century, provided a natural laboratory to investigate possible relations between historical shoreline changes and sea-level rise.
Island-wide and regional historical shoreline trends were calculated for the islands using shoreline positions measured from aerial photographs and survey charts. Shoreline positions were manually digitized 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). Shoreline movement through time was measured using GIS software. Historical shoreline data were optimized to reduce anthropogenic influences (e.g., constructing seawalls or sand mining) on shoreline change measurements. The researchers controlled for influences other than sea-level rise to determine if sea-level rise remains as the best explanation for observed changes. They also utilized a series of consistency checks to determine if results are significant and to eliminate other possible explanations.
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.
Photo by Zoe Norcross-Nuu.