Historical shoreline trends and management implications: Southeast Oahu, Hawaiʻi

Abstract

Here we present shoreline change rates for the beaches of southeast Oahu, Hawaii using recently developed polynomial methods to assist coastal managers in planning for erosion hazards and to provide an example for interpreting results. Polynomial methods use data from all transects (measurement locations) on a beach to calculate a shoreline change rate at anyone location on a beach. These methods are shown to produce rates with reduced model uncertainty compared to previously used methods and can detect acceleration in the shoreline change rates. An information criterion, a type of model optimization equation, is used to identify the best shoreline change model for a beach. Polynomial models that use Eigenvectors as their basis functions are identified as the best models most often. Using polynomial models that are constant (linear) in their rates, we find erosion along 36% of the study area beaches, including North Bellows Beach, South Waimanalo Beach, and at most beaches between Kaiona and Kaupo Beach Park in the south of the study area. The ability to detect accelerating shoreline change with the polynomial methods is an important advance as a beach may not erode or accrete at a constant (linear) rate. Acceleration models may detect erosion hazards not detected by other methods that use linear models. Using polynomial models that include acceleration in their rates, we find accelerating erosion at 33% of transects, including the south of Kailua Beach, much of northern Bellows Beach, and in the south half of Kaupo Beach Park.