Are beach erosion rates and sea-level rise related in Hawaii?

The islands of Oahu and Maui, Hawaii, with significantly different rates of localized sea-level rise (SLR, approximately 65% higher rate on Maui) over the past century due to lithospheric flexure and/or variations in upper ocean water masses, provide a unique setting to investigate possible relations between historical shoreline changes and SLR. Island-wide and regional historical shoreline trends are calculated for the islands using shoreline positions measured from aerial photographs and survey charts. Historical shoreline data are optimized to reduce anthropogenic influences on shoreline change measurements. Shoreline change trends are checked for consistency using two weighted regression methods and by systematic exclusion of coastal regions based on coastal aspect (wave exposure) and coastal geomorphology. Maui experienced the greatest extent of beach erosion over the past century with 78% percent of beaches eroding compared to 52% on Oahu. Maui also had a significantly higher island-wide average shoreline change rate at − 0.13 ± 0.05 m/yr compared to Oahu at − 0.03 ± 0.03 m/yr (at the 95% Confidence Interval). Differing rates of relative SLR around Oahu and Maui remain as the best explanation for the difference in overall shoreline trends after examining other influences on shoreline change including waves, sediment supply and littoral processes, and anthropogenic changes; though, these other influences certainly remain important to shoreline change in Hawaii. The results of this study show that SLR is an important factor in historical shoreline change in Hawaii and that historical rates of shoreline change are about two orders of magnitude greater than SLR.