Multi-decadal shoreline change and beach connectivity in a high-energy sand system

Understanding how and why the world's coastlines are changing is a pressing international concern in a context of rising sea levels, increased climatic variability and intensifying coastal development. Medium to long-term records of coastal change are rare worldwide and often limited to individual beach compartments. This study presents a 70-year (1940-2010) aerial imagery record to compare decadal changes in shoreline position across four high-energy west coast beaches near Auckland, New Zealand/Aotearoa: Whatipu, Karekare, Piha and Te Henga (Bethells). The common exposure of these adjacent mesotidal beaches to changing wind and wave conditions might, if they present the dominant controls on shoreline position, be expected to produce synchronous change. Whatipu (935 m), Piha (32 m) and Te Henga (52 m) showed net overall progradation for the study period, while Karekare retreated slightly (-4.1 m). All except Whatipu underwent periods of beachwide erosion. Shoreline change was not coherent between beaches, despite similar exposure to variations in wind, wave and sea level. Variable sediment supply from northward littoral drift is implicated as the primary control on decadal-scale shoreline change for these beaches, highlighting the importance of local context in influencing shoreline response to changing environmental conditions.