Annual and interannual changes on a reef-fringed pocket beach: Kailua Bay, Hawaii

Historical aerial photographs and topographic survey sheets are used to establish a 70-year shoreline history (1926–1996) for Kailua Beach, Oahu, Hawaiian Islands. The shoreline has migrated seaward over this period at an average rate of 0.5 m/yr, with a maximum net accretion along the beach of 58.7 m and a maximum net erosion of −13.2 m. Net accretion has taken place even while sea levels have risen on the order of 0.1 m. Semi-annual and monthly beach profile surveys (1995–1999) at seven transects reveal short-term variations of shoreline position, sand volume, and beach shape. A relationship between beach width and corresponding sand volume fluctuations, established from the beach profile data, is applied to the historical shoreline change data to establish a history of sand volume fluctuations. Results show that Kailua has experienced a net accretion of 673 000 m3 of sand over the period 1926–1996, with average annual rates of volume change varying between 6.8 m3/m/yr and −0.1 m3/m/yr. The most recent period (1989–1996) shows a net volume increase of 41 000 m3. Given the lack of sand inputs at the ends of the beach, exchange with offshore deposits is a likely mechanism for long-term accretional trends. Seasonal fluctuations in Kailua Beach morphology dominate the variability with a response to seasonal wave state that varies along the length of the beach in magnitude and sign. At least four alongshore zones are observed, with the first and third zones exhibiting high/low sand volumes during the summer/winter, and the second and fourth zones exhibiting opposite behavior with high/low volumes during the winter/summer. Although seasonal sand accumulation varies along the beach, the overall beach profile is largely maintained. Moreover, changes in sand volume occur in phase over the subaqueous and subaerial sections of the beach. This behavior suggests that longshore rather than cross-shore sand transport is important at annual time scales. A simple seasonal transport pattern is proposed to account for these observed fluctuations, which depends in part on the topography of the offshore reef. [Copyright &y& Elsevier]