Waikiki: Historical analysis of an engineered shoreline

Short-term and long-term shoreline change at Waikiki Beach, Hawaii, is analyzed to enhance resource management. Bi-monthly beach profiles reveal short-term variations of the shoreline. Increased wave heights from south swells between May and October often correspond to beach volume increase, while short-period wind waves predominating between November and April correspond to volume losses. A total mean volume of 167,000 m 3 is estimated for Waikiki Beach, with an uncertainty of 15 to 40%. A net volume loss of approximately 5,200 m 3 is found between October 2000 and May 2002. The Royal Hawaiian littoral cell accounts for 93% of the loss. Historical aerial photographs and NOAA T-sheets establish a 76-year shoreline history (1925-2001). The shoreline has migrated a mean distance of 12 m seaward over this period, reflecting a high level of human intervention. Likewise, average beach width has increased by 32% since 1951. Four of seven littoral cells, however, are characterized by recent erosion at a mean erosion rate of 0.3 ± 0.1 m/yr. Of the remaining three littoral cells, two have experienced long-term accretion and one has exhibited stability. A relationship between beach width and corresponding sand volume change, established from beach profile data, is applied to historical shoreline changes to establish a history of sand volume fluctuations. Early volume fluctuations are traced to beach nourishment, typically with subsequent beach loss. Volume gains are documented across the entire shoreline between 1975 and 1985. Widespread chronic erosion characterizes the years after 1985. Despite past beach nourishment, a sediment budget for Waikiki reveals a sand volume deficit of at least 77,000 m 3 for the time period between 1951 and 2001, owing to permanent offshore losses.