- Waikiki - Analysis of an Engineered Shoreline

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2002-12

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University of Hawaii at Manoa

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Short-term and long-term shoreline change at Waikiki Beach 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 a period of volume increase, while short-period wind waves predominating between November and April regularly correspond to volume losses. A total mean volume of 167,000 m3 is estimated for Waikiki Beach, with an uncertainty of 15 to 40%. A net volume loss of ~5,200 m3 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 the high level of human intervention. Likewise, overall beach width has increased by 32% since 1951. Four of seven littoral cells, however, are characterized by erosion over more recent time scales, showing 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 frequent beach nourishment, a sediment budget for Waikiki reveals a sand volume deficit of at least 77,000 m3 for the time period between 1951 and 2001, owing to permanent offshore losses.

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xiv, 107 leaves

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Theses for the degree of Master of Science (University of Hawaii at Manoa). Geology and Geophysics; no. 3750

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Table of Contents

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