MONITORING AND GEOSPACIAL MODELING OF WEST KAUAI’S CARBONATE SAND BEACHES IN RESPONSE TO ANNUAL AND DECADAL SCALE WAVE DYNAMICS

Although only the 4^th largest of Hawaii’s eight main high islands, the geologically mature, > 5 my, shield volcanic island of Kauai possesses 1/3 of the sand beaches. Over 45% (82 of 179 km) of Kauai’s perimeter is rimmed by beaches, falling into three main categories. Excepting the 5 km-long volcaniclastic Waimea beach complex, beaches are composed predominately of bioclastic carbonate sand. The eastern 2/3 of the island’s perimeter displays 65% of the beaches, generally categorized as relatively confined bay head and back reef carbonate deposits, whereas the western quarter of the island presents 30% of its beaches, predominately carbonate, distributed sparsely along the Na Pali coast and nearly continuously along the Mana Coastal Plain. Sand comprising the West Kauai beaches is derived mainly from abrasion of the fringing reef complex of Haena and transported south and east around the island via ocean wave-generated alongshore currents as far as Kekaha, over 45 km distant.

Initiated late 2012, study of the distalmost 10 km of the Mana carbonate beaches has provided monthly beach profile surveys, daily monitoring of wave heights approaching all sides of the island, and GIS analyses of historical coastal zone aerial imagery over the past 65 years. Variation in beach morphology in response to seasonal variation in wave dynamics is dramatic. Over a 10 km distance, beach widths vary in alternating phases by 150-200 m. During winter months (Oct-Apr), when North Pacific storm waves exceed 3 m, and range to over 10 m, sand is moved south and east. Through summer months (May-Sept), persistent E-NE trade winds and S ocean swells move the sand back west and north.

The magnitude of seasonally alternating, wave-generated alongshore-current beach erosion and accretion far exceeds other short or long-term processes. Beach monitoring data, however, has revealed a longer term component of sand transport, possibly related to fluctuation in phases of the Pacific Decadal Oscillation (PDO). Hindcast analyses of N Pacific wave systems, 1980-present, suggest a close correlation between PDO cycles and West Kauai beach morphology. Locally, beach loss on a decadal time frame has been significant. Fluctuation in wave dynamics and beach geometry is continuing to be evaluated in order to assist coastal zone use planning for the islands of Hawaii.