KAUAI ISLAND’S FLUCTUATING SANDY BEACHES: RESPONSE TO VARYING WAVE ENERGY AND APPROACH AZIMUTH OVER SEASONAL AND DECADAL TIME SCALES

The size and seasonal variability of ocean waves that approach the central N Pacific islands of Hawaii are well known; however, associated beach response has not been documented with any precision. Kauai, a near circular, 45 km diameter volcanic island, is impacted on all sides by sizeable ocean waves generating onshore/offshore and alongshore sediment movement. West Kauai’s Mana Coastal Plain beaches, Hawaii’s longest and largest volumetrically, are composed of skeletal carbonate sand sourced from marine reefal systems.

We present wave monitoring and beach surveys recorded over the past two years demonstrating dramatic beach response displaying a seasonal periodicity. N Pacific winter (Oct-Apr) storm waves approaching from the NW with heights frequently greater than 3 m result in alongshore movement of tremendous volumes of sand, narrowing the beach at Majors Bay by 185 m, nearly 85% of its maximum width. Sand movement is most intense Dec-Feb when wave heights exceed 5 m. Much of the sand migrates southeastward over 6 km to widen the beach at MacArthur Park, Kekaha, by nearly 95 m or almost 80% of its maximum width. Through the summer months (May-Sept), persistent E-NE trade wind waves and S swells move the sand northwestward back to Majors Bay. The magnitude of seasonally alternating wave-generated alongshore-current erosion and accretion of these beaches far exceeds any other short or long-term processes.

Additionally, hindcast analyses of N Pacific wave systems, 1980-present, suggest that NW swell, and thus beach geometry fluctuation, is strongly related to phases of the Pacific Decadal Oscillation (PDO). While NE trade wind-generated waves and S swells continually move sand W and NW, equilibrium relies on powerful winter NW swells to move sand back to the southern terminus of the sediment cell. The number and size of NW wave events is strongly related to the phase of the Pacific Decadal Oscillation (PDO) due to the fluctuation in number and depth of transient lows in the Aleutian Low pressure system. Beach loss becomes significant with successive years of fewer and weaker NW swells during low wave PDO phases. Fluctuation in wave dynamics and beach geometry, on both seasonal and decadal scales, are continuing to be evaluated in order to assist coastal zone use planning for the islands of Hawaii.