COASTAL PROCESSES AT POIPU BEACH PARK: THE EFFECTS OF A BIMODAL WAVE CLIMATE ON THE STABILITY OF THE POIPU TOMBOLO

Poipu Beach Park is a major attraction on the south shore of the island of Kauai. The prominent tombolo, which separates the adjacent Poipu and Waiohai embayments, became an intermittent feature after the direct hit of Hurricane Iniki in 1992. With the tombolo absent, a strong current flows between the two embayments. At the request of Kauai County, Sea Engineering, Inc. (SEI) conducted oceanographic and engineering studies in the vicinity of the park. Topographic surveys conducted over a year’s span showed pronounced movement of the tombolo axis of up to 12m, a small net accretion of beach sand, and a normal variation of up to 600 m³ in sand volume. Wave spectra and currents were measured at two offshore locations using Acoustic Doppler Current Profilers (ADCP’s) and correlated with WaveWatch III hindcasts. Numerical wave transformation models (CGWave and Bouss2D) and a flow model (CMS-Flow) were used to examine wave behavior and wave-induced current formation in shallow water. The wave climate is strongly bimodal, with the two embayments responding uniquely to each spectral mode. The Poipu embayment was found to be more influenced by trade wind generated waves, while the Waiohai embayment responds to southern swell. The numerical modeling indicates that the Poipu embayment experiences an approximate 1 cm rise in water level during normal trade wind wave conditions that is sufficient to drive the current between the two embayments when the tombolo has been breached.