COASTAL PROCESSES ON A TYPICAL WINDWARD, FRINGING REEF SHORELINE - KAPAA, KAUAI

The shoreline at Kapaa, on the windward coast of Kauai, consists of a calcareous sand beach 1,830 m (6,000-feet) long, interrupted by canal jetties, and a broad fringing reef offshore. Kapaa Beach has a history of erosion, and shoreline armoring. Sea Engineering, Inc. conducted a detailed 14-month study to investigate sand transport processes and beach erosion. Work included: (1) recurring beach profile measurements along 18 established transects over a one year duration; (2) measurement of waves and currents through in situ instrument deployments; (3) offshore side scan and sub-bottom profile surveys; (4) sand deposit sampling and diver probing at selected locations; (5) numerical modeling of wave transformation over the complex reef structure, and the resulting wave-driven circulation.

Wave and current data, and numerical wave modeling indicate that the shallow reef complex and deep channels produce complex wave patterns, that nevertheless become aligned parallel to the shoreline for all incident wave directions. In response, the beach shape is largely consistent throughout the year. Trade wind waves breaking on the fringing reef drive a persistent circulation pattern that is dictated by the reef complex. Water is transported across the broad reef flats to the north and south, then flows alongshore toward a convergence in the Kapaa Channel, then eastward through the channel to the ocean. Sand transport paths follow this general pattern, where sand is transported along the shoreline, and then out into Kapaa Channel. This appears to be the primary mechanism of beach sand loss. Consistent with this finding, geophysical and diver surveys found an extensive deposit of beach quality sand close to shore in the channel. The persistent trade wind wave-driven circulation and the steep sides of the deep channel prevent possible return back to the beach by natural processes. This suggests that future steady erosion of the beach is likely.

Similar morphologies in other windward island locations, including Oahu, suggest that Kapaa can serve as a model for reef and channel sand transport in trade wind dominated areas.