Offshore sand prospecting in : History and recent efforts

Sea Engineering, Inc. (SEI) has continued long- standing efforts to find exploitable offshore sand resources for the purpose of nourishing Hawaii´s beaches. Investigations by University of Hawaii researchers in the 1970´s established the potential presence of large bodies of sand in ancient offshore alluvial channels and wave-cut terraces. Sampling efforts showed that much of the sand was too fine-grained to be useful as beach nourishment. Prospecting was revived in the early 1990´s with funding for new surveys provided by the U.S. Bureau of Mines through the Marine Minerals Technology Center at the University of Hawaii. The realization that existing instrumentation was not adequate for useful imaging of Hawaii sand deposits led to a team effort by SEI and Precision Signal, Inc., funded by CEROS to develop a state-of-the-art sub-bottom profiling system. The new chirp profiler resulted in excellent sub- bottom imagery. SEI successfully mapped selected sand deposits with the new instrument around the island of Oahu for the United States Geological Survey (USGS) in 1996 and 1998. The continued erosion of Hawaii´s beaches has led to recent programs by both private concerns and State and County agencies to find offshore sand for specific target beaches. SEI has conducted extensive surveys off Kaanapali and Kahului on Maui, off Poipu and Kapaa on Kauai, and off Waikiki. The State of Hawaii Department of Land and Natural Resources (DLNR) has funded a project for the recovery of 24,000 cy of sand from offshore deposits to be placed on Waikiki Beach. The project deposits are nearshore and in shallow water. Most were mapped using aerial photographs and jet probing as well as sub-bottom profiling. A 2008 survey at Kahului Bay for Maui County showed the potential for vast resources. The sub-bottom imagery shows a complex geology, but preliminary sampling indicates that beach quality sand is available. Surveys completed off Kaanapali for the Kaanapali Operations Association- (KOA), found large amounts of sand. Extensive hand-coring and vibracore sampling showed that most deposits were composed of very fine sand produced by halimeda (calcareous green algae). However, two areas off Kekaa Point had reasonably good quality sand, one in shallow water depths of 30 to 60 ft, and one at depths of 100 to 130 ft. The deeper deposit occurred in an area characterized by large sand wave bedforms, indicating the presence of strong bottom currents. The Kaanapali deposits are considered a reasonable option for the future nourishment of Kaanapali Beach.