Southwest Kauai’s unique, 4 km-long, Waimea/Kikiaola volcaniclastic sand beach complex, Hawaii’s largest terrigenous beach sand reservoir, represents the westward alongshore current distribution of detrital sand contributed to the coast by the Waimea Canyon watershed. Over 65 years, the seaward protruding Kikiaola Small Boat Harbor structure, emplaced 1960, 2.7-3.0 km west of the Waimea River sediment source, has blocked, like a river damn, the natural movement of sand along the coast. While Waimea Beach east of the harbor’s lava rock breakwater prograded over 100 meters and increased sand volume by over 350%, 0.8 km-long Kikiaola Beach down drift (west) of the harbor retreated over 60 meters and suffered a decrease in sand volume of over 50%. Over a period of three months, mid-2014, in order to replenish Kikiaola Beach, 45,000 cubic meters of sand was moved, by excavator and truck from Waimea Beach east of the harbor to the 0.3 km portion of Kikiaola Beach immediately west of the harbor. Monitoring the sand bypass operation since several months prior to sand movement, employing tape/transit beach profile surveys and GIS analysis of coastal zone aerial imagery, has provided an evaluation of the beach replenishment program as well as enhance understanding of how this alongshore detrital sand transport beach system works. Results to date have been enlightening.
Waimea Beach east of Kikiaola Harbor displayed considerable stability. The two beach localities from which 30,000 and 15,000 cubic meters of sand was excavated have not regained sand volume. The excavation sites regained their classic backshore-berm-offshore beach morphology, but each site continued to lose sand. Such character is a reminder that it took 65 years for the beach to build up, via alongshore sand movement, east of the harbor. For over two years following the bypass operation, the 0.3 km portion of Kikiaola Beach immediately west of the harbor maintained a moderate degree of stability; however, since that time it has begun to display erosion of the bypass sand pile. The 0.5 km portion of Kikiaola Beach west of the bypass sand deposit displayed only minor fluctuation in beach character, suggesting a lack of impact of the replenishment program on the beach’s down drift portion more than two years following the bypass operation.