Wave-induced scour burial experiments in carbonate sediments near Oahu, Hawaii

Abstract

Wave-induced scour burial experiments were conducted at two shallow-water sites along the southern coast of Oahu, Hawaii. Sediments at both sites were moderately well-sorted, fine carbonate sand. Instrumented cylinders were deployed at the Kilo-Nalu coastal observatory during the summer-fall of 2007 and in the Halekulani Sand Channel off Waikiki Beach during the summer of 2008. Water depth at both sites was 12 meters. Two different patterns of burial were observed during the experiments at the Kilo-Nalu observatory. Two instrumented cylinders buried to approximately their diameter (53 cm) after exposure to a single high-wave event (greater than 1.25-m significant wave height). The remaining two instrumented cylinders buried more slowly and to approximately 50% of their diameter (25 cm). Burial measured as surface area covered by sediment varied (5 to 50%) as scour pits developed and were infilled as a result of local sediment transport. The different burial patterns were controlled by sediment layer thickness, with the shallow-buried cylinders actually resting on the coral basement by the end of the experiments. Burial predicted by models for wave-induced scour were in close agreement with burial measured on cylinders deployed in the thicker sediments but the predicted burial was much greater than measured burial for the cylinders limited by sediment thickness. Measured and predicted burial during the second experiment in Halekulani sand channel was less than measured and predicted burial at the Kilo-Nalu experiments. Three instrumented cylinders buried to 40–60% of their diameter after two events with significant wave heights of approximately 1 meter. However the fourth cylinder buried to only 10% of its diameter. We have been unable to explain the shallow burial of the single cylinder. Predicted burial was between 40–50% The greater measured and predicted burial during the first experiments at Kilo-Nalu compared to burial during the Halekulani Sand Channel experiments were related to the increased scour from the higher significant wave heights during the first experiment (1.25 m vs. 1.0 m). Other variables such as sediment type, mean grain size, and water depth were the same for both experiments. These results demonstrate the applicability of the wave-induced scour burial model to model predict burial in unconsolidated carbonate sediments.