Morphologically driven sedimentation patterns on a coral reef

Abstract

Abstract Coral reef sediment research focused on quantifying production sources, suspended sediment, and trapping or accumulation rates, overlooking the role of hydrodynamics and reef morphology in determining these. We used an interdisciplinary approach, focusing on the links between physical features and processes and hypothesized how they affect reef recovery. Typhoon Bopha hit Ngederrak Reef (Palau) in 2012, significantly reducing coral cover. The reef is characterized by spur and grooves (SAGs) and bordered by two tidal channels, but SAGs are not present in its northernmost part, near one of these channels. The reef has recovered in the SAG area, but its recovery was slower in the north. Flow measurements and sediment samples taken in SAG and non-SAG areas were used to calculate the threshold for sediment movement due to mean flows and wave orbital velocities. Sediment on SAGs was mainly suspended by waves, but the direction of net transport was determined by the mean flow. The threshold for sediment movement due to mean flow was reached 80–100% of the time on spurs, in grooves it was reached 60% and 33% of the times during flooding and ebbing tides respectively. This tidal asymmetry suggests that sediment was removed from spurs and transported seaward in grooves to be stored at depth. The steeper slope in grooves (8%) relative to the non-SAG area (4%), favors rubble accumulation and stabilization. This information can help predict localized sediment impacts as well as describing the role of the widely distributed SAG morphology in promoting coral reef recovery.