2018 CSDMS meeting-110
Influence of wave-induced currents on sediment transport and coral growth of atolls
As one of the three major Asian marginal seas in the western Pacific, the SCS occupies less than 1% total ocean area while accommodating 15% atoll (25434.6 km2) in the globe (GSA, 2009), which mainly distribute in the Xisha, Zhongsha and Nansha Islands. Atolls in the SCS are generally ellipse-shaped with a longer axis extending in the NE-SW direction and a wider southwest reef platform compared to the northeast. One possible explanation ascribed such features to the monsoon circulation (northeast and southwest monsoons blow alternatively in winter and summer) over the SCS (Zeng, 1984). Waves and currents influence the atoll development by (1) sediment suspension and transportation that can influence the transparency of the water, thus the symbiotic algae and the coral growth, (2) supply of dissolved oxygen and nutrient and (3) removal of metabolic wastes under normal weathers, while storm waves can cause large-scaled breakage, transportation and reconfiguration of reefs (e.g. Chappell, 1980; Storlazzi et al., 2005). Yet, little data was available regarding the hydrodynamic conditions of the forereef of the SCS atolls. Here, we conducted in situ tripod mooring observations (ADCP, ADV & CTD) for at least one tide cycle in 15-18 m water depth at the southeast forereef of three typical atolls – Xiaonanxun (NX), Anda (AD) and Kugui (KG) Reef – in the SCS, respectively, and collected coral sediment samples at different zonation of atolls in September 2017. During the observation periods, tide elevations varied by ca.1 m in all the three sites, with the highest 1.16 m in AD and lowest 0.96 m in KG. Mean flow velocity turns out to be as weak as about 0.1 m/s, with the weakest ~0.05 m/s in KG. Wave influence appears to be strongest in NX, with the significant wave height of ~1 m, in contrast to the 0.6 m and 0.4 m in AD and KG, respectively. The hydrodynamic observations under normal weathers should be able to transport the fine reef debris alone, with limited sediment transport rates of 0.61, 0.01 and 0.64 m3/m per tidal period in the observations in NX, AD and KG, respectively. Coarse coral rubbles and gravels might be only transported during extreme weathers. More observations and modeling work are needed, e.g. simulations of waves’ influence on atoll sedimentary systems’ development with XBeach.