Sedimentary zonation shift of tidal flats in a meso-tidal estuary

Abstract

Understanding the spatial-temporal pattern of sedimentary dynamics on tidal flats is important for examining their ecological function and evolutionary trend. In this study, the dynamic state of sediments across an open-coast tidal flat of the meso-tidal Changjiang Estuary, China, on a monthly scale from December 2014 to June 2016 was examined, based on a high-spatial-resolution sedimentological-topographic survey and a collection of related meteorological-hydrological data. The results revealed a continuously distinct zonation of sediments, which transformed seaward from sand-dominance to silt dominance over a short distance. A zoning, to divide the tidal flat into a sand zone, a mixture zone and a silt zone, was delimited based on the cluster analysis of grain size frequency distributions. The zonation was highly dynamic, with the mixture zone migrating upward during summer and experiencing an uplift of 1 m in its lower boundary over the time period examined. Within each zone, the medium size and sorting coefficient of sediments tended to be smaller in winter than those in summer. Alterations of the mean tidal level likely controlled the zonation shift and sediment variations. A higher mean tidal level indicated a landward shift of the location exhibiting shear stress maxima and the mixture zone had to migrate upward to maintain an appropriate stress for the development of mixed sediments. The impacts of waves were masked under normal conditions, but during storms strong waves dominated an embedded downward displacement of the mixture zone. Characteristics of sediments covering a large region of the tidal flat changed along with zonation shifts, resulting from altered proportions among sandy, mixed and silty sediments. The study highlights that sedimentary zonation shift, under the impacts of seasonal variations in tidal level, can be a major manifestation of tidal flat sedimentary dynamics.