The encountering dune fields in a bidirectional flow system in the northwestern South China Sea: Pattern, morphology, and recent dynamics

Abstract

Dune fields are composed of differently scaled of dunes and can reflect the external environmental conditions in both eolian and subaqueous environments. However, little is known about the interplay between dune fields when they meet together. Based on recent field observations, we identified new dune fields that encountered with other dunes migrating oppositely under a bidirectional flow system on the tidal shelf in the northwestern South China Sea. The dune fields, whose front boundary was outlined by the reversal of dune asymmetry, encountered each other end-by-end or side-by-side. Six encounter patterns were identified through the relative relation between dunes over the field boundary, which was associated with the net sand transport strength and sand supply. Under the control of the residual sand transport regime, dunes became steeper and more symmetrical towards the dune-field boundary and migrated gradually slower to the minimum. The regional net bedload transport determined the convergent migration of dunes in the encountering fields and shaped the linear, symmetrical, and steep shapes of dunes over the field boundary. The field boundary was shifting from 2014 to 2016, and most dunes close to the field frontier moved to their stoss side, thereby weakening the asymmetry extent or reversing the asymmetry. The field boundary shifts reflected location inconsistency between the long-term accumulative transport balance (symmetrical shape) and the recent transport balance (non-migration) on dunes near field boundary which was ascribed to the modulation of residual sand transports associated with the systematic changes in the regional flow conditions. These results suggest that the behavior of dunes near the field boundary can sensitively reflect the field-field interplay and reveal the relevant minor environmental changes, thus providing new insights into the surface processes in a bidirectional flow system of a planet with eolian or subaqueous environments.