BATTLES BETWEEN HEAD-TO-HEAD ENCOUNTERING DUNE-FIELDS UNDER REVERSING FLOWS

Dunes and dune-fields, being indicators and recorders of environmental conditions, have attracted extensive attentions and are massively studied on the individual behaviors and dune-dune interactions. However, little is known on the behaviors of dune fields when they meet together. The processes of field-field interaction are still elusive. Using the latest high-resolution bathymetric datasets of 2014 and 2016, we presented the new-found dune fields with opposite-inclined dunes developing under reversing tidal currents on a shallow shelf, northwest South China Sea. We recognized that the dune fields were head-to-head colliding and had a coarse-coarse pattern and a coarse-fine pattern in view of the sediment character. Dune-field fronts, which were outlined using the transition of dune asymmetry, presented convergent bed load transports coexisted with divergent suspended load transports. Towards the dune-field fronts, the dunes obtained steeper shapes due to the different responses of height and length, which were benefited from the bidirectional sand supply and the comparable reversing current speed. Dunes can become larger around dune-field fronts as bed load move together, however, some largest dunes occur inside the dune field rather than near the fronts. Such distinct variations towards the dune-field fronts may suggest the past dominant southward migration of the north fields and the resistance of the south fields. From 2014 to 2016, dunes inside the dune fields mostly moved to their inclining direction which was accordant with the regional net flow distribution. Notably, dunes around the dune-field fronts migrated oppositely, which were caused by the rebalance of regional sand transports. We inferred that the rebalance of regional sand transport is attributed to the regional flow changes associated with recent tide modification, while surface waves and storms cannot be the reason for the apparent changes of sediment transport in the small certain areas. Moreover, the dune fields exhibit various shifting rate in different regions. We identify that the shifting rate is largely influenced by the magnitude of grain size, water depth, dune height, and current speed. This reminds us the behaviors of field-field interactions can indicate and also record the environmental changes, which brings new insights in detecting local environment using dune system both on the Earth and other planets like Mars.