Wave-supported fluid mud and sediment vertical mixing under winter storm

Abstract

Data from 5 monthly field observations of the modern Yellow River delta demonstrate the formation of the fluid mud layer (FML) above the seabed under continuous storms and the sediment resuspension and vertical mixing over the FML. It was observed that the storm wave-related oscillatory flows during winter were mainly in energetic regime (400<ReΔ<1200). The high concentration sediment is trapped in a thin layer stably stratified above the seabed due to the formation of the wave-supported fluid mud (WSFM) layer. Above the WSFM, the total velocity spectral flux E(f) at z=0.7 m is decreased by approximately 14%∼22%, indicating a significantly decreased sediment carrying capacity of the flow. To explore detailed flow and sediment dynamics within the WSFM, a series of experiments in a wide wave flume with WSFM generated were carried out. The velocity and sediment concentration profiles measured over the water depth suggested that the sediment was confined in a thin FML even in high ReΔ regimes (ReΔ>480), and the behavior of FML can transition between a laminar to turbulent with varying wave orbital velocity and Rib. Laminar fluid mud dominates when uw<45 cm/s and ReΔ<518, resulting in exceptionally high viscous dissipation of wave energy. This can explain the observed significant decrease of E(f) during the onset of storm wave recession. Once the fluid mud layer forms, stronger flow with uw>45 cm/s (Rib<0.2) can disturb the interfacial wave of the fluid mud and the flow above it, which cause strong entrainment of the fluid mud layer and sediment mixing into the water column.