Two relatively comprehensive and long‐term observation campaigns of bottom sediment dynamics have been conducted at the subaqueous Yellow River delta in the winters of 2016 and 2018. High suspended sediment concentration (SSC) up to 10 g/L were observed in 2016 and active events of wave‐supported fluid mud (WSFM) were found to exist at the abandoned river delta during storms in 2018. A set of calibrated optical backscatter sensors measured the near‐bed SSC >30 g/L with a maximum WSFM thickness >60 cm. WSFMs emerging at the abandoned river delta were classified into three types: single‐peak, multi‐peak, and hybrid according to the time‐varying characteristics of WSFMs. The dominant factors triggering the WSFM generation and dissipation are storm‐wave actions and tidal effects, with limited contribution from current‐induced turbulence. In this study, several lines of evidence were put forward to prove the presence of seabed liquefaction to a depth of 10–16 cm, and thus we can infer that another significant factor enhancing the WSFM thickness scale may be wave‐induced liquefaction. Observations showed obvious signs of WSFM motion, or in other words, wave‐supported gravity flow (WSGF) moved downslope. The downslope velocity (around 1.9 cm/s) was estimated by solving a buoyancy‐friction force balance. This study observed frequent WSFM events at the abandoned Yellow River delta for the first time and demonstrates the need to attach more concerns about its roles in sediment transport and offshore engineering in the future.

The sloping silty sediments in estuarine deltas are frequently affected by extreme storms, and they are prone to liquefaction instability. The unstable liquefied sediments of the slopes can subsequently form a sediment gravity flow (SGF), which can seriously endanger offshore engineering facilities. To better understand the characteristics and mechanism of wave-induced liquefied sediment gravity flow (WILSGF), a flume experiment was conducted to reproduce the formation, movement, and deposition processes of the WILSGF and analyze their controlling factors using natural silty sediment samples collected from the Yellow River Delta in China. The results show that the mass of the WILSGF comes from the sediment in the liquefied layer, and the movement of the WILSGF in these experiments was significantly affected by the wave orbital velocity and the relative outflow position. At the initial stage of the formation of the WILSGF, the phase and amplitude of the WILSGF were the same as those of waves, and the maximum velocity of the WILSGF reached 2.39 cm/s. The velocity of the WILSGF decreased continuously with the downward evolution of the liquefaction interface. When the liquefaction depth reached its maximum value, there was no WILSGF. We also found that the median particle size of the WILSGF was greater than that of the original seabed due to wave-induced seabed coarsening and the intrusion of ambient water. This study has guiding significance for in-depth understanding and prediction of the geological hazards caused by WILSGF.