Dynamic Response of the Fluid Mud to a Tropical Storm

Fluid mud (FM) is a unique sedimentary feature in high-turbidity estuaries, where it can make a rapid contribution to morphodynamics. Insufficient field measurements and fixed-point monitoring lead to deficient understandings of the formation, transport, and breakdown of the FM under extreme weather conditions. A field survey was conducted in the Changjiang Estuary during the period of turbidity maximum, just after Typhoon Haikui. The measurements captured the formation of the FM beneath the suspended layers, particularly around the lower reach of the North Passage. The thickness of the observed FM gradually decreased landward along the channel, with the maximum value reaching similar to 0.9 m. The major features of the observed storm-induced FM were simulated using the Finite-Volume Community Ocean Model. The results indicated that the initial appearance of the FM was the result of a typhoon-intensified, salinity-induced stratification in the outlet region. The subsequent landward propagation of the FM was driven by the combined effects of the FM-induced mud surface pressure gradient force and saltwater intrusion near the bottom. Weak mixing during the subsequent neap tidal period sustained the FM as it rapidly extended into the middle region of the North Passage. This produced a large velocity shear at the interface of the FM and upper suspension layer, increasing the entrainment from the FM to the upper suspension layer. As a result of the increased tidal mixing, the FM weakened and then finally broke down in the subsequent spring tidal period. Plain Language Summary The environment along the large river to estuary continuum is generally turbid and frequently produces highly concentrated benthic sediment suspensions, that is, fluid mud (FM). The FM is a sediment feature, with a concentration mostly in the range of 10 to >100 g/L. It is difficult to track the FM's movement and breakdown. The response of the FM to extreme atmospheric and oceanic conditions is therefore not well understood. In this study, a comprehensive field campaign identified the large-scale formation of the FM after a severe tropical storm in the Changjiang Estuary. The life cycle of the FM in this estuary cannot, however, be determined through the field observation alone. A two-layer FM model was developed to achieve this goal. The experiments suggested that the key physical factor was the stratification resulting from the typhoon-enhanced saltwater intrusion. This led to the formation of the FM in the near-bottom layer. After the FM formed, it extended onshore along the channel under the influence of an ambient saltwater intrusion. The FM was constrained in the benthic layer as a consequence of weak mixing from the saltwater intrusion. The breakdown of the FM was governed by enhanced tidal mixing in the subsequent spring tide.