Dynamic Response of the Fluid Mud to a Tropical Storm

Abstract

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 ~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.

Impact of Vegetation on Lateral Exchanges in a Salt Marsh‐Tidal Creek System

“…The time steps for the external and internal modes were 1.0 and 10.0 s in the large domain and 0.1 and 1.0 s in the small domain, respectively. The time step of 10 min used for the wave module was the same as that used by Ge et al (2013Ge et al ( , 2020. Meteorological forcing was constructed using ERA-Interim data from the European Center for Medium-Range Weather Forecasts.…”

“…Tidal forcing was predicted from the TPXO 8 database (Egbert & Erofeeva, 2002). The other configurations used in the large domain were the same as those applied by Ge et al (2015Ge et al ( , 2020. To examine the impacts of vegetation on physical processes in the tidal creek-salt marsh complex, we ran the model with and without the inclusion of vegetation.…”

“…The time step of 10 min used for the wave module was the same as that used by Ge et al. (2013, 2020). Meteorological forcing was constructed using ERA‐Interim data from the European Center for Medium‐Range Weather Forecasts.…”

“…In mudflats and tidal creeks where vegetation is absent, the Partheniades-Krone equation is used for suspended load transport, and the Meyer-Peter-Müller formulation is chosen for bedload transport. The original and default settings in the FVCOM sediment module include these formulations (Chen et al, 2013;Ge et al, 2015Ge et al, , 2020Warner et al, 2008).…”

“…An unstructured-grid model, for example, the finite-volume community ocean model (FVCOM), can resolve complex irregular coastal and estuarine geometries. The FVCOM has been successfully applied to simulate water changes over tidal creek-salt marsh complexes with high resolutions of up to a few meters in coastal regions encompassing salt marshes (e.g., Chen et al, 2008Chen et al, , 2013Ge et al, 2015Ge et al, , 2018Ge et al, , 2020Lai et al, 2015;Zhao et al, 2010).…”