Observation and modeling of the storm-induced fluid mud dynamics in a muddy-estuarine navigational channel Wan Yuanyang a,b, ⁎, Dano Roelvink b , Li Weihua a , Qi Dingman a , Gu Fengfeng a a Shanghai Estuarine and Coastal Science Research Center, Shanghai 201201, China b UNESCO-IHE Institute for Water Education, 2601 DA Delft, The Netherlands abstract article info Article history: Received 13 January 2013 Received in revised form 18 March 2014 Accepted 25 March 2014 Available online 18 April 2014 Keywords: Fluid mud Storm-induced Stratified flow Modeling Yangtze Estuary Navigational channel Observations of storm-induced fluid mud dynamics have been conducted at the North Passage deepwater navigational channel (DNC) of the Yangtze Estuary in October to December 2010, during the occurrence of a cold-air front. The measurement data reveal that just after the critical wind wave event, a large amount of fine sediment was trapped in a state of fluid mud along the channel. The observed thickness of the fluid mud was up to about 1–5 m, which caused some significant economic and safety problems for shipping traffic in the Yangtze Delta area. The mechanisms and transport processes of the storm-induced fluid mud are analyzed and presented from the angles of both process-oriented and engineering-oriented methods. With the help of hydrodynamics and wave modeling, it could be inferred that the behavior of the storm-induced fluid mud event mainly depends on the overall hydrodynamic regimes and the exchanges of sediment, which is released by storm-wave agitation from adjacent tidal flats. These sediments are accumulated as fluid mud, and subsequently oscillate and persist at those locations with weaker longitudinal residuals in the river- and tide- dominated estuary. In addition, the downslope transport of fluid mud is also thought to have stimulated and worsened the fluid mud event observed in this study. Our modeling results and observations demonstrate that: (1) the transport of fluid mud is an advective phenomenon determining the central position of fluid mud layer along the channel, and it's also a tidal energy influenced phenomenon controlling the erosion and accumulation of fluid mud; and (2) both suspended particulate matter availability and local residual flow regime are of critical importance in determining the trapping probability of sediment and the occurrence of fluid mud. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The occurrence of fluid mud is widely covered and commonly witnessed in many locations, such as estuaries (e.g. Mehta, 1989; Winterwerp, 1999), lakes (e.g. Li and Mehta, 2000; Bachmann et al., 2005), rivers (e.g. Wang, 2010), waterways (e.g. Li et al., 2004) and even open sea (e.g. Puig et al., 2004). Fluid mud exists in the water column (Fig. 1, Table 1) as a transitional stage (McAnally et al., 2007a), when the net rate of sediment falling from the upper suspen- sion layer into the bottom layer exceeds the dewatering rate of the high-concentration sediment–water mixture, and the bonds of the interconnected matrix structure are not strong enough to form an erosion-resisting consolidated layer. The characteristics of fluid mud differ significantly from those of both suspensions above and the consolidated bed below. The temporal transition status varies quickly in response to sediment availability and intensity of currents (when fluid mud is ‘left alone’ it will consolidate). Considering the condition of sediment availability or supply, it may relate to micro-scale sediment mixing, such as flocculation and hindered settling (Le Hir et al., 2000); it could form a stepped vertical profile of suspended sediment concentration (SSC) and trap sediment in the near-bed layer. At the same time, the sediment supply is also associated with macro-scale sediment movement and circulation (Shi, 2010), where transport of enough fine sediment mass from nearby shoals and beaches to the navigational channel favors the formation of a fluid mud layer. The current dynamics can also be divided into micro- and macro-scale processes, where the micro-scale processes include turbulence damping, drag reduction and some stratification effects of flow, while the macro-scale refers to the regime of currents, residual circulation, tidal asymmetry and so on. Therefore, there are two types of viewpoint from which to study the dynamics of fluid mud. The first approach is process-oriented or micro- mechanism driven, which is conducted primarily by sedimentologists, geomorphologists and oceanographers; they focus on some responses and influences on sedimentary processes and vertical profiles of currents and SSC, such as flocculation, re-suspension, deposition, ero- sion, turbulence damping, drag reduction, density flow, and turbidity maximum. The second method can be called engineering-oriented or Geomorphology 217 (2014) 23–36 ⁎ Corresponding author at: Shanghai Estuarine and Coastal Science Research Center, Shanghai 201201, China. Tel.: +86 21 68909900 243; fax: +86 21 68905318. E-mail address: sway110@qq.com (Y. Wan). http://dx.doi.org/10.1016/j.geomorph.2014.03.050 0169-555X/© 2013 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph