Observation and modeling of the storm-induced fluid mud dynamics in a muddy-estuarine navigational channel
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 suspension 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, erosion, turbulence damping, drag reduction, density flow, and turbidity maximum. The second method can be called engineering-oriented or macro-mechanism driven, which is the approach chosen mostly by hydraulic and coastal engineers, who are concerned with the horizontal and overall regime of currents and sediment; typically the keywords in this type of research are residual circulation, tidal asymmetry, sediment availability, flow regime and so on.
Many complex physical processes are related to the formation of fluid mud, such as flocculation, settling and mixing, deposition, re-entrainment, gelling, consolidation, liquefaction and erosion (Winterwerp, 1999). In addition, McAnally et al. (2007b) showed that physics, such as rheology, as well as chemical oceanography and microbiology also play a large role in fluid mud behavior. Many efforts (Wolanski et al., 1988, Kineke et al., 1995, Ali et al., 1997, Shi, 1998, Le Hir and Cayocca, 2002, Vinzon and Mehta, 2003, Guan et al., 2005, Winterwerp, 2006, Hsu et al., 2007, McAnally et al., 2007a) have been dedicated to investigating the formation of fluid mud. Among those studies, the effect of wave and storm processes on fluid mud has attracted considerable attention in recent years. McAnally et al. (2007a) pointed out that fluidization of soft sediment beds by wave agitation is one of three principal mechanisms of the fluid mud formation. Li et al. (2004) suggested that the formation of fluid mud phenomena may fall into three categories: slack water, storm and salt wedge. Through measurement data from two moored tripods, Traykovski et al. (2000) showed that the fluid mud could be trapped within the wave bottom boundary layer. Based on tripod data, Puig et al. (2004) also provided a clear picture of the influence of surface–wave activity on the rapid generation of a sediment gravity flow (fluid mud) by development of excess pore water pressure during storms. Warner et al. (2008) utilized ROMS and SWAN models to reveal that bottom sediment resuspension is controlled predominantly by storm-induced surface waves and transported by the tidal- and wind-driven circulation. With the aid of some laboratory experiments van Kessel and Kranenburg (1998) showed that the wave-induced liquefaction (fluid mud) of subaqueous mud layers may be a mechanism of rapid sedimentation observed in navigational channels after storms. In summary, wave energy has the potential to resuspend, release and load sediments in a submarine layer, which facilitates the formation of fluid mud; in particular, if it can stir up large quantities of sediment over mudflats and in shallow areas under wind wave conditions. In short, storm generation is considered one of the most significant causes of fluid mud.
In this paper, firstly, a storm-induced fluid mud event in a muddy-estuarine navigational channel is studied. Secondly, wind wave propagation is modeled to examine the condition of sediment availability under a cold-air front, and three-dimensional (3D) hydrodynamics are simulated to achieve a better understanding of the major mechanism determining the dynamics of fluid mud. Finally, both process-oriented and engineering-oriented methods are employed to investigate the possible factors influencing the mechanisms and transport processes of this storm-induced fluid mud event.
Section snippets
The fluid mud event
Recently, a 90 km-long and 12.5 m-deep (all elevations, heights and water depths in this paper are referred to the Lowest Astronomical Tide) deepwater navigational channel (DNC) in the North Passage of Yangtze Estuary, China was completed. The engineering construction of the DNC was launched in 1998, the water depth of the navigational channel was developed in three steps from 8.5 m in 2002 (Phase I), 10 m in 2005 (Phase II), to 12.5 m in 2010 (Phase III) (see Fig. 2).
In recent years, according to
Modeling
We know that sediment is mainly agitated by wave effects and transported by tidal currents during some extreme wave weather events, for instance storm surge and typhoon (Kineke et al., 1995, Dalrymple et al., 2008). So here numerical evidence for the availability of suspended particulate matter (SPM) is provided by wave modeling for examining the sediment source of the fluid mud event. At the same time, the net transport of tidal currents and sediment transport pathway is investigated by 3D
Process-oriented methods
It is still unclear how the fine sediment–current interaction would affect the basic erosion–deposition process near the bed layer, but a number of commonly accepted understandings (e.g. Ali and Geoprgiadis, 1991, Le Hir et al., 2000, Traykovski et al., 2000, Madsen and Wood, 2002, McAnally et al., 2007a, Manning et al., 2010) have been achieved by detailed measurements and experiments. In this study, we examine the physical processes of fluid mud (i.e. fluidization, flocculation and downslope
Conclusions
A storm-induced fluid mud event is investigated in a muddy-estuarine navigational channel from the views of both process-oriented and engineering-oriented approaches by the means of observations, hydrodynamic modeling and wave propagation simulation. A non-dimensional parameter, wave breaking factor is proposed in this paper to investigate the wave-induced sediment availability. Besides, the Lagrangian residuals, bed shear stress and sediment transport pathway are all analyzed to study the
Acknowledgments
This work is jointly supported by UNESCO-IHE Partnership Research Fund (UPaRF) under contract no. 60038881, National Key Technology R&D Program of China under contract no. 2013BAB12B00 and Shanghai Municipal Natural Science Fund of China under contract no. 11ZR1415800. Prof. John Z. Shi from Shanghai Jiao Tong University is thanked for his insightful advice on hindered settling effect on fluid mud. Prof. Richard Burrows from University of Liverpool is also thanked for having sent the copies of
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