Dynamics of mud transport
Contents
Introduction
Mud in coastal areas is mainly found in intertidal deposits [1].
Migniot [2] was probably the first to present a comprehensive overview of all the processes involved in mud dynamics.
In order to understand the dynamics of mud in coastal environments, it is necessary to properly define mud and its properties, in contrast to sand and other non-cohesive particles.
Definition
Mud is defined as a mixture of mainly fine-grained sediments (clays, silt and sand), organic matter and water [3], where the cohesive properties of the clay fraction, enhanced by the properties of the organic matter, dominate the overall behaviour. Studies on erosion behaviour of sand-mud mixtures indicate that the bed exhibits cohesive behaviour for clay contents above 15-20% [4]. In daily language "mud" refers to the deposited state of mud particles. In this state mud can occur as a fluid-like of soil-like entity. The dynamics of mud then refers to the formation, deformation and erosion of such layers.
Flocculation
A key feature of mud particles is their cohesive nature that distinguishes them from non-cohesive solid particles such as sand.
Clay particles in an aquatic environment tend to stick together (coagulate) as the result of the van der Waals forces into aggregates or flocs. This process is enhanced by slimes (EPS) and mucus produced by micro-benthos and bacteria (that feed on decaying organic matter).
The size, structure and density of flocs are determined by the forces the aggregate-particles undergo. These forces comprise: hydrodynamic forces (especially shear), collisions between particles and electrochemical forces (determined by the composition of solid particles and dissolved ions in the ambient water). The latter explains also why mud particles in fresh and saline water have a different structure.
The basic building blocs of flocs are the primary particles and/or flocculi (compact aggregates, O(10 µm), which rarely break down into primary particles), which form micro-flocs (silt size), macro-flocs (fine-sand size) and organic-rich mega-flocs (coarse-sand size, linked to seasonal biological events, such as algae-bloom). The simultaneous occurrence of micro- and macro-flocs is attributed to the tidal dynamics, and/or sometimes to the mixing of sea- and river-born aggregates. Further research is necessary to understand better the dynamics of the different floc populations.
Despite the trend to characterize the floc structure by a fractal number [5], the structure is not self-similar. In general, the fractal dimension decreases with increasing floc size, which implies that the floc structures becomes more and more open and its strength decreases.
Settling velocity
Since size, structure and density of a mud particle vary dynamically, the same applies to its settling velocity. The value of the parameter “settling velocity” actually should correspond to the averaged value for the entire local floc population, i.e. the settling flux per unit concentration. It has been demonstrated that the sediment flux can much more accurately be calculated when considering the major floc populations individually [6].
For hindered settling of mud particles, it is important to express the hindrance correction factor in terms of the effective volumetric concentration occupied by the aggregates, including the immobilized water captured in the porous structure of the floc.
Mud Bed Formation
Accumulation of mud particles on the bottom generates an open-spaced soil, which network structure is sometimes compared with a card-house (or sponge). When the layer grows in thickness, the overburden will cause a stress on the structure by which it will slowly collapse. During this process, known as self-weight consolidation, pore water is expelled to the surface and the pressure of the pore water increases relative to the hydrostatic pressure, generating excess pore pressures. The process continuous until the strength of the soil skeleton (the effective stress) is in equilibrium with the submerged weight is has to carry and the excess pore pressures are dissipated.
The critical volumetric concentration at which soil formation starts is called the gel point. In principle it could be predicted if the effective volume of the flocs is known. For North-Sea mud, the gel point is found for soil bulk densities of about 1100 kg/m³, which implies a solids concentration of the order of only 8%. This explains the apparent fluid-like behaviour of freshly deposited mud, since its structure is easily disturbed. This is better understood in the light of the rheological characteristics of fluid mud.
Due to flocculation and hindered settling, instant bed formation is retarded and a near-bottom high-concentrated (HC) suspension layer (also known as high-concentrated benthic suspension, HCBS) is formed.
Mud Bed Destruction
Considering the fact that the soil consists of aggregated particles, which are bound by electrochemical forces of varying origin and strength, these bonds may be broken by various mechanical forces, either shear forces (at the surface or internally) or excess pore pressures that exceed the effective stress (usually due to wave action, but also by e.g. earthquakes). Local micro-cracks may grow to larger cracks and eventually create failure planes in the bed. Erosion is the process where the structure is broken to such degree that the loose parts may be picked-up by the flow and transported. A distinction is furthermore made into the following three erosion modes:
- Surface erosion = particle-by-particle erosion at the surface;
- Mass or Bulk erosion = erosion of a patch of mud above a failure plane;
- Cliff erosion = break-up of solid clumps of over consolidated mud by (boat) waves, which after mechanical rolling erosion may become rounded mud pellets.
Transport modes
Suspended Load Transport of Mud
Traditionally, it has been assumed that cohesive sediments have such low settling velocities that the dominant transport mode is by suspended load. Usually, only dilute suspension transport is considered.
The importance of high-concentrated (HC) suspension transport in the inner layer above the bed (often named “fluid mud”, but this term is more consistently restricted to another state > see Fluid Mud) is often underestimated or ignored. However, the amount of sediment transported in this layer can be very significant. Research on this topic is still ongoing.The thickness of the HC suspension layer above the bottom can be significantly larger than in the case of sand. A relatively sharp interface, a lutocline, can be found between this layer and the dilute layer above. Instabilities can be observed along this lutocline in the form of internal waves.
Contrary to HC sand suspension layers, HC mud suspension layers usually exhibit strong turbulence damping (or even laminarization) and drag reduction. A well known example is that of the Yellow River (China), where roughness values have to be taken corresponding to smoother than a smooth glass plate, in order to predict the hydrodynamic resistance correctly.
Turbidity maximum
Due to interaction of heavier saline sea water and lighter fresh river water, a zone with high suspended loads of cohesive sediments, an estuarine turbidity maximum (ETM), is formed in estuaries. The turbulence due to the interaction keeps particles in suspension, while large fresh water flocs may alter into more compact flocs with a lower settling rate due to the increased salinity. The ETM location moves up- and downstream with the tide, depending on the strength of the tide and the river discharge. In some estuaries, more than one ETM can occur.
Bedload Transport of Mud
Despite the traditional approach of only considering suspended transport for mud, two observations suggest that bedload transport of mud should not too easily be discarded.
- High-concentrated near-bottom transport of mud should not be treated as suspended load, since the theory for the latter requires fully-developed turbulent flow, while the former typically shows the properties of low-Reynolds-number flow, requiring another mathematical treatment (which is still under development). Because of the high amounts of sediment transported in this layer, and the physical similarities with bedload transport of non-cohesive sediments, it seems justified to consider this mode as bedload transport of mud.
- Large clumps of overconsolidated may break off from mud layers that are exposed to the air for longer time such that they dry and show cracks. Once the water line reaches these areas in combination with significant wave action (either wind waves or boat traffic induced waves), large clumps may break off, resulting in cliff erosion. Pulled up and down by the tidal currents, they role over the bottom, they may break up into smaller clumps, erode more or less, and eventually become rounded mud pebbles which can role even better and be transported over longer distances or accumulate in deeper areas. Mud pebbles are formed along the low water line of the intertidal flats of the Yzer Mouth (Nieuwpoort, Belgium) and found back in the mud dredged from the navigation channel. – Laboratory observations suggest that bulk erosion may also generate submarine mud pebbles. As far as known, no evidence from the field on the latter is available.
Fluid Mud Flow & Density Currents
Mud bottoms in underconsolidated state are prone to fluidization and/or liquefaction. The resulting fluid mud can deform under wave action and, when there is no barrier, even flow driven by gravity.
As long as the density remains below the gel point, the effective viscosity is so high that the flow behaviour will remain laminar. When accelerating, the shear and resulting instabilities at the interface will cause entrainment in the two directions (i.e. water into the fluid mud and mud particles into the water column), resulting in dilution of the fluid mud below the gel point, and the water-sediment mixture should now be considered a (highly-concentrated) suspension, which can flow turbulently.
When depositing on a sloping bottom (e.g. river banks on dredged channel slopes), self-weight induced shear may avoid consolidation and keep the deposit liquefied, such that the deposit may flow as a gravity current to the lowest point.
This knowledge is used in low-cost agitation dredging, where a mud deposit is mechanically disturbed and liquefied in low-energy locations, such as lock entrances, with the purpose to accumulate the fluid mud in deeper areas where the mud, if necessary, can be removed during maintenance dredging.
Mud gravity currents (or avalanches) may also be induced due to liquefaction by earth-quakes.
Mud gravity currents may carry large amounts of mud to the deeper ocean where rivers end in a canyon. Example: It has been estimated that about 50% of the mud from the Amazon flows down the continental slope in front of its mouth. About 20% is transported to the west by wave action as migrating mud banks along the Guyanas coast and eventually flows down the Orinoco river canyon into the deeper ocean.
Mud Rheology
The deformation and flow of fluid mud requires a rheological description, i.e. mathematical expressions (closure laws) relating stress to deformation and/or deformation rate.
Deformation under wave action has traditionally been described by visco-elastic or plastic models, which allow analytical solutions under (over)simplified (1D) conditions.
However, considering the link between strength and bed structure, a description by a thixotropic characterization seems more promising. This approach shows (logical) analogies to the structural kinetics approach to flocculation [7].
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References
- ↑ Eisma, D. et al. (1997). Intertidal Deposits: River Mouths, Tidal Flats and Coastal Lagoons. CRC Press, Boca Raton (FL), 525 pp.
- ↑ Migniot, C. (1968). Etude des propriétés physiques de différents sediments très fins et leur comportement sous des actions hydrodynamiques. La Houille Blanche, 1968(No.7):591-620 (in French).
- ↑ Berlamont, J., Ockenden, M., Toorman, E. & Winterwerp, J. (1993). The characterisation of cohesive sediment properties. Coastal Engineering, 21:105-128.
- ↑ Mitchener, H. & Torfs, H. (1996). Erosion of mud/sand mixtures. J. Coastal Engineering, 29: 1-25.
- ↑ Kranenburg, C. (1994). On the fractal structure of cohesive sediment aggregates. Estuarine, Coastal and Shelf Science, 39:451-460.
- ↑ Lee, B.J., Fettweis, M., Toorman, E., Moltz, F. (2012). Multimodality of a particle size distribution of cohesive suspended particulate matters in a coastal zone. J. Geophysical Research, 117, C03014, 17pp. (doi:10.1029/2011JC007552)
- ↑ Toorman, E.A. (1997). Modelling the thixotropic behaviour of dense cohesive sediment suspensions. Rheologica Acta Vol.36 (No.1):56-65.
