Difference between revisions of "Dynamics of mud transport"
(→Flocculation) |
(→Flocculation) |
||
| Line 12: | Line 12: | ||
<br> | <br> | ||
<p> | <p> | ||
| − | == Flocculation == | + | === Flocculation === |
A key feature of mud particles is their cohesive nature that distinguishes them from non-cohesive solid particles such as sand.<br> | A key feature of mud particles is their cohesive nature that distinguishes them from non-cohesive solid particles such as sand.<br> | ||
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).<br> | 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).<br> | ||
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. <br> | 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. <br> | ||
| − | 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)<ref name ="ref 2 "/> , 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 [[Algal_bloom|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. | + | 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)<ref name ="ref 2 "/> , 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 [[Algal_bloom|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. <br> |
| + | Despite the trend to characterize the floc structure by a fractal number <ref name = "ref 5">Kranenburg, C. (1994). On the fractal structure of cohesive sediment aggregates. Estuarine, Coastal and Shelf Science, 39:451-460.</ref>, 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. | ||
| + | <p> | ||
| + | <br> | ||
| + | <p> | ||
Revision as of 13:09, 3 September 2012
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)[5] , 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 [6], 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.
- ↑ 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.
- ↑ Cite error: Invalid
<ref>tag; no text was provided for refs namedref 2 - ↑ Kranenburg, C. (1994). On the fractal structure of cohesive sediment aggregates. Estuarine, Coastal and Shelf Science, 39:451-460.
