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Characteristics of muddy coasts

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This article provides an introduction to and summary of habitats derived from fine sediments suspended in tidal waters.

Muddy coasts are only found in environments that are fairly calm with respect to wave conditions, or where there is abundant supply of fine sediments. They are normally vegetated - for example, with mangroves fronted by very flat slopes or tidal flats. A muddy coast with mangrove vegetation is characterized by a muddy shoreface, sometimes in the form of muddy tidal flats, and the lack of a sandy shore.

Some characteristics

The area exposed to tidal variation in a muddy coast is often vegetated by mangrove, and this coast type occurs in tropical climates where rivers supply abundant fine material to coastal zone (CZ). Wave exposure is normally low to moderate; the tidal regime can be any. Silt and clay are not stable in the littoral zone and are washed offshore. If large amounts of fines are supplied, mud flats and mangrove areas may develop.

Mangrove constitutes an important part of muddy coast profile, biologically and for stability. Cutting can cause severe problems, decreasing biodiversity and causing erosion and flooding.

The connected coast type is often low wetland exposed to flooding. Mixed environments with wave-exposed shores or sandy tidal flats alternating with mud-dominated tidal flats and deeper muddy areas are common, and require special management consideration.

Salt marsh areas and intertidal mud flats

Salt marsh. Source: USGS
Salt marshes and intertidal mud flats act as natural coastal defences against a rising sea level. It is however questionable whether these areas are able to keep track with a changing sea level at the same time as adjusting to various anthropogenic impacts including stress caused by herbicides, nutrient loading etc.

Sustainable development of an ecosystem requires the system to be fully understood and the future development to be adequately anticipated in order to make the right basis for management In order to predict the future development of salt marshes under a changing climate, process descriptions of the bio-geo-chemical and geo-morphological processes influencing this area are needed as well as an understanding of the present ecosystem vulnerability. The interactions between human impacts and natural changes in the ecosystem include the aspects of bio-diversity and global change.

State of the art

Processes

Estuarine areas and coastal lagoons are sinks for fine-grained sediment on an annual time scale. The processes responsible for this net import of sediment to estuaries are well described in literature. The processes involved include:

  1. Settling and scour-lag[1][2];
  2. Estuarine circulation driven by horizontal density gradients;
  3. Shoaling of the tidal wave causing an asymmetric distribution of the velocity and sediment concentration;
  4. The aggregation of fine-grained particles either by electrochemical or biological processes[3].


These processes together have the effect of accumulating fine-grained sediments in shallow coastal lagoons, generally keeping pace with the magnitude of the local rise in sea level[4]. However, there is large spatial variation in accumulation rates with the highest rates in the inner parts of the lagoons and the lowest rates close to the tidal inlet and in deeper parts of the area[5]. Therefore, where accommodation space is available, salt marsh areas develop, especially fringing the inner parts of estuarine areas.


Local hydrodynamics

Tidal currents and waves dominate the local hydrodynamics and thus determine the physical, morphological and biological characteristics of a mudflat. The degree of wave activity depends upon both the fetch and the nature of the prevailing wind, and can vary significantly within an estuary. It is well known that even small waves are able to erode large amounts of surface sediment; this material can be carried shoreward with the advancing tide[6]. The level of wave attack at a point on the mudflat is the result of wave attenuation and the relationship between the mudflat slope and the water level and is thus sensitive both to rise in sea level and storm frequency. The mudflat profile will change in response to different forcing, altering the feedback between the morphology and hydrodynamics, whilst evolving to reach some new equilibrium.

Eco-morphology of muddy coasts

Mud shrimp. Source: USGS

Biological processes such as the effect of the macrofauna living in the mud and algae growing on the sediment surface producing EPS (Extracellular Polymeric Substances) are of prime importance for mudflat stability and erodibility[7][8]. The net effect of these processes on erosion and deposition on the intertidal mudflats is only partly understood. Furthermore, there is a gap in knowledge about how a changing climate will affect key species of the intertidal ecosystem in particular[9]. The biological, sedimentary and physical processes are closely inter-connected, and complex relationships control the nature and movement of surface sediment across the intertidal zone. Therefore, alterations in the activity of key species on the tidal flats due to climate change may lead to significant changes in salt marsh development.

Effect of storm events

The altering processes of inundation and drying are on an average believed to establish a steady state situation with salt marshes being both eroded and fed during the inundation events. Therefore, it is not a straightforward problem to foresee what will happen to a specific salt marsh area if the storm frequency increases or if mean sea level rises faster than it has up till now[10]. Salt marsh areas build up vertically when inundated by turbid estuarine waters. This means that episodic events like storm surges and extreme high water levels are important and one storm event (as on 3rd December 2000, in the North Sea, also influencing the Danish Wadden Sea) may substantially contribute to the annual sedimentation rate. The timing and frequency of such events are likely to be very different in warmer climates than in colder climates with important implications for the stability of the system.

See also

Mud

External links

Sand and Mud flats --- description and information of coastal habitat restoration

References

  1. van Straaten, L. M. J. U. and Kuenen, Ph. H. (1958) Tidal action as a cause of clay accumulation. Journal of Sedimentary Petrology. Vol. 28, 406-413.
  2. Postma, H., 1967. Sediment transport and sedimentation in estuarine environment. In: Estuaries. Ed. by G. H. Lauff. Am. Assn. Adv. Sci., Washington, D. C., 158-179.
  3. van Leussen, W. (1994) Estuarine macroflocs and their role in fine-grained sediment transport. Proefschrift from university of Utrecht, Holland. 488 p.
  4. Nichols, M. M. (1989) Sediment accumulation rates and relative sea level rise in lagoons. Marine Geology. Vol. 88, 201-219.
  5. Pejrup, M,. Larsen, M. and Edelvang K. (1997) A fine-grained sediment budget for the Sylt-Rømø tidal basin. Helgoländer Meeresuntersuchungen. Vol. 51, 253-268.
  6. Christie, M. and Dyer, K. R. (1998) Measurements of the turbid tidal edge over the Skeffling mudflats. In: Sedimentary Processes in the Intertidal Zone (Eds: Black, Paterson and Cramp). Geolgical Soc. Lon. Vol. 139, 45-55.
  7. Holland, A.F., Zingmark, R.G. & Dean, J.M. 1974. Quantitative evidence concerning the stabilization of sediments by marine benthic diatoms. Marine Biology, 27, 191-196.
  8. Nowell, A.R.M., Jumars, P.A. & Eckman, J.E. 1981. Effects of biological activity on the entrainment of marine sediments. Marine Geology. 42, 133-153.
  9. Asmus, H. and Asmus, R. (1998) the role of macrobenthic communities for sediment-water material exchange in the Sylt-Rømø tidal basin. Senckengergiana Maritima. Vol. 29, 111-119.
  10. Dyer, K. R. (1994) Estuarine sediment transport and deposition. In: Sediment transport and depositional processes. (Ed: Pye, K.). Blackwell sci. Pub. 193-215.
The main author of this article is Edelvang, Karen
Please note that others may also have edited the contents of this article.