Characteristics of muddy coasts
This article provides a short introduction to and summary of habitats derived from fine sediments suspended in tidal waters.
Muddy coasts are common in environments that are fairly calm with respect to wave conditions or where there is abundant supply of fine sediments. A global inventory of coasts dominated by fine-grained sedimentary deposits (silts and clays) forming flat surfaces, revealed that 14% of the world’s ice-free coastline is muddy, about 60% of which is located between 25°N and 25°S. The total length of muddy coastlines is estimated at about 90,000 km[1].
Contents
[hide]Mangroves
Coastal areas exposed to tides in tropical and subtropical climates are often covered with mangrove forests. This type of coast occurs where rivers bring abundant fine material to the coastal zone. Wave exposure is generally low to moderate, as silt and clay are easily washed offshore by wave action. The tidal regime can be any. Mangrove forests stabilize the muddy subsoil. Mangrove clearing causes erosion and biodiversity loss and increases the flood risk of inland areas, which often consist of low-lying wetlands.
Salt marsh areas and intertidal mud flats
Sustainable development of an ecosystem requires adequate anticipation to future developments. In order to predict the response of salt marshes to climate change, 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 biodiversity and global change.
State of the art
Processes
Estuarine areas and coastal lagoons are net sinks for fine-grained sediment. The processes responsible for this net import of sediment to estuaries are well described in the literature. The processes involved include (see e.g., Dynamics of mud transport, Estuarine turbidity maximum):
- Settling and scour-lag[2][3];
- Estuarine circulation driven by horizontal density gradients;
- Tidal asymmetry resulting from the interaction between estuarine morphology and tidal wave propagation;
- Aggregation of fine-grained particles either by electrochemical or biological processes[4].
These processes together have the effect of accumulating fine-grained sediments in shallow coastal lagoons. In most estuaries and coastal lagoons, accretion of fine sediments is currently keeping pace with the local sea level rise[5]. A large spatial variation in accumulation rates is often observed, 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[6]. Therefore, where accommodation space is available, salt marsh areas develop, especially fringing the inner parts of estuarine areas. The global mud coast inventory[1] found that in the period 1984-2016, about 1/3 of the muddy shorelines worldwide were about stable, about 1/3 were accreting and about 1/3 were eroding.
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 strength of the prevailing wind, and can vary significantly within an estuary. Even small waves are able to erode large amounts of surface sediment which is subsequently transported by tidal currents[7]. Wave action on the mudflat depends on the relationship between the mudflat slope and the water level and is thus sensitive both to rise in sea level and storm frequency. Tidal channel morphodynamics also play an important role in the morphological evolution of mudflats.
Eco-morphology of muddy coasts
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[8][9]. 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[10]. 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. See Biogeomorphology of coastal systems for further details.
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[11]. 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 may substantially alter the net annual sedimentation[12]. 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.
Related articles
- Mud
- Salt marsh
- Mangroves
- Fluid mud
- Flocculation cohesive sediments
- Estuarine turbidity maximum
- Coastal mud belt
- Dynamics of mud transport
- Coastal and marine sediments
- Dynamics, threats and management of salt marshes
- Sediment deposition and erosion processes
- Estuarine circulation
- Tidal channel meandering and marsh erosion
References
- ↑ Jump up to: 1.0 1.1 Hulskamp, R., Luijendijk, A., van Maren, B., Moreno-Rodenas, A., Calkoen, F., Kras, E., Lhermitte, S. and Aarninkhof, S. 2023. Global distribution and dynamics of muddy coasts. Nature Communications 14, 8259
- Jump up ↑ van Straaten, L. M. J. U. and Kuenen, Ph. H. 1958. Tidal action as a cause of clay accumulation. Journal of Sedimentary Petrology 28: 406-413
- Jump up ↑ 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.
- Jump up ↑ van Leussen, W. 1994. Estuarine macroflocs and their role in fine-grained sediment transport. PhD thesis Utrecht University, Netherlands, 488 p.
- Jump up ↑ Nichols, M. M. 1989. Sediment accumulation rates and relative sea level rise in lagoons. Marine Geology. Vol. 88, 201-219.
- Jump up ↑ Pejrup, M,. Larsen, M. and Edelvang K. 1997. A fine-grained sediment budget for the Sylt-Rømø tidal basin. Helgoländer Meeresuntersuchungen. 51: 253-268.
- Jump up ↑ 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. 139: 45-55
- Jump up ↑ 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
- Jump up ↑ 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.
- Jump up ↑ Asmus, H. and Asmus, R. 1998. The role of macrobenthic communities for sediment-water material exchange in the Sylt-Rømø tidal basin. Senckengergiana Maritima 29: 111-119.
- Jump up ↑ Dyer, K. R. 1994. Estuarine sediment transport and deposition. In: Sediment transport and depositional processes. (Ed: Pye, K.). Blackwell sci. Pub. 193-215.
- Jump up ↑ Andersen, T.J. and Pejrup, M. 2001. Suspended sediment transport on a temperate, microtidal mudflat, the Danish Wadden Sea. Marine Geology 173: 69-85
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