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Mud flat State 2 - eroding
 Identifying
eroding mud flats is difficult. This is due to the cohesive nature of the fine-grained
sediments which make up the substrate. The resistance of mud to erosion is referred
to as the 'yield-strength' i.e. the amount of stress the mud surface can withstand
before it breaks down. The process by which this takes place involves the mass
failure of 'lumps' of sediment which subsequently break up and become dispersed
(Pethick 1984). In the short term mudflats
may appear to be accreting but over a longer period erosion may be the predominating
force.
Caption:
A typical eroding mud flat in Essex, south east England. Note the mud-mounds
with steeply sloping sides in the middle distance. Although the picture appears
to show eroding mudflats there are no measurements to confirm this is the situation.
Assessing the state may require other factors to be taken into account, not
least the presence of eroding saltmarshes.
Factors which can cause the erosion of mud flats include:
- Freshwater river flows;
- Waves and storms and storm surges;
- Sea level rise;
- Reduction in sediment supply.
The principle effects are erosion along the river and creek banks as scouring
action takes place. Without new sediment it is possible, especially in areas
where sea level is rising, for an accreting sand/mud flat to be turned into
one which is eroding. Disturbance from the wake of ships and boats can have
a similar impact. The value
of the mudflats may or may not be impaired, depending on the nature of the erosion
i.e. whether there is a loss of habitat or a lowering of the level of the tidal
flats (or both).
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General guidance: It is important to recognise that erosion is
a natural process in the formation of sand and mud flats. This state is
defined when the overall balance of erosion over accretion results in
a loss of habitat, rather than a gain as in State
1.
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References
Pethick, J.S.
1984.
An Introduction to Coastal Geomorphology. Edward Arnold, London, 260 pp.
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