Sand and mud flats - State 1, Accreting

Sand and mud flats naturally accrete. Sediment settles out on the surface when the velocity is lowest. This tends to occur at the top end of the tidal cycle. As the tide recedes and velocities increase the sediment is remobilised and drawn back down the the mudflat slope. The period over which this process takes place includes a 'settling lag' which results in "each sediment grain moves 'two steps forwards, one step backwards' during each tidal cycle..". This increases the concentration of sediment in suspension towards the shore and increases the deposition rates on the higher flats. The resulting morphology is of a flat upper surface and a steeper lower section to the tidal sand or mud flat (Pethick 1984). The value of this habitat can be described in terms of its inherent interest as well as its contribution to the overall intertidal ecosystem.

Caption: Extensive tidal sand flats on the Norfolk coast at Brancaster

Caveat: Tidal flats develop where there is abundant sandy sediment on exposed open coasts, sufficient to withstand the natural erosive forces. In more sheltered situations finer sediments of muds and silts settle out to form tidal mudflats. The tidal flats may go through several periods of natural erosion or accretion and it is the overall overall balance towards sediment deposition which helps to define this state. In these circumstances the area of habitat increases in extent (compare with State 2 where there is a decrease in area).

References

Pethick, J.S. 1984. An Introduction to Coastal Geomorphology. Edward Arnold, London, 260 pp.