Tidal Processes • Waves • Sediments • Flocculation

Estuaries and bays can be divided into muddy systems, dominated by a suspended sediment load comprised mainly of silts and clays (less than 63µm in diameter), and sandy systems dominated by bed load comprised of sands and gravels (> 63µm). The largest areas of mudflats in England and Wales are found in Essex, Kent, Humberside, Norfolk, Lincolnshire, Hampshire and Gloucestershire/Gwent; reflecting the generally muddy character of the upper intertidal flats in the estuaries of eastern and southern England. The intertidal flats of north-west and north-east England are mainly sandy, with mud generally restricted to the highest intertidal zone and the inner parts of estuaries. Extensive sandflats occur in the lower intertidal zones of most east coast estuaries, including the Humber, The Wash and Outer Thames Estuary. Relatively little sediment enters the estuaries of England and Wales from fluvial sources in present times.

If conditions are suitable, sediments transported into estuaries via fluvial processes can also form large deposits called deltas. Where the coast is dominated by tides, with little wave activity, the delta is likely to be typified by shoals (small islands) surrounded by water. Deltas are constantly changing owing to their constant sediment input and its rearrangement by local currents.

Saltmarshes within estuaries and bays are ultimately dependent on the energy provided by the tides and waves capable of transporting coastal sediments and changing the shape of the landforms. This change in shape may mean that the landforms gradually adjust to the applied forces in order to absorb the imposed energy without further morphological changes taking place; achieving so-called morphodynamic equilibrium.

The processes of sediment erosion and deposition are controlled by the relationship between the size of particles and the strength of the currents imposed upon them. When the velocity of the water flowing over a surface becomes fast enough, the sediment particles on that surface will be eroded and transported, to be deposited when the flow velocity decreases (i.e. the capacity of the flow to move sediments reduces). The initiation of transportation, and the type of transportation (suspended or bedload) that follows, will depend to a large extent on the particle size and the shear stress applied to the sediment by the flow. Deposition occurs when the force of gravity acting on the particles is sufficient to overcome the fluid forces causing movement.