A distinctive coastal form – tidal flats

On many low-energy coasts around the world tidal flats are ­formed when large amounts of clay, silt and fine sand particles are imported by rivers. These tidal flat areas, however, do not look the same everywhere. A distinction can be made between “closed” tidal flats, characterized by plant growth, and “open” tidal flat areas where the sediments are exposed. The largest tidal flat area in the world extends across broad stretches of the Dutch, German and Danish coasts of the North Sea and is an “open” tidal flat. It has been listed as a World Natural Heritage Site by UNESCO (United Nations Educational, Scientific and Cultural Organization) since 2009. The intertidal area here contains the typical mud consisting of 30 per cent clay, 30 per cent fine silt and more than 30 per cent sand as well as dead biomass. But technically this area cannot be referred to as a muddy coast because of the relatively high sand content in most areas. Thus, in the strict sense, this tidal flat is considered to be a sandy coast.
A true “open” muddy coast, on the other hand, is found in the South American country of Suriname, where the coastal Atlantic currents are very weak. Here, even the finest clay and silt particles can be deposited to form thick muddy sediment packages. The bulk of these are transported over a distance of around 600 kilometres from the mouth of the Orinoco in Venezuela, through the Atlantic and into the calm waters off Suriname.
On the east coast of the USA, however, the situation is quite different. Salt marshes have formed at many locations between Florida and the peninsula of Cape Cod in Massachusetts, which defines them as “closed” tidal flats. These form along low-energy segments of the coastline where rivers import large ­volumes of material that are primarily deposited in shallow areas near the shore. The tidal flats grow upward on the order of decimetres through time, and thus become less frequently inundated by water. Specialized salt-resistant plants can then colonize here. These salt marshes are important stopover and breeding sites for birds and thus represent a crucial habitat within the tidal flat environment.
Tidal flats often form between the mainland and offshore islands. Because of the low-energy currents here, fine particles can be deposited on the sea floor. A prerequisite for the formation of these island or backshore tidal flats is a significantly large tidal range, the difference in the water level between low and high tide, so that the area is regularly flooded and exposed as in the western European Wadden Sea. As a rule, the tidal range here is between 3 and 3.5 metres. Island tidal flats are also found on the Pacific coast of Colombia, for example. These, however, are not “open” tidal flats, but covered by salt-resistant mangrove trees.

fig. 1.25 > The Wadden Sea on the margin of the North Sea is very popular with tourists. Many people are fascinated when they walk across the muddy sea floor at low tide for the first time. © Thomas Ebert/laif

fig. 1.25 > The Wadden Sea on the margin of the North Sea is very popular with tourists. Many people are fascinated when they walk across the muddy sea floor at low tide for the first time.