Mud Flats and Salt Marshes

Mud flats are typically found in areas where the tidal waters flow slowly, such as sheltered bays, estuaries, rias and along gently shelving coasts.

A mixture of very fine silts from tidal waters and alluvium from rivers dropping their load as they reach the sea is deposited, causing a build up of mud layers, called mud flats. Mud flats are covered at high tide and exposed as the tide drops. In rias, mud flats are often found in the remains of the valleys that were tributaries to the main submerged river. In such areas, remains of the original watercourses can sometimes be seen as channels carved out of the mud, down which a little fresh waters may be seen flowing at low tide. All mud flats are usually crisscrossed by winding channels that are kept open by tidal action. Unless these channels are fed by active water sources, such as streams and rivers, they will usually dry out at low tide and contain no water.

Where the muds are sufficiently stable to support vegetation, salt marshes may form, or in tropical areas, mangrove swamps.

Mud flats are well named. They are muddy, and often very flat! The reflective surface of the mud flat is created by very high water content in very fine slit, producing a mirror like surface. This mud is very unstable and anyone trying to walk on it would sink up to the knees within seconds. Further out into these mud flats you would sink without trace!

Despite not being a good place for heavy humans, the mud is teeming with life and is a popular feeding ground for birds.

Salt marshes form in coastal areas that already have mud flats. They usually form in areas that are well sheltered, such as creeks, inlets and estuaries where fine sediments can be deposited. They also form behind spits and artificial sea defences where tidal waters can flow gently and deposit fine sediments. Salt marshes are exposed at low tide and at least partially flooded at high tide, so only salt resistant forms of vegetation can grow there. They are typically very flat, with numerous muddy channels and creeks cutting through them, making them very dangerous places to be as the tide comes in.

In some areas salt marshes are left as natural coastal defenses because they can be safely flooded by high tides and thus protect the more valuable commercial, housing or agricultural areas lying further inland. Salt marshes can be reclaimed as agricultural land, as happened in Norfolk, but they are also very important in their natural state. They form a specialised ecosystem supporting numerous species of insects, birds and plants, as well as shellfish, and invertebrates such as lug worms.

Human use of salt marsh areas varies but can include tourism in areas designated as nature reserves, bait collecting areas for fishermen, and they also provide ideal conditions for the farming of some species of shellfish. Hundreds of young shellfish are placed in large sacks that allow the water to flow through whilst stopping the creatures from wandering off. The sacks are pegged out on the mud and left for a year or two until the creatures have grown to a marketable size.

Dunes

Sand dunes are small ridges or hills of sand found at the top of a beach, above the usual maximum reach of the waves. They form from wind blown sand that is initially deposited against an obstruction such as a bush, driftwood or rock. As more sand particles are deposited the dunes grow in size, forming rows at right angles to the prevailing wind direction. If vegetation, such as Marram Grass and Sand Couch, begins to grow on the dune its roots will help to bind the sand together and stabilise the dunes.

Dunes are subject to different forms and sizes based on their interaction with the wind. Most kinds of dune are longer on the windward side where the sand is pushed up the dune, and a shorter "slip face" in the lee of the wind. The "valley" or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as ergs.

Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds.

Sand dunes can be important ecosystems supporting unique plant life and a healthy population of small animals and insects. An example of a sand dune ecosystem is found at West Wittering on the South East coast of the UK. Dune habitats provide niches for highly specialized plants and animals, including numerous rare and endangered species.

Dunes are very vulnerable to erosion by natural processes and by human activity. It is common to see vulnerable sections of dunes fenced off to prevent public access, or for paths to be laid to prevent people from eroding the dunes. Due to human population expansion dunes face destruction through recreation and land development, as well as alteration to prevent encroachment on inhabited areas. Some countries, notably the U.S., New Zealand, Great Britain, Australia, Canada and the Netherlands have developed extensive programs of dune protection. In the UK, a Biodiversity Action Plan has been developed to assess dunes loss and prevent future dunes destruction.

One of the biggest problems posed by sand dunes is their encroachment on human habitats. Sand dunes move through a few different means, all of them helped along by wind. One way that dunes can move is through saltation, where sand particles skip along the ground like a rock thrown across a pond might skip across the water's surface. When these skipping particles land, they may knock into other particles and cause them to skip as well. With slightly stronger winds, particles collide in mid-air, causing sheet flows. In a major dust storm, dunes may move tens of meters through such sheet flows. And like snow, sand avalanches, falling down the steep slopes of the dunes that face away from the winds, also moving the dunes forward.

Sand threatens buildings and crops in Africa, the Middle East and China. Drenching sand dunes with oil stops their migration, but this approach is highly destructive to the dunes habitat and uses a finite resource. Sand fences might also work, but researchers are still analyzing optimum fence designs. Preventing sand dunes from overwhelming cities and agricultural areas has become a priority for the United Nations Environment Programme.