Shoal

In oceanography, geomorphology, and geoscience, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material, and rises from the bed of a body of water close to the surface or above it, which poses a danger to navigation. Shoals are also known as sandbanks, sandbars, gravelbars, or bars. Two or more shoals that are either separated by shared troughs or interconnected by past or present sedimentary and hydrographic processes are referred to as a shoal complex.

The term shoal is also used in a number of ways that can be either similar to, or quite different from, how it is used in geologic, geomorphic, and oceanographic literature. Sometimes, the term refers to either any relatively shallow place in a stream, lake, sea, or other body of water; a rocky area on the seafloor within an area mapped for navigation purposes; or a growth of vegetation on the bottom of a deep lake, that occurs at any depth, or is used as a verb for the process of proceeding from a greater to a lesser depth of water.

Shoals are characteristically long and narrow (linear) ridges. They can develop where a stream, river, or ocean current promotes deposition of sediment and granular material, resulting in localized shallowing (shoaling) of the water. Marine shoals also develop either by the in-place drowning of barrier islands as the result of episodic sea level rise or by the erosion and submergence of inactive delta lobes.

Shoals can appear as a coastal landform in the sea, where they are classified as a type of ocean bank, or as fluvial landforms in rivers, streams, and lakes.

A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as:

The term bar can apply to landform features spanning a considerable range in size, from a length of a few meters in a small stream to marine depositions stretching for hundreds of kilometers along a coastline, often called barrier islands.

They are typically composed of sand, although they could be of any granular matter that the moving water has access to and is capable of shifting around (for example, soil, silt, gravel, cobble, shingle, or even boulders). The grain size of the material comprising a bar is related to the size of the waves or the strength of the currents moving the material, but the availability of material to be worked by waves and currents is also important.

Wave shoaling is the process when surface waves move towards shallow water, such as a beach, they slow down, their wave height increases and the distance between waves decreases. This behavior is called shoaling, and the waves are said to shoal. The waves may or may not build to the point where they break, depending on how large they were to begin with, and how steep the slope of the beach is. In particular, waves shoal as they pass over submerged sandbanks or reefs. This can be treacherous for boats and ships.