Shoal

A sandbar is a deposit (sedimentation) of sand or gravel at the bottom of seas and rivers, formed by the tide and current. A "free beach wall" is formed when an offshore sandbar rises above sea level. The narrow passage between adjacent sandbars is called a kille. As sandbars grow, they become islands. One example is the Kachelot Plate in the East Frisian Wadden Sea between Borkum and Juist. Sandbanks that dry out daily are also called plate or sandplate.

A good example of a free beach barrier in Germany is the sandbank at the North Sea resort of Sankt Peter-Ording. Sandbanks can migrate due to the influence of currents and therefore pose a danger to shipping. One example of a "migrating" sandbank is Blauort off the coast of Büsum, another is Noorderhaaks off Den Helder in the Netherlands. Sandbanks pose an even greater danger if they consist of particularly fine sand - known as milled sand in mariners' parlance - which traps ships that have run aground and usually causes them to break through. Such a sandbank is, for example, the Große Vogelsand north of the main fairway of the Outer Elbe, where several hundred wrecks already lie.

Other well-known examples of dangerous sandbanks are, for example, in the Strait of Dover, there near the English coast the Goodwin Sands and about in the middle of the eastern entrance of the Strait of Dover the Sandettie Bank. Such sandbanks are often and to this day secured with fire ships.

For a description of the destructive effects of sandbars on historic and modern ships, see the Goodwin Sands article, where it is estimated that about 2000 ships have been lost.