A gat is a strait that is constantly eroded by currents flowing back and forth, such as tidal currents. It is usually a relatively narrow but deep, up to passage between land masses or shallow bars in an area of mudflats. A gat is sometimes a shallower passage on lagoon coasts, including those without any tidal range. According to Whittow a gat is either an inshore channel or strait dividing offshore islands from the mainland e.g. the Frisian Islands, or it is an opening in a line of sea cliffs allowing access to the coast from inland. It is similar, but not identical, to a gut, which is a narrow river channel or strait prior to joining an open ocean or estuary. Leser restricts its use to deep, but relatively narrow inlets in the Wadden Sea that are scoured out by currents, giving the example of the gap between the Frisian islands of Juist and Nordeney.
Description
The comparatively large quantities of water that flow quite quickly through a gat cause heavy erosion that results in a channel deeper than the rest of the surrounding seabed and also endangers neighbouring islands. When the water masses from mud flats behind the islands surge out again into the sea as ebb currents, they flow rapidly again through the narrow gat. But as these water masses break out into the open sea, they spread out and slow down. As a result, on this seaward side of the gat, the particles of sand and mud carried with the water settle and form an ebb delta with its shallower waters between the islands. The sandbanks so formed are often known in Germany as plate. The point where the water pouring out of the gat runs over these banks, which often lie in an arc between the islands, is the sand bar. This is the shallowest part of the gat for shipping, but also the deepest point on the shallowest line between the islands. A flood delta is formed in a similar way on the landward side of the gat. A navigation channel to the open sea is usually marked out in the gats by the waterway and shipping authorities. The area of the bar is usually the most dangerous spot; this is where rip tides and, especially when the current flows against the wind, very dangerous ground swells may occur. Passages between inner and outer coastal waters, such as at the ends of spits of lagoons or along bodden coasts are also referred to as gats. The term "gat" is primarily applied to waterways of the North Sea and Baltic Sea coasts of Europe. A similar term of related but not identical meaning, gut, is mainly applied to channels of the coastal waters of the Atlantic coast of North America.
