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A beach is a landform along the coast of an ocean or sea, or the edge of a lake or river. It usually consists of loose particles, which are often composed of rock, such as sand, gravel, shingle, pebbles, or cobblestones. The particles comprising a beach are occasionally biological in origin, such as mollusc shells or coralline algae. Some beaches have man-made infrastructure, such as lifeguard posts, changing rooms, and showers. They may also have hospitality venues (such as resorts, camps, hotels, and restaurants) nearby. Wild beaches, also known as undeveloped or undiscovered beaches, are not developed in this manner. Wild beaches can be valued for their untouched beauty and preserved nature. Beaches typically occur in areas along the coast where wave or current action deposits and reworks sediments. ==Overview== Although the seashore is most commonly associated with the word ''beach'', beaches are found by lakes and alongside large rivers. ''Beach'' may refer to: * small systems where rock material moves onshore, offshore, or alongshore by the forces of waves and currents; or * geological units of considerable size. The former are described in detail below; the larger geological units are discussed elsewhere under bars. There are several conspicuous parts to a beach that relate to the processes that form and shape it. The part mostly above water (depending upon tide), and more or less actively influenced by the waves at some point in the tide, is termed the beach berm. The berm is the deposit of material comprising the active shoreline. The berm has a ''crest'' (top) and a ''face'' — the latter being the slope leading down towards the water from the crest. At the very bottom of the face, there may be a ''trough'', and further seaward one or more long shore bars: slightly raised, underwater embankments formed where the waves first start to break. The sand deposit may extend well inland from the ''berm crest'', where there may be evidence of one or more older crests (the ''storm beach'') resulting from very large storm waves and beyond the influence of the normal waves. At some point the influence of the waves (even storm waves) on the material comprising the beach stops, and if the particles are small enough (sand size or smaller), winds shape the feature. Where wind is the force distributing the grains inland, the deposit behind the beach becomes a ''dune''. These geomorphic features compose what is called the ''beach profile''. The beach profile changes seasonally due to the change in wave energy experienced during summer and winter months. In temperate areas where summer is characterised by calmer seas and longer periods between breaking wave crests, the beach profile is higher in summer. The gentle wave action during this season tends to transport sediment up the beach towards the berm where it is deposited and remains while the water recedes. Onshore winds carry it further inland forming and enhancing dunes. Conversely, the beach profile is lower in the storm season (winter in temperate areas) due to the increased wave energy, and the shorter periods between breaking wave crests. Higher energy waves breaking in quick succession tend to mobilise sediment from the shallows, keeping it in suspension where it is prone to be carried along the beach by longshore currents, or carried out to sea to form longshore bars, especially if the longshore current meets an outflow from a river or flooding stream. The removal of sediment from the beach berm and dune thus decreases the beach profile. In tropical areas, the storm season tends to be during the summer months, with calmer weather commonly associated with the winter season. If storms coincide with unusually high tides, or with a freak wave event such as a tidal surge or tsunami which causes significant coastal flooding, substantial quantities of material may be eroded from the coastal plain or dunes behind the berm by receding water. This flow may alter the shape of the coastline, enlarge the mouths of rivers and create new deltas at the mouths of streams that formerly were not powerful enough to overcome longshore movement of sediment. The line between beach and dune is difficult to define in the field. Over any significant period of time, sediment is always being exchanged between them. The ''drift line'' (the high point of material deposited by waves) is one potential demarcation. This would be the point at which significant wind movement of sand could occur, since the normal waves do not wet the sand beyond this area. However, the drift line is likely to move inland under assault by storm waves.〔Blair and Dawn Witherington (2007), ''Florida's Living Beaches, A Guide for the Curious Beachcomber'', (Pineapple Press)〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「beach」の詳細全文を読む スポンサード リンク
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