Berm

A berm is a level space, shelf, or raised barrier separating two areas. It can serve as a border barrier. The word berm originates in the Middle Dutch and German berme and came into usage in English via French.

In medieval military engineering, a berm (or berme) was a level space between a parapet or defensive wall and an adjacent steep-walled ditch or moat. It was intended to reduce soil pressure on the walls of the excavated part to prevent its collapse. It also meant that debris dislodged from fortifications would not fall into (and fill) a ditch or moat.

In the trench warfare of World War I, the name was applied to a similar feature at the lip of a trench, which served mainly as an elbow-rest for riflemen.

In modern military engineering, berm has come to mean the earthen or sod wall or parapet itself. The term especially refers to a low earthen wall adjacent to a ditch. The digging of the ditch (often by a bulldozer or military engineering vehicle) can provide the soil from which the berm is constructed. Walls constructed in this manner are an effective obstacle to vehicles, including most armoured fighting vehicles, but are easily crossed by infantry. Because of the ease of construction, such walls can be made hundreds or thousands of kilometres long.

Berms are also used to control erosion and sedimentation by reducing the rate of surface runoff. The berms either reduce the velocity of the water, or direct water to areas that are not susceptible to erosion, thereby reducing the adverse effects of running water on exposed topsoil. Following the 2010 Deepwater Horizon oil spill in the Gulf of Mexico, the construction of berms designed to prevent oil from reaching the fragile Louisiana wetlands (which would result in massive erosion) was proposed early on, and was officially approved by the federal government in mid-June, 2010, after numerous failures to stop and contain the oil leak with more advanced technologies.

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