Inundation (from the Latin inundatio, flood) is both the act of intentionally flooding land that would otherwise remain dry, for military, agricultural, or river-management purposes, and the result of such an act.
The noun "inundation" refers to both the act of inundating; an overflow; a flood; a rising and spreading of water over grounds; and to the state of being inundated; flooding. It should therefore be distinguished from other types of flooding, not caused by man. It should also be recognised that this type of flooding would not take place without the intentional human act. So e.g. tidal flooding of areas that fall dry at ebb tide, and periodical river floods are not inundations in the sense intended here.
The latter condition implies that in most topographies the act usually applies to causing an overflow of a river or stream by damming it. This is also the only method foreseen in the 1888 British military textbook that discusses the technique correctly under "military obstacles". However, in areas that have been reclaimed from the sea, marches or lakes, and are artificially protected from the waters (either accumulated precipitation, or the water source that would flood the reclaimed area, if it were not artificially protected), as in e.g. the artificial hydrological entities, known as Polders, may be inundated by simply giving the water source access again. In the latter perspective inundation may simply be a form of hydraulic engineering.
Before we limit the concept to military uses, it should be recognised that there are at least two other possible uses of inundation as a human act:
Military inundation creates an obstacle in the field that is intended to impede the movement of the enemy. This may be done both for offensive and defensive purposes. Furthermore, in so far as the methods used are a form of hydraulic engineering, it may be useful to differentiate between controlled inundations (as in most historic inundations in the Netherlands under the Dutch Republic and its successor states in that area and exemplified in the two Hollandic Water Lines, the Stelling van Amsterdam, the Frisian Water Line, the IJssel Line, the Peel-Raam Line, and the Grebbe line in that country) and uncontrolled ones (as in the second Siege of Leiden during the first part of the Eighty Years' War, and the Inundation of Walcheren, and the Inundation of the Wieringermeer during the Second World War). To count as controlled, a military inundation has to take the interests of the civilian population into account, by allowing them a timely evacuation, by making the inundation reversible, and by making an attempt to minimize the adverse ecological impact of the inundation.[Note 1] As Vandenbohed discusses, that impact may also be adverse in a hydrogeological sense if the inundation lasts a long time.[Note 2]
From a legal point of view, a military inundation should therefore be seen as a government "taking" of private property under the constitutional protection of private property in many countries. In the Netherlands, this was the subject of art. 152 of the Dutch constitution of 1887.