The Nieuwe Hollandse Waterlinie was a post-Napoleonic military system of inundation zones engineered with fine specification to allow rapid flooding of a north-south line under shallow cover of salt-and-freshwater in the incidence of threat from overland attack.
The purpose of the sophisticated arrangement of apparati and floodplains extending from the Zuiderzee (now Ijselmeer) to the "great rivers" was to isolate the rich, politically-powerful, and densely-populated western provinces from overland attack.
This "New Hollandic Waterline" was new relative to the Oude Hollandse Waterlinie, which had been unable to encompass Utrecht, one of the principal Dutch cities — a part of the randstad, or "Ring-city" agglomeration of metropolises. At the time of construction of the old Dutch waterline (1672,) Utrecht had been under control of the Napoleonic French.
The Nieuwe Hollandse Waterlinie, begun in 1815 after the fall of Napoleon, was a refinement on the concept. A sophisticated arrangement of dikes, canals and waterworking apparati stood ready to flood the entire line in a couple of weeks to about 40 centimeters — not deep enough to sail, too deep to charge.
In areas where flooding was impossible, due to elevation or crossings of structures like railways, roads, canals and rivers, a series of forts defended against intrusion.
The infrastructure along the waterline, largely extant in 2009, is obsolete in the modern age.
Although deployed in 1939, as it had been in the Franco-Prussian War and in World War I, the military floodplain was no longer effective. When Germans bombed Rotterdam in 1940 and forced a Dutch surrender under threat of greater airborne destruction, it became clear that preventing movement on the ground was no longer useful as a primary defense.
