The Netherlands

Dutch military floodline

  The Dutch boezem, a dispersed reservoir

Boezemstelsel of Rotterdam and
The Hague

A boezemstelsel in the Netherlands is a system that buffers excess volume at a variable level above that of a polder bottom and on its way out to sea. It stores excess water until release is timely, and it can augment groundwater in a time of drought. It's the place to put polder-bottom water, and from which to draw it back down if it's needed.

Usually shared by several polders, a boezemstelsel normally comprises diverse bodies of water that are openly connected - canals, lakes, ditches, shipping waterways. A local authority determines the aspirational level of a boezem system, and the true level fluctuates in service of regulation at polder-bottom where strict maintenance is important.

Electric pumps — previously wind, steam and then diesel — elevate water from the polder floor to the boezem. The boezem holds the water until its release, controlled for volume and quality - pumped again, usually into a river by which it goes to sea.