Dikes (or levees) of the Netherlands have been used for more than 2000 years to protect the land from the natural invasion of the ocean’s waters. Dikes are part of life for the Dutch. Our class visited a dike at the end of the Wageningen University physical geography tour and again at Kinderdijk. This interactive map displays all ~22,000 kilometres of the dike network.
Here is the link to the website: http://dutchdikes.net/map/
One can see that the only large un-diked areas in the country are where the Utrechtse Heuvelrug and Wageningen-Ede push moraines sit. Being where the highest elevations in the Netherlands are, dikes are clearly unrequired.
If all of these dikes were absent or breached, I estimate that the entire western half of the country would be lost underwater.