Exactly! Dutch dunes are mostly natural: beach sand is blown onto the land and started to pile up, eventually forming dunes. Even in the places where there are buildings facing the sea, they are at least 100(‘s) meters away from the coastline.

The man-made dikes are much more than just a pile of sand. To quote wikipedia:

Artificial levees require substantial engineering. Their surface must be protected from erosion, so they are planted with vegetation such as Bermuda grass in order to bind the earth together. On the land side of high levees, a low terrace of earth known as a banquette is usually added as another anti-erosion measure. On the river side, erosion from strong waves or currents presents an even greater threat to the integrity of the levee. The effects of erosion are countered by planting suitable vegetation or installing stones, boulders, weighted matting, or concrete revetments. Separate ditches or drainage tiles are constructed to ensure that the foundation does not become waterlogged.

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