Today I learned that a dyke requires a gravel or sand layer on the land side of the dyke to prevent it from breaking due to water that gets through underneath. According to the explanation by the “Sendung mit der Maus” during a flood the water pressure on a dyke is so high that water flows underneath it. In order to prevent this water from taking parts of the dyke with it, a ditch on the land side filled with gravel and sand is in place to make it easy for this water to pass through and being collected for transportation to a pumping station. This way the dyke is much more sustainable than one that is just a massive wall.
I learned about that while browsing throw the “Sachgeschichten”-section (might be translated as “documentary”-section) of the “Sendung mit der Maus” (it’s a very famous German entertainment and learning show for kids). While doing that I came across the story about dyke building which I can highly recommend to watch.