The tidal-shaped bathymetry between Frisian islands revealed by breaking waves
There’s a lot to see over the North Sea. A few weeks ago I flew to London on a beautifully sunny day and got a good view of the Frisian islands. I noticed white “skirts” on the islands that fanned out on the gaps between the islands. I had an idea of why it looked like that, but the explanation wasn’t overtly obvious..
Today, we explain the wave formation around Frisian islands., which altogether turns out to be quite simple on high-level.
1. What we see
The Frisian islands are a fascinating barrier island chain starting from Danish coast and stretching past Germany to the Netherlands. Between the islands and continent is the Wadden sea.
The Frisian barrier islands in 2020. Today’s study area market with the white rectangle.
The gaps between the islands are called tidal inlets. On these inlets, we can see jet-like openings forming a tree-shaped, white wave-breaking pattern around all of the inlets. It is clear these are breaking waves, and breaking waves must be explained by a shallower depth on the white areas.
Wave formation on April 6th 2026 a few hours before low-tide. See the white “jet” flowing out through the inlet on right.
The natural explanation is that water flowing and in an out through the inlets shapes the sea floor around these inlets.
Timing matters
This is not a rare sight, but timing does matter. As I flew past the islands, tide charts confirmed it was a few hours after low-tide. The satellite image above was taken on the same day, but a few hours before low tide - judging by the cloud shadows indicating near-midday capture.
In the satellite image then, water is actively draining out of the Wadden Sea so currents through the inlets are strong and focused. Also, water levels are dropping, bringing seabed features closer to the surface - but high enough to be fully submerged.
This is when the effect becomes most visible from orbit.
2. What it tells us
A coastline built by tides
In practice, these white wave skirts reveal well-structured coastal systems which are called Ebb tidal deltas. This was a new word for me and the formation is well documented.
Each inlet behaves like a hydraulic system:
On rising tide, water flows into the Wadden Sea
On falling tide, it flows out, accelerating through the narrow gaps
Naturally, the strong tidal flow moves and deposits sediment, but the back and forth means the channel does not become blocked and the delta is constantly being shaped.
The resulting physical structure ” and Google’s historical imagery gives an excellent “below” surface view of this. See below a side-by-side of one inlet displaying the tidal channels, sand ridges and how the wave breaking patterns follow that structure precisely.
Comparison of wave formation (left) with bathymetry revealed by a mosaic image (right).
The role of the Wadden Sea
The shallow basin behind the islands - the Wadden Sea - is necessary for this structure to form.
It acts as: A tidal reservoir, exchanging vast volumes of water each cycle as well as a a sediment source and sink, constantly redistributing sand and silt.
While the tidal flow goes back and forth, what comes out is more sediment-heavy so the offshore part is the side growing.
The dynamic repeats in similar conditions, which is why we see the same white wave breaking pattern on virtually every inlet on the Frisian islands.
All visible inlets on the Frisian islands exhibiting similar wave formation on April 6th 2026, a few hours before low-tide.
Conclusion: Why this matters to us
This is a good example of an edge case where:
The signal (breaking waves) is highly visible
The structure (submerged sand bodies) is otherwise hidden
Without waves, this would look like a flat, slightly murky sea. With waves, the seabed reveals itself in detail. By combining spot and mosaic satellite images, we were able to see both layers and arrive to an explanation as long as we understood basic tidal dynamics.
See you,
Orbital Vantage
