In short
- Mud flats consist of more than half very fine particles, clay and silt. They form where the water barely moves, right in front of the dikes and in calm bays.
- Sand flats lie where waves and currents are strong, and they are usually firm. Mixed flats are in between.
- You sink into mud, and every step costs strength and time. That is why you go onto the flats with a guided walk.
How the three kinds form
Every flood tide carries sand and mud onto the flats. What settles depends on how strong the current is. Heavy sand grains sink even in moving water, which makes sand flats. Where the water is calmer, mixed flats of sand, clay and organic matter form. Right in front of the dikes and in bays where the water stands still for long, even the finest particles settle. Those are mud flats.
The kinds are told apart by their share of clay and silt, particles smaller than 0.063 millimetres. Sand flats have less than 10 per cent, mixed flats 10 to 50 per cent, mud flats more than half.
How to recognise mud flats
Mud flats are soft and dark, and the ground gives way under your feet. Sand flats are lighter and usually firm enough to walk on easily.
The mud is dark because it lacks oxygen. A few millimetres to centimetres below the surface there is none left. Bacteria break down dead plants and animals there without oxygen, producing black iron sulphide and hydrogen sulphide, which smells of rotten eggs. Anyone who has stood deep in the mud knows the smell.
What lives in the mud
The dark ground is full of life. One square metre of flats holds millions of diatoms and thousands of tiny crustaceans, mussels, snails and worms. The diatoms cover the ground with a brownish, slippery film and hold the mud together; without them there would be no mud flats. The laver spire shell feeds on them, for example.
Common cockles and sand gapers are especially at home in mixed flats. One floor down, about 30 centimetres deep, lives the lugworm that leaves the familiar sand casts. For birds the flats are a laid table: in spring and autumn a good ten million migrating birds stop over.
What it means for a walk on the flats
You sink into mud flats, and every step costs strength and time. Together with tidal creeks and the returning flood, that is dangerous without local knowledge.
The national park administration advises going onto the flats only with experienced, qualified mudflat guides. Plan the tour with the local tide times so that it ends at low tide. Whether you walk in rubber boots, trainers or barefoot is something to agree with your guide. The times for your place are on the tide map.
Mud flats are the soft, dark flats in front of the dikes and in calm bays. They form where the finest particles settle, and they are full of life. Only walk on them with a guide.
See the tide for your placeFrequently asked questions
What are mud flats?
Flats made of more than half very fine particles, clay and silt. They form where the water barely moves, above all right in front of the dikes and in sheltered bays.
Why does the mud smell of rotten eggs?
A few millimetres to centimetres below the surface there is no oxygen. Bacteria break down dead material without it, producing black iron sulphide and hydrogen sulphide, which smells of rotten eggs.
What is the difference between sand flats and mud flats?
Sand flats form in strong currents, have less than 10 per cent fine particles and are usually firm. Mud flats form in calm water, consist of more than 50 per cent fine particles and are soft.