Two hands with purple gloves take a sediment sample from a body of water using a small spoon and a glass jar
Under water

Sand, mud, sludge, silt - a little sedimentology

The River Elbe is subject to currents from two directions: the upper reaches of the river and the sea. Whilst the upper reaches of the Elbe carry suspended matter towards the harbour, the flood washes sandier material from the North Sea into the city. In the harbour, the particles settle to the bottom as sediment.

When it comes to the Elbe, the Port of Hamburg and the navigation channel, the terms mud and sediments almost automatically come to mind. But what actually is mud? What is behind the term "sediments"? And what is the difference to sludge or sand? For a better understanding, we have written a short "sediment science":
 

What are sediments?

Sedimentum is the Latin word for sediment. Sediments are small particles from the water that settle to the bottom of a body of water due to gravity. They consist of sand (2 - 0.063 mm), silt (0.063 - 0.002 mm), even smaller clay particles and plant remains, for example dead algae, which have decomposed over time. Sediments are therefore a natural component of bodies of water - they enter the river via side arms and the air, are deposited at the bottom, are stirred up again and transported further by the current. If they are in motion, the particles are called suspended matter; if they settle, we speak of sediments.

Sand patterns with wave-like structures created by water currents and small animal tracks running across the sand ribs
The sediments in the Elbe are constantly in motion. They form sandbanks, mudflats, tidal creeks and channels.

What is sand?

Sand is formed from weathered rock, usually quartz. It can also consist of limestone if it is formed from mussel shells or coral. Sand is defined by its size: a grain of sand measures between 0.063 and 2 millimetres. If it is larger, we speak of gravel. Sand that occurs in tidal waters is rounded and further crushed by the constant movement of the water and friction with other grains.

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What is silt?

Silt can be categorised between sand and clay: Its grain size is 0.063 to 0.002 millimetres. It consists mainly of very fine weathered rock such as feldspar and quartz. Just like clay, it can bind water well due to its small size. Pure silt is rare in bodies of water - it is usually part of a mixture with sand and clay.
 

What is clay?

Clay particles are extremely fine and small, smaller than 0.002 millimetres. Clay consists mainly of minerals, i.e. non-organic components. When dry, it is very hard - then it shrinks and cracks. As soon as clay combines with water, it swells and has adhesive properties. It is an excellent moisture reservoir, but also binds other substances such as heavy metals or pesticides.</p

A child in a blue sun hat and mud trousers plays in the wet mud, sticks a hand deep into the ground and splashes up sludge.
Sediments are regularly analysed for quality by independent laboratories. Sediments relocated to relocation sites in the tidal Elbe and North Sea also fulfil the legal requirements for playgrounds

What is mud?

Silt is a natural component of the water and consists mainly of fine-grained sediments, i.e. clay and mud, as well as organic components such as algae, bacteria or plant residues. When this combination of suspended matter settles to the bottom of the water, mud is formed. Due to its high clay content and fine grain size, mud binds a lot of water - up to 50 per cent. And this is precisely what gives it its characteristic "silty" consistency.</p

<p>By the way: several million tonnes of mud are dredged in the Port of Hamburg every year to ensure the necessary depth for shipping Here you can find out how we test the material and where it ends up.

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What is sludge?

Like mud, natural sludge is a mixture of particles of different grain sizes and water. However, sludge is not deposited in the tidal Elbe, but mainly in lakes and inland rivers. This is why there is sludge in the inland Elbe, but not in the tidal Elbe - where it is called mud.  

 

FAQ on sedimentology

The most important questions and answers at a glance.

 

The Elbe is influenced by two flow directions: The upper reaches transport very fine suspended matter towards the harbour, while the flood washes coarser sandy-silty material from the North Sea into the city. As a result, particles of different sizes and origins come together in Hamburg's tidal Elbe.</p

 

 

The particles from the upper reaches and the North Sea sink to the bottom in the harbour as soon as their transport conditions change, i.e. when the current decreases. These sediments are then repeatedly stirred up by the tide and current and transported further.</p

 

 

Coarse particles such as sand settle more quickly and are only displaced by stronger currents. Fine particles such as clay and mud remain in suspension for longer, bind a lot of water and form the typical silt of the tidal Elbe as they settle.

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Mud is formed in tidal waters such as the Elbe from clay, silt and organic material. Sludge, on the other hand, is mainly formed in lakes and slow-flowing inland rivers. This is why we speak of mud in the tidal Elbe - not sludge.

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Fine-grained sediments are deposited as mud all over the harbour and have to be removed regularly. Coarser sediments such as sand change channels, tidal creeks and structures in the tidal area. The interplay of sediment types determines how deposits are handled and the requirements for sediment management.

 

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