A large dredger (hopper dredger) with yellow cranes and the name "JAMES COOK" is sailing on the Elbe. In the background, a smaller HADAG ferry can be seen on the water. The water is calm and some seagulls are flying near the ship.
Under water

How does dredging work? Always with the right equipment - from hopper dredgers to silt plows

To ensure that ships from all over the world can call at the Port of Hamburg, we dredge many tons of silt and sand from the navigation channel and the harbor basin. To do this, we flexibly deploy the appropriate equipment: from hopper dredgers and grab dredgers to silt plows.

Deck of a hopper dredger with large pipes, cranes and safety equipment. Several workers in safety clothing are on the ship, while the port of Hamburg with containers and cranes can be seen in the background.
Suction hopper dredgers (also known as hopper dredgers) are ships that usually have an open cargo hold (hopper) and one or two suction pipes on the sides.
Close-up of a hopper dredger in operation. The suction head, a large industrial device, is lowered into the water and shows complex hoses, metal structures and mechanisms for dredging operations.
During the journey, centrifugal pumps suck up the sediment via the suction head and transport it through the pipeline into the cargo hold.

A characteristic of the Tidal Elbe is that it regularly sediments wash into the harbor. To keep the river navigable for shipping, we have to dredge continuously. Sounding vessels, which continuously survey the bottom of the harbor, indicate where dredging is required. Before the dredging work begins, samples are taken of the sediments and analyzed for their pollutant content. The majority of the dredged material is clean enough and remains in the water, where it is merely relocated. More highly contaminated sediments are treated on land and disposed of safely.

 

Hopper dredger & Co.: Floating dredgers for sludge removal

Attentive observers will have noticed the large dredgers that constantly travel up and down the Elbe, removing excess silt from the river bed. We would like to explain here what special ships these are and what equipment they use.

As in most large ports around the world, dredgers, also known as hopper dredgers, are mainly used in the Port of Hamburg. They usually have an open hold - known as a hopper - and one or two suction pipes on the side. This type of ship is used for large dredging volumes and specializes in the shifting or transporting of sediments. The suction pipes can be lowered and suck a mixture of sediment and water from the bottom of the Elbe into the hold during the journey using centrifugal pumps. The largest hopper dredger ever used in Hamburg has a capacity of around 18,000 cubic meters. Once the hold is full, it travels downstream and releases the load back into the water at designated points via bottom flaps. There, the sediments are distributed by the natural currents. Sometimes the hopper dredger also sprays its load in a high arc onto the shore. This spectacular "rainbow process" is used from time to time to replenish the beach, for example on the Falkensteiner Ufer. However, only sand is used for this purpose.

<p
A dredger with a bright orange bridge and the name "KEES JR" sails on the Elbe in the port of Hamburg. It has a high mast with signal lights, antennas and a small German flag. A striking red clock tower, numerous construction cranes and a parking lot with cars can be seen in the background. The sky is gray and cloudy.
The "Kees Jr." silt plough has a special plough at the rear with which it can gently move sediment that is difficult to access, e.g. from the berths to deeper channels. There, the silt is picked up by the hopper dredgers and transported away.
A large dredging bucket of a yellow dredger dredges sediment from the Elbe in the port of Hamburg. The dredger is named "Gian Lorenzo Bernini" and is registered in Luxembourg. Stacked containers can be seen on the left.
In canals and canals with shallow water depths and bridges to be crossed, hydraulic excavators are often used, as they are known from road construction.

Cutterhead and grab dredger for firmer water bottoms

If the river bed has a very firm bottom that is difficult to loosen or contains large stones, other dredgers are used whose mode of operation is digging and grabbing - for example cutterhead or grab dredgers. In Hamburg, grab dredgers are usually used to remove sediments with large amounts of stone or scrap in shallow water areas as well as in large water depths. Bucket dredgers, which used to be one of the most frequently used devices for water depth maintenance, are now only used to a limited extent, if at all.

Where grab dredgers or bucket chain dredgers are used, the dredged material is transported using folding or flushing barges, whose load volume usually ranges from 250 to 1,000 cubic meters. The flat ships can also be used in narrow and difficult to access areas. Flushing barges are double-walled and can be emptied using a crane or vacuum cleaner. Hinged barges have a hinged floor and can discharge their cargo directly into the water.

<p
The blue dredging vessel "AKKE", a water injection device, with a white bridge and yellow mast is sailing on the Elbe. A wooded bank with a sandy beach and walkers can be seen in the background.
The "MS Akke" is a so-called water injection device. It presses water into the silt layer, which loosens the sediments and transports them downstream with the current. Such devices are also indispensable for smoothing the water bottom after hopper dredging or in the case of large sand ripples.
A workboat named "NJÖRD" from Cuxhaven sails on the Elbe in the port of Hamburg. It is equipped with various technical devices and signal masts. Large harbor cranes and container ships can be seen in the background.
The special ship "Njörd" can remove sediments in hard-to-reach areas, e.g. between piles, using water pressure like a high-pressure cleaner with a flushing pipe.

Soil leveling and silt displacement with water injection technology

Another method of moving silt and sand in a targeted manner is water injection: this involves gently loosening the bottom of the Elbe using water jets so that the loosened material can flow into deeper areas with the force of gravity and the current. The corresponding vessels are called water injection devices. They are used in particular to level the bottom of the water after the work of a hopper dredger and to remove small mounds of silt and sand.

A silt plough works in a similar way: it pulls excess silt over short distances into deeper areas without stirring it up using a special plough that is pulled over the water bed on ropes - for example from a quay wall into deeper channels, where a hopper dredger can then pick up the silt easily and effectively. The silt plough is used, for example, to keep berths at depth.

<p

 

<p>Sometimes the dredged sediment is also used to wash up beaches. The so-called rainbow method is used here, as the film shows.</p

Questions and answers on the use of devices

 

Different dredgers are used in the Port of Hamburg, including hopper dredgers (suction hopper dredgers), grab and cutter dredgers as well as water injection equipment and silt plows. Depending on the ground conditions and task, these are used for excavation, relocation or soil leveling.</p

 

 

The HPA must be able to react very flexibly to the different requirements and boundary conditions. The best way to do this is to rent modern large-scale equipment as required, as is common practice worldwide. Own dredgers and transport vessels would initially not be fully utilized all year round and would therefore be unused for long periods of time, especially in years with low sedimentation. In the event of defects or illness, large dredging companies can quickly procure replacements. And even during rest periods (e.g. closed seasons), large dredgers would have long downtimes. In addition, the construction of large ships is a high risk that can only be worthwhile if they can be in continuous and long-term use. Studies have confirmed that, under the current conditions, hiring equipment on demand is the safest, most efficient and most cost-effective approach for Hamburg.</p

 

 

This is exactly what is done on a small scale, e.g. dredgers on pontoons are combined with transport barges. However, long distances would result in long downtimes for the dredging vessel when the transport barges are underway, and berths would have to be available all year round. Even if there is little sedimentation, as in years with high headwater discharge, several large ships would remain unused for long periods. In addition, dredgers and transport ships are less maneuverable and efficient during dredging and loading in a longitudinal combination in the port. Large modern hopper dredgers, on the other hand, are very flexible, cost-effective and are usually considered the best solution for dredging and transportation worldwide. This is also the case in Hamburg.

 

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