History of dredging

Dredging is the process of digging or deepening of rivers, canals, harbours, and sees. Mostly, because of obstacles such as sandbars, which obstructs navigation. But also because of silting up of rivers or canals. Dredging is not only the deepening of rivers, but also the expansion of land, or making a river, sea or canal less deep.

History

Long ago, dredging was done by hand. People used a long stick, with a bucket on the end. When the bucket scratched along the bottom, a lot of rubbish was taken along with the bucket. Yet, this was a huge, time consuming job. Later on, horses were used, and dredging ships. These wrenched and grubbed the ground on the bottom. By the flow, the dredged ground was transported to other parts of the river of canal.

Dredger

Nowadays, dredging is done by a dredger, a large ship with a cycling chain, to which lots of buckets are attached. These buckets scratch the mud of the ground. The bucket chain could be compared to the Ferris wheel. One bucket could easily fit 600 litres of mud. When the bucket chain starts cycling, the buckets scratch the bottom. On top, the buckets are emptied in a chute, which is something like a long slide that leads to a container in a ship on the side. In this ship, all the mud is collected. The mud from this ship could be used the expansion of land, or the augmentation of embankments, but also other locations are possible. The ship with mud is drained away by a dredge drain, which leads the mud through an iron tube to the place of destiny. If the mud is really fertile, it could even be used as fertilizer for agriculture.

Dredging in The Netherlands

A large part of the bottoms in Dutch rivers, canals, and the sea is polluted by heavy metals and insecticides. Some parts of the ground are even poisonous, and therefore have to be cleaned beforehand. Because the cleaning of polluted ground is very expensive, the mud is first stored in depots. The Netherlands dispose of five depots, below the water, as well as on land.

Yearly, about 3 millions of cubic metres are dredged in The Netherlands. Sometimes, the layer of dredge is tens of metres deep. The chain with the buckets has to be made longer, so deeper dredging can be done. With help of a depth and location indicator, the shipper knows exactly how deep, and where he already has been dredging.

Boat types

Currently, dredging companies make use of the two boats described below.

Suction cutter dredger

The suction-cutter dredgers thanks it’s name for the English word for ‘snijkop’: cutter. It is especially suitable for hard bottom, because it consists of a long tube, with a cutter at the end. Attached to this cutter are steal messes, which turn quickly in order to loosen the ground. The loosened ground is drained away by a long tube, and is transported through a pipeline to the main land. Sometimes, when the ground is too hard, dynamite is used to loosen the ground, or ‘break’ the ground. Next, a dredge craine is used, which ladles large pieces from the bottom, after which the suction cutter dredger can continue dredging.

Trailing suction hopper dredger

In contrast with the suction cutter dredger, the trailing suction hopper dredger is not suitable for hard and solid ground. The trailing suction hopper cutter has two tubes on board, which can be lowered into the water. Slowly, these tubes move along the bottom, and drain away the mud. The sand is separated by the water, and the sand lands in the loading platform, which is called the hopper.

In the course of years, the capacity of the trailing suction hopper dredger has changed tremendously. Around 1900, the loading platform had a capacity of 2000 cubic metres. Nowadays, the capacity is more like 20.000 to 30.000 cubic metres.

There are three ways a trailing suction hopper dredger can get rid of its load. First one is also the most easiest one, which is: in the water. It only has to open it’s doors. Another possibility is the so called ‘rainbowen’. From a distance, the mud is sprayed on land. The third possibility has a lot to do with rainbowen, if the distance between mainland and the boat is too large, the mud is transported through a pipeline to the place of destiny.

New techniques

Recently, on many places people make use of new techniques, which are often ecofriendly as well:

• ‘Waterinjection dredging’. With this technique, water is injected into the ground. Because of the pressure of the water, the mud is transferred. Next, the ground is being loosened and is being pushed towards parts of the bottom that are deeper.
• ‘Grubbing dredger’. This looks much like an enormous rake with pins on all ends. This is a rather cheap solution, because no mud has to be drained away, or being transported, for this is all done by nature.
• ‘Wurmwheel suction dredger’. This suction dredger can remove polluted mud, to the centrimetre accurate. It owes is name to the most important part, the wurmwheel.

External links

www.baggermuseum.nl– This museum, which lies in Sliedrecht, contains of a lot of information concerning dredging. On site, info about the entry, and opening hours can be found. More information about expositions can be found on the side as well.

Sources: www.todio.nl