What is dredging?
Dredging is the removal of a layer of mud, sand, or silt (dredged material) from the bottom of a watercourse (ditch) or body of water (e.g., a lake). Dredged material consists mainly of dead aquatic plants, leaves from trees along the banks, and soil particles (sediment) that wash into the ditch via drainage from agricultural land and banks. Soil material that ends up in the water due to subsidence and/or crumbling of the bank zone (erosion) and soil material from surrounding plots carried into the water by the wind is also specified as dredging. The formation of dredging is a natural process. The layer of dredging grows by one to three centimeters per year.
Why dredge?
By dredging, we ensure that watercourses remain sufficiently deep and maintain the right water quality. If the watercourse is no longer deep enough because dredging has not been carried out in time, there is a high risk that excess water cannot be drained away sufficiently. Or that it cannot be replenished sufficiently in the event of a water shortage. This can lead to flooding or drought damage (exceeding national standards).
In waterways along a shipping route, dredging is necessary to maintain sufficient navigable depth, for example for cargo ships.
A thick layer of silt also leads to a decrease in oxygen in the water due to decomposition processes, especially in shallow water. Shallow water also experiences more extreme temperature fluctuations (higher maximum temperatures and lower minimum temperatures). This can be particularly detrimental to underwater life.
The purpose of dredging is to:
- ensure the supply and removal of water;
- keep waterways navigable;
- maintaining and, where possible, improving water quality and life in and around the water.
How to dredge?
Usually, excavators dig the dredged material out of the ditches from the land and spread it over the strip of land where they are driving.
This often works well in rural areas, but usually not in built-up areas. There is often no room for an excavator or dredger along the ditch because there are houses or gardens. In that case, the dredged material is pushed by a push boat to a place where an excavator loads it into a truck or tipper trailers behind a tractor. These then transport the dredged material to a destination outside the built-up area, such as a maturation depot (dredging depot).
In wide or deep water, such as canals and reservoirs, dredging is sometimes carried out by an excavator on a pontoon (large raft). This fills barges (small cargo ships without engines) and tows them to a destination further away, such as a deep pool along a river. Another technique used in wide or deep water is a cutter suction dredger. This vessel loosens the silt layer with a cutting head and simultaneously sucks up the surrounding water with silt. The released silt flows into the suction nozzle and is transported via a pressure pipe to a temporary depot further away.
Preconditions for dredging
Regulations governing natural values set requirements for how and when dredging may be carried out. For example, dredging is not permitted when the water temperature is too high or when there is a layer of ice. Dredging is avoided as much as possible during the breeding season. If it is necessary, an ecologist is always called in, just as outside the breeding season. There are also rules governing the direction in which dredging is carried out. Fish must be able to escape, so dredging is not permitted towards a dead-end ditch. Environmental legislation also imposes requirements on dredging, such as quality requirements for dredged material that is spread on land and dredged material that is reused after maturation. In addition, there are various other rules, such as certification of vessels, but these are too numerous to mention here. More regulations can be found on the page Dredging yourself.
