What happens the day after we stop dredging? Why is dredging necessary for navigation and trade?



F. Aerts, Department Head, Flemish Community, Maritime Access Division, Antwerp, Belgium



Most ports were built in locations which naturally provided the necessary maritime access. Access to these ports was not a problem for the types of ships which were present at the time. The issues which ports are facing at the moment are multiple and complex.

Globalisation, trends in logistics, scale increases in vessel size and a new port hierarchy in the European port system mean that it is necessary to enlarge the fairway to these ports so that they can continue to play a role as motors of economic prosperity. For the access to most ports dredging has become a fundamental activity.

In this respect it is necessary to consider whether the social benefits of increasing the size of the fairway, or even merely the maintenance dredging, are higher than  the costs incurred by this. For example, it might be more profitable for a region to move the port itself, although this is rather unlikely, in view of the enormous costs of port infrastructure. By way of example, I would specifically like to look at the case of Antwerp.

Scheldt Estuary

It is important to make a distinction between maintenance dredging and capital dredging. Capital dredging for navigation purposes is the excavation of sediments to increase depths in an area to accommodate the draft of vessels. Maintenance dredging is concerned with maintaining the depth of the channel.  Maintenance  dredging in estuaries (an estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage – Pritchard, 1967) is a continuing and unrelenting expense.  Naturally, estuaries tend to silt up rather than become eroded. This is because the river flowing into it transports sediments to the estuary, while a great deal of sediment also comes in from the sea. In many places in the world this process is accelerated by human activity. This applies to dykes, polders and dredging itself.

In an estuary the ratio between what is transported away by the river and the tidal volume is important. In fact, there is an individual type of sediment management for  every ratio. In the Scheldt estuary the tidal volume strongly predominates. Therefore changes in the volume at high and low tide are particularly important. These actually determine whether the estuary will become silted up or enlarged. The water and sediments are transported by the channels. When there is an increase in the
volume at high tide, the channel will become broader and/or deeper. When it decreases, the channel will become smaller. If it becomes silted up with river and sea sediment, the volume at high tide will become smaller, creating smaller channels. Flooding results in a larger volume at high tide and therefore in larger channels.

Therefore dykes and polders result in smaller channels. Dredging enlarges the channels, while the tidal volume does not increase proportionately. This leads to more rapid silting up of areas at the side, so that the volume at high tide decreases again. As a result, it then becomes necessary to dredge more downstream.
The dredging work in the maritime access channel to the port of Antwerp concentrates mainly on the places where the river is naturally shallowest, and particularly on the sills. (These are the straight part of the river situated between two successive bends.) Without dredging work the natural depth at the various sills in the Scheldt would vary between five and nine metres at low tide.

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