Maintenance dredging happens to be one of the most significant aspects of the dredging industry. All over the world, there are many kinds of water systems, including rivers, canals, harbours and lakes, which for one or more reasons require additional depth than would be available naturally.
Navigation is one of the most obvious reasons, but a certain depth may also be required for drainage, water storage capacity or simply in order to guarantee the water quality and a healthy water system.
Dredging equipment developments
Maintenance dredging is as old as the water systems used to serve mankind. In the beginning, manual labour and animal force was used. However, with the invention of steam engines, and later diesel engines, mechanical dredgers entered and conquered this domain.
During the first half of the 20th century, maintenance dredging was mainly carried out by bucket ladder and grab dredgers. However, in the 1960s, trailing suction hopper dredgers (TSHDs) started to play an increasingly important role in maintenance dredging.
The advantages of TSHDs
Being self-propelled and requiring no anchors or spuds for dredging, TSHDs offer enormous flexibility. In busy waterways and harbours with regular shipping movements, this can be particularly advantageous. While carrying out their dredging tasks, TSHDs can merge with traffic and create virtually no hindrance to other vessels.
They also have the flexibility to transport dredging material to any relocation site. These can be offshore where the material can be disposed of by using the dredger’s bottom doors or valves or, in case of a split hopper, by splitting.
In some cases, the dredging material has to be stored in an onshore facility, for example if it can be beneficially re-used, if it is contaminated or when offshore disposal isn’t economically feasible due to long sailing distances. In this instance, a TSHD can pump the material to the facility in question by using a shore connection.
Seagoing TSHDs that are used for maintenance dredging range in size from approximately 200 to 12,000m3. For inland hopper dredgers, the size is restricted to about 2,000m3.
The impact of sediment gas
In many cases, mud-like sediments contain organic matter, which turns into gas due to bio-chemical reactions. For a long time, the presence of this gas was a principle challenge during the deployment of cutter suction dredgers (CSDs), since it obstructs the efficient operation of centrifugal dredge pumps.
The introduction of de-gassing systems on board CSDs during the 1980s has successfully tackled this problem. De-gassing systems extract the gas from the flowing sediment mixture before it enters the pump, largely increasing productivity when dredging this type of sediment.
The importance of mechanical dredgers
Another phenomenon that CSDs still struggle with, is the presence of debris and dumped objects. Notorious items include lost anchors, fishing gear, bicycles and shopping carts, and sometimes even complete cars are found on the bottom of rivers, canals and harbours.
Although less productive, mechanical dredgers are vital in places where heavy objects are likely to be found. Also, this type of dredger can be put to good use in confined places, such as small corners in harbours and behind jetties. These lack the necessary manoeuvring space required for TSHDs.
Although bucket ladder dredgers have almost disappeared from the maintenance dredging sector, grab dredgers in all kind of forms are still widely used. The most common of which has the grab attached to a rope crane, or occasionally a hydraulic or balance crane is used. Grab dredgers exist with or without a means of propulsion and, in the latter case, a tug or push boat is required for moving the dredger.
For the transport of the material dredged by grab dredgers, hopper barges can be used. The unloading of these barges takes place by splitting, if they are of the split barge type. Alternatively, it can also be done by barge unloading dredgers.
In addition, some grab dredgers can have a hopper of their own, in which case they are known as grab hopper dredgers. Being self-propelled, they offer a large degree of flexibility and are widely used by port authorities around the world.
Recently, a multipurpose type of hopper dredger, known as the trailing suction/grab hopper dredger, equipped with both a grab crane as well as a trailing suction pipe, has become a very popular choice for port maintenance.
Auxiliary dredging equipment
Two types of auxiliary dredging equipment have proven their utility in maintenance dredging in recent years: underwater ploughs, also known as bed levellers; and water injection dredgers.
Both are characterised by the fact that they do not bring the dredged material to the surface, but make it move under water, either over the sea floor or in the water column. The biggest advantage of both pieces of equipment, is that they have an extremely low operating cost. However, in order to be effective, they either require the help of nature or of other dredging equipment.
Underwater ploughs, for example, are perfect to operate jointly with TSHDs. They can remove sediment from inaccessible places, such as underneath jetties, and push it to an open area where it can subsequently be dredged. They can also assist with levelling operations and flatten underwater humps that are too costly to be taken care of by larger dredgers.
Water injection dredgers can be efficient when there are specific morphological circumstances. This could include a slope in the ocean bottom that directs down towards the location where the material can be stored or a current that takes care of the further transportation of sediment.
Maintenance dredging in small waters
Stationary suction dredgers can also have a role in maintenance dredging, but mainly in small waters where marine traffic isn’t such an issue, including drainage and irrigation canals, ponds and marinas. In these cases, the dredged material is often pumped to onshore storage facilities or into barges.