Water injection dredging (WID) is a relatively new hydrodynamic dredging technique, developed 25 years ago. WID has been gradually gaining popularity for maintenance dredging and other applications, because it is a cost-effective and environmentally sound solution to some specific dredging challenges. The very first purpose-built water injection dredger – Jetsed, was christened in 1987. Now, more than 25 years later, WID is used all over the world and Van Oord is the market leader in this relatively small but growing segment of the dredging industry.
Van Oord engineering geologist Marcel van den Heuvel is involved with WID projects on a daily basis. He explains: “By injecting a large volume of water under low pressure into the bed, we bring the sediment that has settled on the bottom into suspension and create a discrete current that flows over the bed through the action of gravity. This fluidised sediment layer stays relatively close to the bottom and causes very little turbidity as a result. It flows away down slope or is carried away by natural tidal or river currents. It is an efficient way of maintaining the depth of a port or river. When a port or navigation channel suffers from siltation, and maintenance dredging is requi red on a regular basis, WID should be the first dredging method to consider. When the boundary conditions for WID are in place, it is the most cost-effective way of maintaining navigable depth in the waterways.”
UK sets trend with long-term relationships
Van Oord water injections dredgers, eleven in total, are used to capacity. WID has been used widely in India, Brazil and northwest Europe for many years. Water injection dredger – Jetsed works mainly in the UK, the Netherlands, Germany, and France but occasionally it has also been deployed outside this region. In 2006 for example, it went to Richards Bay, South Africa, to dredge out a ship that had run aground. “This shows how versatile WID is”, says Maurits den Broeder, Van Oord area manager in UK and Ireland. “Every quarter, water injection dredger – Jetsed sets course for the rivers Thames and Medway. We’ve carried out a series of projects in these UK waters for ten years now. The projects are intensive because they are of relatively short duration and carried out for a variety of different clients. The Jetsed works steadily on maintaining breakwaters, locks and harbour basins. The Jetsed can move in any direction and can work very close to shore and even beneath the breakwaters, and uses side nozzles to clean quay walls. The differential global positioning system (DGPS) and digital bathymetric mapping tools are fully integrated in the onboard control systems of the water injection dredger for maximum control of the WID process. In the past years, our database of executed projects has grown to more than 25 separate clients in the UK and Ireland. These are long term relationships, ranging from small local companies to large international corporations.”
Jetsed in great shape
Mr den Broeder continues: “The Jetsed was recently fitted with a new injection bar measuring 13.4 metres wide. The bar has 42 small water jet nozzles that can pump three cubic metres of water per second into the bed. Thanks to a meticulous maintenance regime, the vessel is still in tiptop shape. Right now, the Jetsed is carrying out annual maintenance in the Thames. We celebrated its silver jubilee here. The water injections dredgers – Odin and Baldur also have the UK as their base of operations and perform maintenance dredging there every year at various locations.”
Environmental considerat ions and collaborative working are also key priorities. Mr den Broeder says: “Each and every one of our clients have different requirements, needs and opinions and we tailor our approach to the area we work in. For example, we do not operate the Jetsed during the spawning season for eel, salmon and other migratory fish. The crew of the Jetsed also takes bird migration into account in their plans. On top of everything else, we comply with strict noise level requirements and operate in strong tidal currents. We have close ties with both our clients and the port authorities. That’s absolutely essential given the specialist and labour-intensive nature of the WID work that Van Oord carries out. After so many years, our clients and port authorities have come to know and trust us. The clients know that we deliver quality, and they can count on our flexibility.”
Mr van den Heuvel is convinced that the application of WID has much potential and that, once this technology is fully understood by all stakeholders, a large increase of WID activities will be the result. “WID is an alternative to traditional dredging with a trailing suction hopper dredger. It is often a challenge to introduce this technique to new clients, consultants, legislative bodies and other stakeholders. Trailing suction hopper dredging is proven technology and the environmental impact of this technique is known and accepted. When WID is introduced to new customers today, in a time with more environmental concerns and awareness, resistance from various environmental institutes may arise. The dredging authorities do not always foresee the applications of WID. For instance, often, a dumping area is defined as the end position of the dredged sediment.
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