Working principles

Water Injection Dredging is a technique in which a specially developed WID vessel injects large volumes of water at low pressure into the sediment. This is achieved using jet pumps fitted with a series of nozzles mounted on a horizontal jet bar. The process fluidises the sediment by overcoming cohesion in fine-grained (cohesive) soils or internal friction of coarser- grained (granular) soils. The fluidised sediment remains close to the river or channel bed, flowing down to deeper areas. Fluidised soil is also sometimes referred to as fluid mud, a fluid bed or a density cloud.

A WID vessel lowers a horizontal jet bar to the seabed. This jet bar has a width approximately equal to that of the vessel. Jet pumps, usually positioned below deck, pump large volumes of water at low pressure through the vessel’s piping system to the jet bar. The jet bar is fitted with an array of nozzles on its underside, where the water is released into the seabed of the port, waterway or river.

The combination of high flow and low pressure loosens the sediment, creating a water-sediment mixture at the seabed. When sufficient current is present for transport, this density cloud is stirred up and mixed into the water column. This is referred to a High Energy Environment (HEE), where tidal and/or river currents transport the sediment away from the dredged area.

In contrast, when there is not enough current in the water column, the density cloud remains intact. This is referred to as a Low Energy Environment (LEE). In such cases, the density cloud needs to be brought to a HEE by gravitational forces induced by a gradient in the bathymetry. This gradient can be present naturally or can be created by the dredging method itself. In some cases, WID vessels work together with Trailing Suction Hopper Dredgers (TSHD) where the fine grained sediment is dredged by the WID and the TSHD dredges the coarser sediment.

An understanding of morphology and sediment behaviour regarding transport, deposition and erosion is crucial in assessing whether WID can be successfully applied. That is why WID is often referred to as “dredging with nature”, as horizontal transport is driven entirely by natural phenomena.

Siltation of ports and waterways

In nature, sediments are continuously transported from land to sea by rivers and redistributed by tidal and coastal currents. Through ongoing processes of erosion, transport and deposition, coastal features, such as sandbanks, mudflats, estuaries and beaches are formed. Some coastal systems remain relatively stable, while others change rapidly depending on hydrodynamic conditions. Ports are typically built within these dynamic coastal environments to provide sheltered areas for safe vessel operations. In addition, waterways are deepened to accommodate ships with increasing draught, requiring capital dredging.

The construction of ports and deeper navigation channels disrupts the natural sediment balance. Sheltered port basins experience lower current velocities, increasing the likelihood that suspended sediment particles will settle. Similarly, deepening a river or channel reduces flow velocity, further promoting sediment deposition. These processes lead to siltation within ports and waterways, making regular maintenance dredging essential to keep them accessible.

Siltation is strongly controlled by the relationship between sediment grain size and current velocity. Fast flowing water can transport larger and heavier particles, while calmer water allows fine particles, such as silt and clay to settle. As a result, exposed approach channels often accumulate sandy material, while sheltered port basins mainly silt up with finer sediments. Coarser particles lack sufficient energy to reach deep into port basins, whereas very fine particles will not settle in high energy zones.

The Hjulström diagram illustrates this relationship between grain size and flow velocity. It shows, for example, that sediment erosion requires higher current velocities than sediment transport. During slack water, sediments settle, but when currents resume, the velocity is often insufficient to re erode the deposited material – especially fine or cohesive sediments – causing progressive siltation.

The final destination of dredged sediments cannot be precisely pinpointed, but they follow the same downstream pathways they would have taken without port construction. Technologies such as Water Injection Dredging (WID) simply help bypass man made sediment traps, restoring sediment transport toward its natural end location.

Site conditions

Sediments are solid particles that have been transported and deposited by natural processes, such as water, wind, ice or gravity. In ports and waterways, sediments mainly consist of clay, silt and sand that are carried by river and tidal currents. The type and grain size of deposited sediments are strongly influenced by current velocity and a range of environmental factors, including soil conditions, hydrodynamics, bathymetry, seasonal effects and site geometry.

Water Injection Dredging (WID) is primarily applied for maintenance dredging in low‑strength, fine‑grained sediments. The successful application of WID requires both the ability to fluidise the sediment and the presence of natural forces capable of transporting the resulting density current away from the dredged area.

Tidal and river currents play a critical role in WID effectiveness. In areas with strong currents and high tidal amplitudes, sediment can be transported over long distances once mobilised. Such environments often experience high siltation rates due to the import of fine sediments, but these same currents can remove material after WID application. In estuarine settings, complex interactions between tides and river flow may occur such as salt‑wedge formation, which promotes flocculation and increased sedimentation. Falling tide conditions are often preferred for WID operations to maximise transport efficiency.

In low‑energy environments, bathymetry and site geometry become increasingly important, as density currents follow depth gradients toward deposition areas. Seasonal factors, such as monsoons, storms and hurricanes can cause sudden and significant sedimentation, often requiring targeted dredging campaigns. Overall, WID reintroduces sediments back into the natural system of erosion, transport and deposition by bypassing man‑made sediment traps in ports and waterways.

Energy consumption/emissions

Maintenance dredging can be carried out in different ways. The traditional method of maintenance dredging is to excavate the sediment from the seabed and to lift the sediment out of the water. This can be done mechanically with a grab dredger (GD) or backhoe dredger (BHD) or hydraulically with the dredge pumps of a cutter suction dredger (CSD) or trailing suction hopper dredger (TSHD). The sediment is then either loaded into a barge or hopper and sailed away to a disposal area, or the sediment is pumped through pipelines to a disposal area on land or at sea. In both cases, horizontal transport requires an energy-consuming activity that takes place outside the water column. WID makes use of the free energy provided by tidal and river currents for horizontal transport, which is the main reason why it is a highly energy-efficient dredging technology.

WID vessels are relatively small size vessels making them highly manoeuvrable, which is ideal for maintenance dredging. Their shallow draught makes them well suited for dredging shallow areas and corners within a port. A WID vessel dredges continuously and does not need to sail back and forth to a disposal area. River and tidal currents transport the sediment particles away from the dredged area for free, resulting in a cost-effective process with lower emissions per m3 dredged.

A WID vessel does not require the support of auxiliary equipment or a disposal area. This means that neither pipelines, boosters, a reclamation area with dry equipment, an anchor spread for stationary equipment nor a disposal area are needed. Without this addition plant and infrastructure, the organisation of a WID project is far less complex, leading to considerably lower costs, lower emissions, reduced disruption and less investment.

40 years of WID experience within Van Oord have led to the development of four ultra-low emissions vessels (ULEVs), which have been added to the Van Oord fleet in recent years. Although the WID technique itself already reduces emissions, the introduction of these energy-efficient vessels further enables port owners to achieve their sustainable targets in maintaining their assets.