Physics of Dredging
The Excavation can be mechanically or hydraulically. In both cases sediment is transformed from a solid into a fluid state in a very short time.
The phase change can be regarded as dynamic soil mechanics or high concentrated slurry flow. Soil mechanical parameters as friction angles and permeability as well as fluid mechanical properties as viscosity, and settling velocities play an important role. Deformation rate is very large during excavation. Combined with dilatant behavior this leads to large negative pore pressure or even cavitation of pore water.
Although Vertical Transport can be performed mechanically (for instance with a grab dredge), in most cases a hydraulic mechanism is used using (submerged) centrifugal pumps. The largest part of the sediment dredged is used as building material and is therefore not fine grained. The flow regime typically encountered is therefore heterogeneous. The suction pipes are mostly inclined. Production limiting factors are available pumping power and minimum pressure at the suction inlet of the pump.
For relative short distance sediment is transported by hydrotransport in most cases. With increasing distance (in dredging practice typically from 10 – 100 km) trailing suction hopper dredges (TSHD) are often employed. In that case the sediment is transported in the hopper (cargo hold) of the dredge. A TSHD is a vessel equipped with one or two suction pipes. At the locating where the sediment is extracted the suction pipes are lowered to the seabed. At the lower end of the suction pipe a special designed suction mouth, the so-called draghead is loosening the sediment with the help of water jets and / or teeth. The erosion of sand using high velocity water jets is a very complicated physical process where soil mechanical and rheological properties play an important role.
The sediment-water mixture is transported through the suction pipe and discharge in the hopper. Here the sediment settles and the excess water flows over board. The sedimentation process encountered is very typical. The sediment settles from an overloaded density current in a confined space. Flow velocities are below the deposition limit but still high enough to have an influence on the sedimentation velocity. In most cases a certain part of the incoming sediment will not settle and will flow overboard. This so-called overflow loss is dominated by the particle size distribution, slurry concentration and slurry discharge by unit hopper area.
Deposition of dredged material can be done under water or above water. In both cases hydraulic processes are dominant. Under water density currents develop from which sediment settles and under water bodies are developing. Density and stability of these sand fills are depending on method placed, particle size distribution and operational conditions. Above water sand bodies are forming from highly concentrated flows. Densities and slope angles of the bodies are depending on specific discharge and grain size.
Thorough knowledge of the processes taking place during the above described stages is very important to design dredging equipment (equipment building industry) and to estimate and design dredging projects (dredging contractors & consultancy and engineering companies).