Cohesive Sediments: “Flushing the sediments of sovereign harbor”
GENERAL INFORMATION
Cohesive sediment is a mixture of clay particles, silt, sand, organic material, sometimes gas, and in general, large amount of water. This sediment has cohesive properties because of the electrochemical attraction of clay particles and the organic material. Thus, mud is encountered in the form of mud flocs, both in the water column and within the river or sea bed. As the composition of the sediment mixture varies in place and time, the mechanical properties of the mixture vary in space and time. Moreover, the properties of mud flocs are affected by memory effects with respect to the history in physical, chemical and biological influences. Cohesive sediment is encountered in most water bodies throughout the world. Often, mud is a valuable resource, synonym for fertile land, enriching the natural environment and used as building material for housing and others.
Famous examples are the ancient development in Mesopotamia and the Nile flood plains, which would have been impossible without the regular supply of clay by the rivers. Also the wealth and fertility of the Netherlands is a gift from the River Rhine and North Sea. Currently, many million tons of mud is dredged from harbour basins and navigational channels throughout the world. Unfortunately, today many mud deposits are contaminated, endangering the eco-system and increasing the costs of dredging operations progressively. Thus, proper and sustainable management of our environment requires, amongst other things, a proper understanding of tools to predict the transport and fate of mud in the environment. However, the behaviour of cohesive sediment is still poorly understood and the need for basic research remains large. This motivates the research at the Environmental Fluid Mechanics Section, which is carried out in close collaboration with many institutes and universities within The Netherlands and abroad.
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Amicably, cohesive sediment research at the started in the early 1980s with feasibility study of mud pumping of deposited sediments in the entrance of harbor lock (Cited from, , 1989). The work was continued by developing numerical models, using the Finite Element Method, for fluid mud flow and settling and consolidation of mud (Cited from, , 1992).
Sedimentation, consolidation and liquefaction of mud by Erik Toorman
The laboratory is equipped with temperature controlled, dark room with six transparent settling/consolidation columns as it is 2 m high, 0.1 m in diameter. Measurements are taken of pore pressures at various heights, bulk density and interface height as a function of time. This set-up has been extensively used for research and consulting on the settling and consolidation behaviour of artifical and natural muds, as well as of mud mixtures. A general theory has been developed to unify the classical theories of sedimentation and consolidation (Cited from, , 1999). A computer model has been developed to solve the sedimentation/consolidation problem (Cited from, , 1996). Experimental data have been analysed in order to develop new closure relationships (Cited from, and , 1991; and , 1997; , 1996; and , 2000).
The erosion of sediment bed is governed by the structure of its surface. Its erosion resistance strength changes due to consolidation (strengthening) and/or fluidisation and liquefaction (weakening). During the COSINUS project a first attempt was made to develop a numerical model that could simulate both processes, such as bed dynamics model for soft soils with extremely large deformations (Cited from, ., 2000).
Erosion of sand-mud mixtures by Erik Toorman
The Hydraulics Laboratory is equipped with such straight, re-circulating erosion flume as concentration and velocity profiles can be measured, as well as the water surface slope and bed load transport. The composition of the sediment bed in many estuaries or along the coast are mixtures of sand and mud. Flume experiments have been carried out to study the influence of clay content on the erodibility of mixed sand-clay bottoms. Both homogeneous and layered beds have been investigated.
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The above picture shows ‘’mud ‘beach’ backed with eroding salt marsh at Annapolis Royal, in Annapolis Bay of the Bay of Fundy. Basalt revetment at top of salt marsh is an attempt to halt the erosion’’
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Thus, mangrove can be found between mud flats and salt marsh, and can transmit waves, which erode the adjacent salt marsh behind it. A sloping mud shore, exposed to wave attack often following the destruction of mangrove or other wetland vegetation. The usual cause of wetland or mangrove destruction is property developers, municipal agencies, and individuals wishing to ‘reclaim’ the shore. Natural destruction occurs from the seaward edge, through undermining of the root system by waves and currents.
Erosion Processes on Consolidated Shores
Erosion processes on cohesive shores are distinctly different from those on sandy shores. There are also differences between consolidated and mud cohesive shores. On consolidated shores, the erosion process is irreversible because, once eroded, the cohesive sediment cannot be reconstituted in their consolidated form in the energetic coastal environment. Furthermore, since the sand and gravel content is low in these deposits, erosion is not balanced by an equal volume of deposition within the littoral zone. The eroded fine sediments are winnowed, carried offshore, and deposited in deep water in contrast to the sand fraction, which usually remains in the littoral zone.
The above picture shows ‘’sand beach disappearing into mangrove on the island of Borneo. Sediment within the mangrove is cohesive mud’’
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Consolidated cohesive sediment is eroded by at least four mechanisms:
- Through abrasion by sand particles moved by waves and low currents
- Through pressure fluctuations associated with turbulence generated at various scales such as wave breaking induced turbulence that reaches the lake or seabed and large-scale eddies that may develop in the surf zone
- Through chemical and biological influences
- Through wet/dry and freeze/thaw cycles where exposed to the atmosphere.
However, only when the sand cover is sufficient to protect the cohesive substratum at all times will the shore revert to sandy classification. On consolidated cohesive shores, the rate of lake or seabed downcutting determines the long-term rate at which the bluff or cliff retreats at the shoreline. In other words, while subaerial geotechnical processes may dictate when and where a slope failure will occur, the frequency of failures over the long term is determined by the rate at which the nearshore profile is eroded. Essentially, numerical model of cohesive shore erosion must define the near-bed flow conditions within the surf zone, the movement of any overlying noncohesive sediment cover, as well as the erosion resistance properties of the cohesive sediment.
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