Cohesive sediment, or mud, is a mixture of clay particles, silt, (fine) sand, organic material, sometimes gas, and water. This sediment mixture has cohesive properties because of low permeability and electrochemical attraction of clay particles and organic material. Mud is therefore encountered in the form of aggregates (flocs), both in the water column and in the bed. As the composition of mud generally varies in place and time (for instance, because of seasonal effects), the physical properties of the mixture also vary in space and time. The properties of mud flocs are also affected by memory effects with respect to the history in physical, chemical and biological influences.
Mud deposits are often a nuisance to navigation. Dredging operations are therefore carried out to safeguard accessibility of ports and waterways. Currently, many million tons of mud are dredged from harbour basins and navigational channels throughout the world. Unfortunately, today many mud deposits are contaminated, endangering the ecosystem and increasing the costs of dredging operations progressively.
Mud is also one of the controlling factors for ecological development in the water system. It is often beneficial as an agent for nutrient transport. On the other hand, high turbidity limits the penetration of sunlight into the water and hinders the growth of water plants by inhibiting photosynthesis and affects the sight of foraging aquatic animals.
By interaction with bio-engineering, clay particles in the soil change the erosion and consolidation properties of the soil and therefore the long-term morphodynamic development of the system.
So in both cases, engineering problems and the management of natural water system, knowledge on mud behaviour is important.
specialist areas
Depending on the question and the level of accuracy required, we use a combination of site investigation, sample analysis, desk study, laboratory tests and numerical models to tailor our advice. The following questions are typical of the type of questions we often answer.
impact of dredging
field research
What suspended sediment concentrations are to be expected?
What is the extent of a dredging plume?
What is the optimal dumping location?
How much sediment will return to the dredging site?
siltation
What is the long-term siltation rate in my system?
What is the effect of applying certain modifications (such as mitigating measures) to the system?
What can I do (cost-effectively) to reduce siltation in harbours?
artificial mud deposits
How can sediment be kept in an artificial deposit?
What is the risk of failure in the structure of a slope within a natural mud deposit within the next ten years?
How much dredged material can I store in a depot?
How will consolidation proceed in time?
(natural) system analysis
How do I restore the natural mud dynamics of the system and what time-scale is involved?
What is the cause of the high turbidity level in the water system and what can be done to reduce it?
morphological change
siltation in estuary morphological evolution in estuary
How will the intertidal area change within the next 10 years?
How does increasing the depth in tidal channels for navigation affect the stability of nearby mudflats?
