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  • Parameterisations of a number of concentrated benthic suspensions (CBS) related processes as there are: turbulence damping, flocculation, entrainment, internal waves, erosion, sedimentation and bed co

Parameterisations of a number of concentrated benthic suspensions (CBS) related processes as there are: turbulence damping, flocculation, entrainment, internal waves, erosion, sedimentation and bed co

The management of coastal zones and estuaries requires accurate and detailed knowledge to cope with their problems such as wetland protection and restoration, maintenance of navigation channels, dredging and dredged material relocation, effects of construction works on siltation and turbidity levels, pollutant transport, etc. Development and application of this knowledge requires detailed mathematical models, amongst which full three-dimensional codes. The physical understanding and mathematical description or �modelling� of the processes however was inadequate, especially with respect to the presence of concentrated benthic (near-bed) suspension layers (CBS).

The objective of COSINUS was to establish well validated physical and mathematical descriptions of the behaviour and fate of concentrated near-bed suspensions (CBS or �fluid mud�) and their interaction with the water and the sediment bed. Different processes have been studied in detail: turbulence damping in sediment laden flow; turbulence production due to internal waves in concentrated suspensions; flocculation; generation, properties and entrainment of CBS; bed strength development and erosion of mud beds. Since there seemed to be a lack of experimental data on the role of flocculation and turbulence in the formation and erosion of mud beds and on the formation of CBS, an experimental programme was set up to obtain these data. The detailed process models have been parameterised to obtain relatively simple formulations which can be implemented into currently used 3D and 2DH engineering system models.

The performance of the improved system models has been tested by application of the models to a schematic estuary for which 2DV solutions with the detailed research models were used as a reference. It is felt that great progress has been made in the physically based description of cohesive sediment dynamics with respect to the formulation of turbulence damping functions; the modelling of the rheology of a CBS, incl. consolidation; the modelling of flocculation and the modelling of erosion and entrainment of CBS. Engineering software tools have been improved to enable better predictions of mud dynamics for the benefit of estuarine and coastal managers.

The managing authorities of coastal waters and estuaries face a large number of problems related to cohesive sediment transport, sedimentation and erosion, such as:

-How to maintain safe navigable depths (at minimum cost)?
-Where and how to dump dredged material?
-How can the volume of wetlands be maintained or increased?
-What will happen to the location of the turbidity maximum after constructing new
harbour basins or deepening the navigation channels? Etcetera..

To answer these questions, one needs a model capable of simulating the many different and interrelated (cohesive) sediment processes occurring in coastal and estuarine waters, which can predict natural phenomena and the effects of human interference. Unfortunately, the presently used models were unable to simulate accurately the many different and interrelated cohesive sediment processes occurring in coastal and estuarine waters due to too many simplifications. Therefore there was a need for an integrated sediment transport management model in which all relevant physical processes are integrated. Progress in the understanding and the mathematical description of the different processes and the increasing capacity and speed of modern computers opens new doors toward the operational use of much more detailed models. Therefore, the goal of the COSINUS project was to contribute to the development of an integrated sediment transport management model. "Integrated" refers to the integration of all relevant physical processes over the entire water column and the sediment bed and their interactions.

The major exploitable results of this project are:
-Well validated process modules and their parameterisations.
-A framework for integrated numerical simulations.
-Guidelines for model calibration and a database for model validation.

Exploitation of the results is achieved through their implementation in commercially available engineering models (software codes), currently used in advisory studies for managerial authorities by DHI, LNHE, Delft Hydraulics and H.R. Wallingford and their clients: the engineering models are sold to and used by a great number of consulting engineering bureaus and local and governmental authorities. The parameterisations and their implementation can also be used by research institutes within a more general study of estuarine and coastal system dynamics e.g. for studies related to pollutants exchange between the sediment bed and the water column, studies of morphological stability or morphological changes of mud flats etc.

The different process modules and their parameterisations deal with are:
-The modelling of turbulence damping (buoyancy effect) and turbulence generation (internal wave turbulence production) in concentrated suspensions.

-The development of a floc model which allows the calculation of the settling velocity of flocs in relation to turbulence, concentration, residence time and, if possible, organic content.

-The modelling of the exchange of mass and momentum at the fluid mud/water interface.

-The identification and quantification of relevant processes for generating and maintaining benthic suspensions (CBS).

-The modelling of the development of erosion resistance (strength) in mud beds.

Related information

Reported by

Katholieke Universiteit Leuven
Kasteelpark Arenberg 40
3001 Leuven
Belgium
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