Description of the STOWASUS-2100 project


The overall objective of STOWASUS-2100 is to study severe storms, surges and waves in the present climate and in a scenario with increased CO2-concentration. More specifically the project is a joint atmospheric/oceanographic numerical modelling effort aiming at constructing and analysing storm, wave and surge climatologies for the North Atlantic/European region in a climate forced by increasing amounts of greenhouse gases and to compare with present day conditions. It is investigated whether any systematic anomalies regarding frequency, intensity or area of occurrence are found for these extreme events. Also physical mechanisms responsible for possible scenario anomalies are investigated.


Off-shore industries, fisheries, shipping companies, and the insurance business are highly sensitive to extra-tropical strong wind events and the associated ocean waves and surges. It is likely that impacts of possible future changes in the occurrence of extreme type events like these and others will be more severe than modulations of the long term mean climate. This is the rationale behind the STOWASUS-2100 project which aims at setting up climate change scenarios for storms, waves and surges on a variety of spatial scales. On the larger scales, studies on storminess in the North Atlantic region will be performed, while detailed studies on storminess, surges and waves will be carried out in the Adriatic, The North Sea and the Norwegian Sea. On the local scales, storms and surges will be studied in estuaries, low lying coastal areas along the North Mediterranean and North-western European coasts.

The project builds to considerable degree on the results obtained in another project called WASA, which has been described by “The WASA Group” (1998). In WASA it was found that the storm and wave climate has roughened in recent decades, but that the present intensity of the storm and wave climate seems comparable with that at the beginning of the 20th century. The WASA project furthermore analysed and used the output from a high-resolution (T106 spectral truncation) climate change scenario experiment, mimicking global warming due to increase greenhouse gas concentrations. It was found that storm and extreme wave activity was slightly increased in the Bay of Biscay and in the North Sea in a warmer climate, while this activity was slightly weakened at several other places. The experimental set-up of the climate model simulations on which these results were based has been described by Beersma et al. (1997) who pointed out that the projected anthropogenic changes in storm activity fall well within the limits of variability observed in the past considering the length of the (control and scenario) simulations which was only 5-years.

Recently, two so called time slice simulations with the ECHAM4 model also at T106 horizontal resolution have been performed at the Danish Meteorological Institute (DMI) in a collaboration between the Max Planck Institute for Meteorology in Hamburg and DMI. These simulations each covered a period of 30 years, i.e. 6 times longer than the simulations used in WASA. Thus they should be much more suited for studies of storminess and associated impacts since the sampling problem is considerably reduced. The STOWASUS-project therefore uses these new simulations as a backbone to drive very high resolution regional atmospheric climate models and wave and surge models of different resolution. It is these secondary simulations which will be used to set up climate change scenarios of storms, waves and surges along European shelf and in European Estuaries and to compare these with present day conditions. The project is logically divided into 12 working tasks some of which will be described briefly in the following three sections together with preliminary results – to the extend they are available at this early state of the project.


As mentioned above the backbone in STOWASUS-2100 consists of two 30 year time slice simulations with the ECHAM4 atmospheric climate model at T106 horizontal resolution. The experimental design of these simulations which are not part of the project is described in May and Roeckner (1998). The project includes an analysis of the storm and extreme wind climate in these simulations. Fig. 1 (left column) shows the long term mean sea level pressure (MSLP) in winter (DJF) as obtained from the European Re-Analysis data (ERA), from the control simulation and from the scenario simulation. It is seen, that the ECHAM4 model simulates the atmospheric mass field well except for a too high pressure over and immediately to the west of the Iberian peninsula which leads to a too zonal flow over NE Atlantic region. The figure also shows the difference in the MSLP between the scenario and control simulations in the bottom panel, and it is seen that there is a significant increase in the zonality over the northern part of the area associated with a decrease in the high latitude MSLP in the scenario. The right column in Fig. 1 similarly shows the standard deviation of the band pass filtered (2.5-6 day filter) 500 hPa winter height fields which is commonly used as an estimate of storm activity. Also here the model behaves well and a significant increase in storm activity is seen over Northern Europe in the scenario simulation relative to the control simulation together with a corresponding decrease in storm activity around the east coast of US. Fig.2 shows the 1% percentile wind speed for the winters (DJF). Comparing the ERA data and the control run shows that the ECHAM4 model has more severe storms along the south and east coast of Greenland than the ERA data. The difference between the scenario run compared to the control run (fig. 2d) shows more severe storms in the Atlantic north of 60N, and less severe storms south of this latitude. These changes are in accordance with the changes in the 500 hPa variability (fig. 1d). The changes in near surface wind (Fig. 2) are important to the wave and surge simulations (see below) as one can expect that the enhanced wind speeds will also lead to more severe wave and surge activity. This has, however, not yet been shown in the project.

The atmospheric investigations will also cover atmospheric modelling with very high resolution regional climate models (HIRHAM and BOLAM) to perform of intensive systems that are not well resolved at T106 resolution: intensive extra-tropical baroclinic developments, polar lows and highly convective systems (with some apparent similarities to polar lows) in the Mediterranean. Fig. 3 shows the orography in the T106 model and in the BOLAM model over the Mediterranean region and it is seen that one may expect much larger impact from orographic effects at very high resolution than at T106. All the simulations with HIRHAM and BOLAM will take boundary conditions from the T106 time slice simulations and will to considerable degree focus on analysing and understanding the processes associated with possible changes in scenario cases relative to control cases.

The modelling of cohesive sediment transport

The management of coastal zones and estuaries requires more and more accurate and detailed knowledge of cohesive sediment (mud) transport processes to cope with various problems (e.g. wetland protection and restoration, maintenance of navigation channels, dredging and dredged material relocation, effects of construction works on siltation and turbidity levels, dispersion of pollutants, etc.). Detailed mathematical models, including full three-dimensional codes, are necessary tools for the development and application of this knowledge. Presently, this is becoming practically feasible in view of the current developments in soft- and hardware. The physical understanding and mathematical description of the processes, however, is still lagging behind, especially with respect to the presence of concentrated near-bed suspension layers, as explained below.


The behaviour of mud in coastal zones and estuaries is the result of horizontal and vertical sediment fluxes, i.e. transport, water-bed exchange processes, including the formation of concentrated suspensions, and processes within the bed.

This whole (semi-)cycle is particularly complicated, because it does not necessarily have to be closed and the processes are interdependent. For instance, settling does not always lead to deposition, i.e. when entrainment dominates, and the sediment then remains in suspension; the consolidation processes within the bed are affected by the flocculation processes in the water column and in turn the erosion processes are governed by the consolidation processes and mediated by biota. The actual transport is the net effect of the interactions of different processes, which, until now, have been studied mainly in isolation.

Concentrated benthic suspensions (CBS) and fluid mud

The capacity to transport sediment in suspension by currents and waves is limited by the amount of energy available in the flow. In many cases the suspended matter is not well mixed over the water column and stratification occurs due to settling when the turbulent energy decreases, resulting in a concentrated near bed sediment suspension. They can be maintained by the turbulent energy of the flow when there is equilibrium between the depositional flux and the vertical turbulent transport flux (e.g. Wolanski et al., 1988). These layers are often thin and therefore frequently remain undetected. The concentrations in these layers can be of the order of 5-10 g/l, but also much higher. Because they include a high proportion of the mobile fine sediment, the total amount of sediment that is transported in these sheet flows can still be enormous. Research has revealed that these near-bed layers are the major mechanism of the transport of fine-grained sediments in coastal zones and estuaries (e.g. Faas, 1984; Odd et al., 1992). Therefore, they must be considered in transport models. These near-bed layers will hereafter be called concentrated benthic suspensions (CBS). This terminology is used for the regime where the sediment/water mixture behaves as a fluid. The concept has a broader meaning than that of “fluid mud”, which is associated with high concentration suspensions with a sharp mud/water interface, which can be detected by echo sounding (Parker & Hooper, 1994). The term “fluid mud”, moreover, is often also used for soft consolidating mud layers, which do not behave as a true fluid, and therefore can be misleading.

At concentrations above a critical value of a few 10 g/l, the particle interactions start to modify the properties of the suspension, i.e. the particle interactions lead to hindered settling and at higher concentrations the rheological behaviour of the suspension becomes non-Newtonian (Faas, 1984). To maintain CBS of higher concentrations (above the critical value), much more energy is required as turbulence is being damped by buoyancy effects and by the dissipation of energy due to interparticle collisions and its resulting effect on the floc structure. Therefore, when the energy level is too low to maintain the CBS, this layer will deposit and form a denser fluid mud layer.

The interface between a CBS layer and the water column above can be very distinct, but also very unstable. Helmholtz-Kelvin instabilities may occur (internal waves), leading to interfacial mixing, which contributes to the entrainment of sediment into the water column and of water into the CBS layer (i.e. diluting it) (Scarlatos & Mehta, 1992; Mehta & Srinivas, 1992; Le Hir, 1994; Winterwerp & Kranenburg, 1994). The vertical fluxes involved in this process are not yet fully quantified.

Pressure gradients (e.g. induced by density differences) may cause gravity current flow of CBS. Lower concentration gravity currents are very unstable and generate turbulence. Density currents of dense, visco-plastic fluid mud layers on the other hand are generally laminar.

When the sediment concentration in a dense suspension exceeds a second critical value, the gel point or structural density, the flocs form a continuous network structure and develop effective stresses: a weak saturated soil is formed. At rest this structure will slowly collapse under its own weight and the interparticle bonds will increase in strength: it consolidates. The compression of the bed and the increase of its strength will be determined both by the development of effective stress and by separate time-dependent processes such as creep and thixotropy (Sills, 1994 & 1995). However, under the influence of internal or external forces (e.g. wave action, particularly in storm conditions), the structure of a weakly consolidated bed may fracture and eventually break-up into mobile aggregates and the sediment behaves again as a dense non-Newtonian suspension: the bed is liquefied under shear forces or fluidized under excess pore pressures. The resulting fluid mud layers can easily be eroded, entrained into the water column and transported as gravity currents: this is a second mechanism for the formation of CBS layers (Mehta et al., 1994).

The transition between CBS and the bed is poorly defined at the moment. The erosion happens by different processes depending on the developing rheology of the CBS and on the degree of consolidation. The entrainment of the CBS contrasts with the erosion of the bed by failure of the interparticle bonds of the flocs on the bed.
Flocculation of fine sediment

The formation of concentrated benthic suspensions, their structure and subsequent evolution is governed by the settling flux of suspended sediment towards the bed. This is the product of the mass concentration and the settling velocity. Both the settling velocity and the size of the flocs are determined by a balance between the forces of particle coagulation and disruption. Coagulation is a function of sediment concentration, salinity, organic material content, particle mineralogy and physical processes like Brownian motion, differential settling and turbulent shearing. At low concentrations a small amount of shear will help to bring small flocs together to form larger ones. Higher shear will tend to pull the flocs apart, and which is enhanced by collisions at high concentrations. The flocs most prone to disruption are the largest ones (macro flocs) that have the lowest density, but which have the greatest settling velocity and contain the greatest mass. Their break-up creates a number of smaller micro flocs which, though they together contain the same total mass, individually have higher densities, but lower settling velocities. The aggregation and disruption functions have not been independently determined for natural suspensions. This is a major shortcoming in predictive models which currently only use relationships between settling velocity and concentration. Though these relationships are determined by laboratory measurements in still water on natural samples, and empirically account for mineralogy, salinity and organic content, they are only a first approximation. Because of the interaction between the processes due to concentration and turbulent shear, their combined effect can only be derived from in-situ measurements.


We identify the research needs for cohesive sediment transport processes from the requirements of the transport models which are used by the managing authorities and engineering consultants.

The modelling of cohesive sediment transport requires the numerical solution of the basic conservation equations of mass, momentum and turbulent energy. Several model parameters are supplied by semi-empirical closure equations for input quantities which depend on position and time (Teisson, 1994), amongst which: the settling velocity of flocs, the eddy diffusivity and the sediment flux at the bed (deposition, erosion). For practical applications fine-sediment suspensions can be modelled as a continuous (single) phase fluid (Le Hir, 1994).

In the state-of-the-art engineering models for simulating the transport and fate of CBS layers, a three-layer approach is often applied in which the lower layer is formed by the consolidated bed, the upper layer by the water column and the layer in between by the fluid mud (e.g. Odd & Cooper, 1989; Kusuda & Futawatari, 1992). The exchange of mass between the two upper layers is described through deposition and entrainment formulae, whereas the exchange of momentum is limited to an adaptation of the internal friction coefficient. The exchange between the two lower layers is described in the form of consolidation and erosion formulae. All formulae are highly empirical often with dimensional empirical coefficients, indicating little physical background. This is cumbersome, as the transport and fate of fluid mud layers, predicted with these models, is highly dependent on the actual value of these coefficients. For instance, the settling velocity is taken constant in general, whereas in reality it is strongly affected by the level of turbulence. The entrainment process was studied by Mehta & Srinivas (1993) and Winterwerp & Kranenburg (1994). The latter derived an integral entrainment model by a rigorous integration of the equation for kinetic turbulent energy over the water depth. However, even in that case many empirical coefficients are still required, especially for the fluid mud properties.

In summary, in the state-of-the-art models the interaction between the water movement and the fluid mud layer is neglected, the properties of the sediment in suspension (settling velocity) and of the fluid mud (viscosity and yield strength) are given by highly empirical relationships and the occurrence and effects of CBS is even entirely ignored.

Turbulence modelling of sediment-laden flow

Turbulence models of various levels of sophistication have been developed, from simple algebraic models to Reynolds stress models (Rodi, 1980; Schumann & Gerz, 1995). Turbulence models for estuarine flows should take into account the damping effect of vertical density stratification. Existing models satisfy this requirement in one way or another. Stratification may not only result from the ubiquitous salinity gradients, but also from gradients in the sediment concentration. In the latter case an interaction exists between turbulence and sediment transport. This interaction in near-bed layers has been analysed numerically by Le Hir (1994) (algebraic mixing-length model), Uittenbogaard et al. (1996) (k-epsilon two-equation model) and Teisson et al. (1992) (Reynolds stress model), amongst others. Uittenbogaard (1995) argues that the production of turbulent kinetic energy by internal waves should be included when modelling CBS. Turbulence modelling in the case of fluid mud is complicated by non-Newtonian rheological behaviour and low Reynolds number effects. As sediment transport processes occur on widely different length and time scales, the numerical aspects of turbulence modelling are complex and require special attention.

In summary, turbulence modelling is needed to determine the quantities required by fine-grained sediment transport models, i.e. to estimate the floc settling velocity and deposition rate, entrainment rates of suspended sediment, surface erosion rates, interfacial stability and mixing at lutoclines and the damping of turbulent energy in concentrated suspensions.

Modelling flocculation

A conceptual model of the effect of shear and concentration on median floc settling velocity has been proposed by Dyer (1989). A heuristic formulation for the process, based on laboratory studies, has been advocated by van Leussen (1994), which relates flocculation and break-up to the dissipation rate of turbulent kinetic energy. This formulation has been incorporated into a numerical estuarine model by Malcherek et al. (1994) and appears to be a major improvement. Flocculation models are most easily formulated in a Lagrangian framework. However, for implementation in general sediment transport models an averaged Eulerian formulation is needed, which requires additional turbulence modelling (e.g. Casamitjana & Schladow, 1993).

The various coefficients need to be obtained from field measurements. This approach requires simultaneous measurement of the settling velocities of the various floc size fractions, the suspended sediment concentration, and the characteristics of the turbulence such as the turbulent shear stresses, the turbulence energy and the eddy dissipation rate. An additional factor that has to be quantified is the influence of organic constituents that can act as ‘glue’ in the flocculation process.

A number of new techniques have recently been developed for in-situ measurements of floc size and settling velocity with minimal disruption (Eisma et al., 1990; van Leussen & Cornelisse, 1993; Fennessy et al., 1994; Dyer et al., 1996). These apparatus have been compared with Owen Tubes in an intercomparison experiment during which it became apparent that the Tube performance was dependent on the operator and the sampling protocol, and that the flocs were disrupted by the sampling. The new techniques are able to distinguish individual flocs, and can provide information on the spectra of floc size, settling velocity and effective density under different conditions.

Modelling the interaction between concentrated benthic suspensions and water

A further understanding and considerable improvement of the modelling of CBS layers are only possible by taking the three-dimensional (3D) effects fully into account. Considerable progress has been made recently by Le Hir (1994), Le Normant (1995), Malcherek (1996) and Galland (1996), amongst others, who studied the three-dimensional behaviour of suspended sediment transport and concluded that simulation of observations is indeed only possible by taking the 3D effects into account. Entrainment of CBS by a turbulent flow can be modelled in a similar way as the entrainment of a dense fluid (Winterwerp & Kranenburg, 1996).

Up to now, the important interaction between water movement (turbulence) and sediment could only be accounted for to a minor degree, because the required physical-mathematical formulations are not yet available. In particular the interaction between (turbulent) water movement, damping of turbulence due to buoyancy effects and its influence on the vertical mixing processes, the effects of flocculation and hindered settling on the formation of fluid mud and CBS and their internal properties (non-Newtonian stress-strain relations), the influence of waves on the generation and erosion of fluid mud layers and the stability of fluid mud layers and CBS are still poorly understood.

The effects of surface waves on the fluid mud layer and vice-versa (e.g wave damping) are important phenomena. Their explicit effects, however, lie beyond the scope of this project. But the indirect effects, such as pore pressure build-up, prior to fluidisation, or interfacial mixing has to be accounted for. Also very little is known about the erosion of a consolidated mud bed by breaking waves, which may occur on tidal flats during a storm event following a period of deposition. This also falls outside the scope of this research project.

Modelling the interaction between fluid mud and the bed

For each of the processes which determine the fate of fluid mud or a mud bed, i.e. consolidation, liquefaction and fluidisation, different models have been developed over the past years: a unified theory for settling and consolidation has been developed (Toorman, 1996) and a poro-elastic model is used for fluidisation (Yamamoto et al., 1978). These models allow the computation of the density and/or stress history within the mud layer. In order to erode a mud bed, the bottom shear stress should be larger than the shear strength of the bed surface. Traditionally, erosion of a consolidated bed has been modelled using empirical formulae and the critical erosion stress has been empirically related to the yield stress, a rheological property which is a measure of the degree of bed structure (Gularte et al., 1979). The yield stress, which is difficult to measure (even in the laboratory), is determined as a function of the density. In this way an empirical link has been established between consolidation theory and rheology. A comprehensive theory, which incorporates liquefaction and fluidisation, is still lacking. An attempt in this direction has been made by van Kesteren et al. (1993), who used a geotechnical approach. As a further step in erosion modelling, this approach should be linked with a hydrodynamic approach. The common factor in the bed history is the structure, which should be parameterized. A possible approach is that of the structural kinetics theory, which has been applied successfully to the modelling of the thixotropic behaviour of mud (Toorman, 1995) and the formation of flocs (Winterwerp, 1996).


Since the processes are very complex, the process models and modules generally are parameterizations of the elaborated mathematical-physical descriptions. This is still a common practice to make the computer codes cost and time efficient in order to be applicable to practical managerial problems.
Measuring methodology

Validation of numerical models requires field and laboratory data. Measuring campaigns have been hampered by a lack of understanding of the physical processes and an isolated approach on the research of mud dynamics. For instance, it may appear necessary to measure turbulence properties simultaneously and some time prior to the settling velocity measurement to obtain reliable results. An improvement of measuring methodologies for cohesive sediments will be one of the spin-offs of the proposed research. Vice versa, insight in measuring methodologies and techniques will prevent the development of process formulations containing unmeasurable quantities.

In summary, it is hypothesized that the transport, fate and subsequent behaviour of cohesive sediment suspensions in many coastal and estuarine environments are largely governed by concentrated near bed suspensions and the structure of the aggregates involved. Classical fluid mud appearances are one of types of CBS identified in the literature. The state-of-the-art review leads to the identification of various gaps in our knowledge, of which the major ones can be summarized as follows:

  • The relationship between floc properties, such as strength, and settling velocity in turbulent flow is poorly understood. A tractable model of history effects on flocculation and floc break-up in turbulent flow is not available.
  • An entrainment model for fluid mud should account for all flow conditions depending on the degree of turbulence damping, i.e. the transition from high to low Reynolds number turbulent flow, down to laminar flow, where the rheological properties of become dominant. Such turbulence models are not available.
  • A physically based bed erosion model should take into account the interaction between turbulence and soil-mechanical properties of the bed. A general model is required which allows for the computation of the strength history of the bed, including the effects of consolidation, liquefaction and fluidisation.

Economic & Social Impact of COSINUS project

Estuaries belong to the most developed areas in many countries. They are under severe stress, as the surrounding land is often densely populated and a concentration of industry, harbour activities and ship traffic is encountered. Many other commercial activities take place, such as fishing, recreation, sand mining, dumping of dredging material, disposal of industrial and domestic waste, etc. On the other hand, estuaries are important natural areas for wildlife, nursery for fish. Often large wetlands are found in estuaries. Through their interaction with the sea, estuaries form an important part of large scale eco-systems.

In serving all these needs many conflicts of interest emerge in the management of estuaries. The managing authorities therefore have a need to weigh the various interests and to predict the consequences of managerial measures in the system. Managing authorities are confronted with progressively more conflicting interests, smaller budgets and more severe legislation. In many of the problems they encounter in estuaries and coastal zones, the transport and fate of cohesive sediment, contaminated or not, plays a key role. Thanks to new developments in numerical modelling and measuring techniques, it is apparent now that this transport and fate is governed in many situations by near bed concentrated mud suspensions. However, our physical understanding is yet insufficient to quantify these processes adequately and formulate them properly in mathematical models. As a result large uncertainties exist in any prediction or recommendation that is made with these mathematical tools. This means that presently it is not possible to improve the physical rationale to optimize managing strategies, the minimization of maintenance costs, nor the evaluation of measures for sustainable development.

An important tool in these evaluations is formed by mathematical models. The shortcomings of the present class of models is discussed in the state-of-the-art. The improvement of these models provides the authorities with means to improve their weighing process. More specific, the impact of a series of measures and interference with the system become better predictable.

The COSINUS project aims at an integration of our perception of various individual processes and of the individual process formulations, and the subsequent integrated validation, with special emphasis on major shortcoming in our understanding, i.e. on the behaviour of near bed concentrated suspensions. An important aspect of this work is related to the practical implications of implementing the various formulae in mathematical models. We believe that in spite of the enormous developments in computational power, it is not yet, nor in the near future, possible to incorporate all these formulae fully in the models, as they then become unpractical to run. Therefore additional studies are required to obtain convenient numerical codes and parameterizations of the physics.

This is a challenge, as it involves the quantification of the flocculation process and the bed structure, turbulence modelling beyond a state that is routine in civil engineering, the interaction between Newtonian and non-Newtonian viscous fluids and a poro-elastic medium, thixotropic effects, implementation of the derived formulations, or their parameterizations in global mathematical models suitable for engineering purpose, and last but not least, the measurements within relatively thin layers of high concentrated near bed suspensions.

The results of the proposed research will contribute to a better assessment of the environmental impact of human interference and regulation in estuaries and coastal zones, improving the modelling tools used for environmental management. The major socio-economic advantages of the COSINUS project consist of:
• A better assessment of the siltation patterns, both with respect to the quantities involved, and the spatial and temporal distributions, enables the authorities to optimize the lay-out and maintenance costs of fairways and harbour basins. At the end this implies lower operational costs of harbours, improving their competitive edge in relation to other ports elsewhere in the world.
• A better assessment of the turbidity levels and accumulation patterns of (contaminated) sediment allows the sustainable development of estuaries and coastal zones; typical examples are the restoration of wetlands and intertidal areas. This contributes to a healthier environment within the Community.
• The commercial institutes working together within this project will increase their competitive edge on the international consulting market through the technology step that will result from the proposed research.

Our major innovation will consist of the development of explicit, physically sound mathematical descriptions of the behaviour of concentrated benthic suspensions and its interaction with the bed and the water column, and efficient mathematical formulations to apply this knowledge in everyday advisory work.

The COSINUS project will yield the following innovations:
• Assessment and quantification of the role of concentrated near bed suspensions on the transport and fate of cohesive sediment in estuarine and coastal zones,
• A formulation of the flocculation process of cohesive sediment in a turbulent environment, and its role on the formation of concentrated near bed suspensions and the structure of the bed,
• An integrated model formulation describing the stress response of the video porno, the erosion and re-entrainment process as a function of cyclical loading (waves) and current,
• A unified formulation for the processes of sedimentation, consolidation, liquefaction, fluidization and deformation of the bed.
• Validation, and where necessary modification, of classical turbulence closure models for application in near bed high concentrated suspensions,
• Development of convenient numerical codes and parameterizations enabling engineering applications of the mathematical models, and
• Guidelines for the execution of laboratory and field experiments and procedures for collecting the optimum data set aimed at studying the occurrence and role of near-bed concentrated suspensions.