2.15 – The Physical Analyses of Muddy Sedimentation Processes

• J.C. WinterwerpAuthor Vitae

• Deltares & Delft University of Technology, Delft, The Netherlands

Available online 20 March 2012

20 March 2012

Current as of 25 March 2013

25 March 2013

Reviewed 25 March 2013

Keywords

• Bed properties;
• Buoyancy destruction;
• Consolidation;
• Erosion;
• Flocculation;
• Fluid mud;
• Sedimentation;
• (Hindered) settling velocity;
• Wave damping

2.15.1. Introduction

This chapter focuses on the physical aspects of sedimentation processes for fine sediments in estuaries and coastal areas, with emphasis on cohesive sediments. Although the treatment is largely physical, it is recognized that biology has an important role to play in many aspects of fine-sediment behavior. For example, flocculation is modified by the presence of sticky organic material, which is often referred to as extracellular polymer substances (EPSs – mainly polysaccharides), although some other substances are important as well. These substances are secreted by microphytobenthos (algae) and bacteria. Currently, biological effects can only be taken into account from a predictive, modeling viewpoint by modifying the various parameters that arise in the physical descriptions presented here. The reader is referred to, for example, Le Hir et al. (2007) for an overview. The first part of this chapter presents a classification of fine sediments and summarizes relevant processes in the water column and sediment bed, whereas the last part discusses the formation and behavior of fluid mud.

Estuarine and coastal systems are very shallow in general: horizontal scales (length and width) exceed the water depth by many orders of magnitude, as shown in Figure 1. The sedimentary features in such systems are therefore largely governed by exchange processes at the water–bed interface. Yet, in spite of this shallowness, these exchange processes are often significantly affected by three-dimensional structures in the water column and within the sediment bed. This chapter, therefore, focuses on the processes at the water–bed interface, said three-dimensional structures, and their effect on sedimentation processes, with emphasis on the fine, cohesive fraction of the sediments.

It should further be acknowledged that sediments in estuarine and coastal systems generally consist of mixtures of clays, silts, sand, organic material (alive or dead), often a lot of (pore) water, and sometimes gas (methane and carbon dioxide). The composition of these mixtures may vary in space and time (seasonal effects!), and biology may affect the properties of these mixtures greatly – the latter being treated extensively in Chapter 2.14.

This chapter is further confined by its focus on the shorter timescales (i.e., a spring–neap cycle at most). We treat sedimentation processes as a function of local (hydrodynamic) forcing, the history of that forcing and the accompanied deformations, and the availability of sediments. The important feedback between sedimentation processes and bed geometry, the discipline of geomorphology, is treated in Volume 3, whereas the underlying net transport rates, for instance, as a function of tidal asymmetry, gravitation circulation, etc., are treated in Chapter 2.17.

The sedimentation processes discussed in this chapter cannot be understood properly without a thorough understanding of the driving hydrodynamics in the estuary and/or coastal system. In particular, we refer frequently to the dynamics and properties of (tidal) flow; waves; current–wave interaction; stratification and density currents induced by temperature, salinity, and/or suspended sediment; turbulence and mixing; and the structure of the (turbulent) boundary layer. These dynamics are discussed elsewhere in this treatise (See Section 2.15.4, 2.15.5 and 2.15.7). However, hydrodynamic properties may alter when the suspension evolves from a dilute water–sediment mixture, via a high-concentrated water–sediment mixture, to a soil. A description of these changes requires some background on the so-called non-Newtonian behavior and soil mechanical theory – these are provided within this chapter, when relevant.

In our treatise on sedimentation processes, we follow a hypothetical tidal cycle, possibly modulated by episodic events, describing sediment behavior in the water column, within the bed, and at the water–bed interface. As this chapter focuses on sedimentation processes, we start our cycle in the water column, though we appreciate that most sediment is found in the sediment bed.

In Figure 2 and Table 1, we summarize the various definitions on sediment concentration used in this chapter. We distinguish between mass and volume concentration, mass and volume fractions, and use a number of soil mechanical parameters, such as water content. Furthermore, we use the following superscripts to distinguish between the various mineral constituents: cl, clay; si, silt; sa, sand; and m, mud, in which the mineral content of mud consists of a mixture of clay and silt. Note that more generally, mud is defined as a mixture of minerals (clays and silt, some fine sand), organic matter (alive or dead), a lot of water, and sometimes gas. In addition, we use the following subscripts: s, solids; w, water; f, floc; p, primary particle; 0, neutral conditions; ref, reference conditions; and e, equilibrium conditions.