J.C. Winterwerp

On the flocculation and settling velocity of estuarine mud

Abstract This paper presents a three-dimensional model in an Eulerian reference frame for the evolution of the settling velocity of cohesive sediments in estuarine and coastal environments. The variation in settling velocity is the result of turbulence-induced aggregation and floc breakup processes. The mud flocs are treated as self-similar fractal entities because of which Stokes’ formula relating settling velocity and floc size has to be modified. A new hindered settling formula is proposed which accounts for the gelling processes typical of cohesive sediment at high concentrations. When the floc concentration approaches unity, a space-filling network develops and the settling velocity becomes zero. This state of the suspension is often referred to as fluid mud. The flocculation model is implemented in a 1 dv point model and applied to simulate the processes in a reach of the turbidity maximum in the Ems estuary, The Netherlands, where longitudinal gradients are small. The numerical results compare favourably with observed values of the vertical suspended sediment concentration, of the floc size and the fluid mud concentration. The model predicts large temporal variations in settling velocity and flocculation time, which explains why it was not possible to simulate the observed vertical profiles of suspended sediment concentration properly with a constant settling velocity.

The characterisation of cohesive sediment properties

Abstract This paper describes apparatus, techniques and methods used by participants in MAST G6M project 4 (Cohesive Sediments) for determining cohesive sediment properties. This comparison of methods aims to stimulate a more general discussion on standardisation of techniques which will lead to characterisation of muds in terms of physical parameters. Such characterisation would allow inter-comparison of muds from different sources. Methods are given for sediment properties of grain size distribution, settling velocity and rheological parameters and for water-bed exchange properties of permeability, effective stress and critical shear stress for erosion and deposition. Accuracy and repeatability are discussed. A typical range of values is indicated for each of these parameters.

Shear-induced flocculation of a suspension of kaolinite as function of pH and salt concentration.

The relation between the electrokinetic charge of kaolinite particles and their flocculation behavior has been investigated over a wide range of pH and added salt (for MgCl(2) and NaCl salts). All flocculation experiments have been done with a mixing jar (sediment volume concentration phi=3.84x10(-5)). The electrokinetic charge of particles in different suspensions has been assessed by electrophoresis while laser diffraction has been used to measure the floc size distribution. Mixing jar experiments can be successfully used to investigate the flocculation behavior of kaolinite at shear rates higher than or equal to G=35 s(-1), which is the shear rate used in the experiments. At lower shear rates, the floc size distribution is affected by particle settling. The electrophoretic mobility of kaolinite decreases in absolute value when the pH of the suspension decreases. This is reflected in an increase of both floc size and flocculation rate: the floc size at pH 4 is three times larger than at pH 7 and the flocculation time is one order of magnitude smaller (from 1000 to 100 min). When the ionic strength of the suspension is increased, the electrophoretic mobility and the mean floc size display the same variations. On addition of NaCl (pH 9) both the electrophoretic mobility and the floc size display an optimum around 1 mM of added salt, a feature that has been observed by other authors as well. The equilibrium floc size for a suspension (A) at 1 M of added NaCl and pH 9 is the same as for a suspension (B) at pH 2 with no added salt. However, the time needed to reach the equilibrium for suspension (A) is one order of magnitude larger than for suspension (B). This is due to edge-face Coulombic attraction in suspension (B). The equilibrium floc size obtained by addition of MgCl(2) or sea salt at pH 9 is similar to the size obtained by addition of NaCl. The flocculation rate for a suspension with added MgCl(2) is higher than for suspensions with other added salts.

Pilot Study on the Erosion and Rehabilitation of a Mangrove Mud Coast

Abstract This pilot study describes an analysis of the erosion processes of the Bang Khun Thien mangrove mud coast situated at the Upper Gulf of Thailand, and discusses measures to stop this erosion process and to rehabilitate the area. The rapid erosion observed is the result of the decimation in intertidal area by the dikes directly behind the coastline, constructed to protect the fish and shrimp ponds in the coastal area. The decrease in sediment yield from the Chao Phraya River as a result of the construction of the Bhumipol and Sirikit dams, and the local subsidence due to ground water withdrawal and natural settling will augment the observed coastal erosion, but at a much smaller rate. The latter effects are expected to become important on a time scale of about a hundred years only. The key to stop the erosion processes and rehabilitate the area is therefore the restoration of the intertidal area. This can be done either entirely within the current coastline, but by sacrificing part of the fish and shrimp ponds, or partly within the current coastline and partly in the coastal zone, i.e. in the coastal area recently lost. In the latter case, the coastal area has to be protected from lateral transport of sediment by permeable groynes perpendicular to the coast. It is estimated that an intertidal mangrove belt of about 300 to 500 m is required to re-initiate sedimentation processes, hence to restore a favourable habitat for mangrove forest.

Electrokinetic study of kaolinite suspensions

In this article a new formula for the electrophoretic mobility of a spheroidal colloid is given. This formula and the formula presented in Chassagne and Bedeaux [C. Chassagne, D. Bedeaux, J. Colloid Interface Sci. 326 (2008) 240-253] for the dipolar coefficient of a spheroidal colloid are intertwined. The combination of electrophoresis and complex conductivity measurements (from which the dipolar coefficient can be derived) allows to assess both the zeta potential and the Stern layer conductance. We will in particular show that the values found for the zeta potential from both techniques are similar in the case of the kaolinite suspension studied. Electrophoretic mobility data are also presented and discussed for a wide range of ionic strengths, different types of salt and various pH. This data has been used in flocculation studies on the same kaolinite samples [F. Mietta, C. Chassagne, J.C. Winterwerp, J. Colloid Interface Sci., accepted for publication, doi:10.1016/j.jcis.2009.03.044].

Man-induced regime shifts in small estuaries—II: a comparison of rivers

This is Part II of two papers on man-induced regime shifts in small, narrow, and converging estuaries, with focus on the interaction between effective hydraulic drag, fine sediment import, and tidal amplification, induced by river engineering works, e.g., narrowing and deepening. Paper I describes a simple linear analytical model for the tidal movement in narrow, converging estuaries and a conceptual model on the response of tidal rivers to river engineering works. It is argued that such engineering works may set in motion a snowball effect bringing the river into an alternative steady state. Part II analyses the historic development in tidal range in four rivers, e.g., the Elbe, Ems, Loire, and Scheldt, all in northwest Europe; data are available for many decades, up to a century. We use the analytical model derived in Part I, showing that the effective hydraulic drag in the Ems and Loire has decreased considerably over time, as anticipated in Part I. We did not find evidence that the Upper Sea Scheldt is close to its tipping point towards hyperturbid conditions, but risks have been identified. In the Elbe, tidal reflections against the profound step in bed level around Hamburg seem to have affected the tidal evolution in the last decades. It is emphasized that the conceptual picture sketched in these papers is still hypothetical and needs to be validated, for instance through hind-cast modeling of the evolution of these rivers. This will not be an easy task, as historical data for a proper calibration of the models required are scarce.

Man-induced regime shifts in small estuaries—I: theory

This is Part I of two papers on man-induced regime shifts in small, narrow, and converging estuaries, with focus on the interaction between effective hydraulic drag, fine sediment import, and tidal amplification, induced by river engineering works, e.g., narrowing and deepening. In this part, a simple linear analytical model is derived, solving the linearized shallow water equations in exponentially converging tidal rivers. Distinguishing reflecting and non-reflecting conditions, a non-dimensional dispersion equation is derived which yields the real and imaginary wave numbers as a function of the estuarine convergence number and effective hydraulic drag. The estuarine convergence number describes the major geometrical features of a tidal river, e.g., intertidal area, convergence length, and water depth. This model is used in Part II analyzing the historical development of the tide in four rivers. Part I also presents a conceptual model on the response of tidal rivers to narrowing and deepening. It is argued that, upon the loss of intertidal area, flood-dominant conditions prevail, upon which fine sediments are pumped into the river, reducing its effective hydraulic drag. Then a snowball effect may be initiated, bringing the river into a hyper-turbid state. This state is self-maintaining because of entrainment processes, and favorable from an energetic point of view, and therefore highly stable. We may refer to an alternative steady state.

A stochastic formulation for erosion of cohesive sediments

The linear formulation for erosion E = M(?b??c), often applied in engineering applications, has two properties, which do not always comply with field and laboratory observations, they are as follows: (1) The erosion rate is zero below the critical bed shear stress ?c and increases linearly with bed shear stress ?b, when exceeding the critical bed shear stress; incipient motion (?b

A conceptual framework for shear flow–induced erosion of soft cohesive sediment beds

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Settling velocities of fine suspended particles in the Changjiang Estuary, China

?c) is poorly represented. (2) The erosion rate is constant in time for constant values of M and ?c, whereas observations often suggest time dependency. In this paper we analyze the process of incipient motion and time dependency by using a stochastic forcing (bed shear stress) and a stochastic bed strength (critical bed shear stress). It is well known that the bed shear stress is not constant but varies due to turbulence. This stochastic nature of the turbulent motion is accounted for by a probability density distribution for the bed shear stress, which is based on the formulation of Hofland and Battjes (2006). This distribution is implemented in the linear erosion formulation. An analytical solution for the erosion rate is obtained, which only depends on the mean bed shear stress. A parametrization is made for efficient application in numerical models. The sediment in the bed is considered to be nonuniform. Therefore, it is subdivided into several classes, distinguished by the critical bed shear stress and not necessarily by the grain size. The variability of the critical bed shear stress is treated in a discritized way. Sediment balance equations are solved for each class. Considering different classes, the total erosion rate becomes time dependent, as the erosion depends on the availability of sediment. The model is applied to two annular flume data sets, Jacobs (2009) and Amos et al. (1992a). The results show that with a proper choice of the required parameters, the time dependence of the erosion rate and the concentration can be reproduced. We conclude that the occurrence of incipient motion can be explained from a stochastic forcing. Time?varying erosion rates can be explained from a stochastic bed strength distribution or from a vertical gradient in bed strength. The latter is, however, not likely and not measurable in the top layers of dense consolidated cohesive sediment beds.