Cohesive Sediments in Open Channels: Erosion, Transport and Deposition

By Emmanuel Partheniades

Control the impact of cohesive sediments on open channels by managing the effects of silt, clay and other sediments in harbors, estuaries and reservoirs. Cohesive Sediments in Open Channels provides you with a practical framework for understanding how cohesive sediments are transported, deposited and eroded. One of the first books to approach the subject from an engineering’s perspective, this book supplies insight into applying hydraulic design as well as understanding the behavior of cohesive sediments in a flow field.

• Properties and of the nature and the origin of the interparticle physicochemical forces
• The forces between clay particles and the process of flocculation
• Processes and dynamics of flocculation and the hydrodynamic behavior of cohesive sediments
• Transport processes of sediments by flowing water and related equations are first presented and explained
• Deposition and resuspension of beds deposited from suspension from flowing waters
• Engineering applications of the hydraulics of cohesive sediments

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 23, 2009
• ISBN: 9780080877976

Book preview

Cohesive Sediments in Open Channels

Properties, Transport, and Applications

Emmanuel Partheniades

Brief Table of Contents

Copyright Page

Preface

Acknowledgments

Dedications

Chapter 1. Introduction

Chapter 2. The Mineralogy and the Physicochemical Properties of Cohesive Sediments

Chapter 3. Forces between Clay Particles and the Process of Flocculation

Chapter 4. The Hydrodynamic Transport Processes of Cohesive Sediments and Governing Equations

Chapter 5. Rheological Properties of Cohesive Sediment Suspensions

Chapter 6. Erosion of Cohesive Soils

Chapter 7. Deposition and Resuspension of Cohesive Soils

Chapter 8. Engineering Applications of Cohesive Sediment Dynamics

Index

Table of Contents

Copyright Page

Preface

Acknowledgments

Dedications

Chapter 1. Introduction

1.1. Importance and Distinction of Sediments

1.2. Outline of the Development of Cohesive Sediment Behavior

1.3. Objectives of the Book and Outline of its Contents

Chapter 2. The Mineralogy and the Physicochemical Properties of Cohesive Sediments

2.1. General Properties of Cohesive Sediment Suspensions and of Cohesive Sediment Deposits

2.2. The Bonding Mechanisms

2.2.1. Interatomic or Primary Bonds

2.2.2. Secondary Bonds

2.3. The Nature and Mineralogy of Clay Particles

2.3.1. Introductory Remarks

2.3.2. The Basic Clay Minerals

2.4. Origin and Occurrence of Clay Minerals and Formation of Clay Deposits

Chapter 3. Forces between Clay Particles and the Process of Flocculation

3.1. Introductory Remarks

3.2. The Electric Charge and the Double Layer

3.2.1. Isomorphous Substitution

3.2.2. Preferential Adsorption

3.3. The Theoretical Formulation of the Double Layer

3.3.1. The General Case

3.3.2. Surfaces of Constant Potential

3.3.3. Surfaces of Constant Charge Density

3.3.4. Illustrative Applications

3.4. Interaction of Two Flat Double Layers

3.4.1. Force and Energy Interaction

3.4.2. Illustrative Examples

3.4.3. Potential Energy of Interaction between Two Flat Double Layers

3.4.4. Illustrative Examples

3.4.5. Potential Energy of Interaction of Two Flat Double Layers Due to van der Waals Forces

3.4.6. Total Potential Energy for Two Particles and the Process of Flocculation

3.5. Some Important Properties of Fine Particles and Aggregates

3.5.1. The Counterion Exchange

3.5.2. Limitations of the Gouy-Chapman Theory and the Stern Layer

3.5.3. The Water Phase

3.5.4. Sensitivity and Thixotropy

3.6. Internal Structure and Fabric of Flocs, Aggregates, and Cohesive Sediment Deposits

3.6.1. Particle Arrangements within Flocs

3.6.2. The Microstructure of Deposited Cohesive Sediment Beds

Chapter 4. The Hydrodynamic Transport Processes of Cohesive Sediments and Governing Equations

4.1. The Fundamental Transport Equations for Cohesive Sediments

4.1.1. The Development of the General Transport Equations

4.1.2. Discussion of the Developed Equations

4.2. The Process and Dynamics of Flocculation

4.2.1. Collisions Due to Brownian Motion

4.2.2. Collisions Due to Velocity Gradients

4.2.3. Collisions Due to Differential Settling

4.2.4. Concluding Remarks

4.3. Review of Fundamental Properties of Turbulent Flows

4.3.1. Significant Stresses and Parameters

4.3.2. Collision Rates in Turbulent Flows

4.4. The Properties of the Aggregates and the Aggregate Growth Equation

4.4.1. The Properties of Aggregates and Their Relation to the Controlling Flow Variables

4.4.2. Quasi Steady-State Aggregate Distribution and Maximum Aggregate Size

4.4.3. Some Additional Research Work on Flocculation and Aggregate Properties

4.4.4. Discussion and Concluding Remarks

Chapter 5. Rheological Properties of Cohesive Sediment Suspensions

5.1. Importance of the Subject

5.2. Basic Properties of Sediment Suspensions and Methods of Evaluations

5.3. Concluding Remarks

Chapter 6. Erosion of Cohesive Soils

6.1. Introductory Remarks

6.2. Erosion of Consolidated Cohesive Soils

6.2.1. Early Empirical Information

6.2.2. More Recent Field and Laboratory Studies

6.3. Erosion of Soft Cohesive Sediment Deposits

6.4. Summary and Concluding Remarks

Chapter 7. Deposition and Resuspension of Cohesive Soils

7.1. Deposition of Cohesive Sediments

7.1.1. Early Experiments and Preliminary Conclusions

7.1.2. Detailed Studies on Deposition. Part A: The Degree of Deposition

7.1.3. Detailed Studies on Deposition. Part B: The Rates of Deposition

7.1.4. Variation of Depositional Parameters as the Sediment Sorts during Deposition in Open Conduits

7.2. Hydrodynamic Interaction of Suspended Aggregates with the Deposited Bed

7.3. Resuspension of Deposited Cohesive Sediments

7.3.1. Introductory Remarks

7.3.2. Fundamental Considerations

7.3.3. Experimental Results

7.4. Summary and Closing Comments

Chapter 8. Engineering Applications of Cohesive Sediment Dynamics

8.1. Areas of Application

8.2. Design of Stable Channels

8.2.1. Design for Safety Against Scouring

8.2.2. Design for Safety Against Deposition

8.3. Shoaling in Estuaries

8.3.1. Fine Sediment Transport Processes in Estuaries

8.4. Illustrative Case Histories

8.4.1. The Savannah Estuary

8.4.2. The Delaware River Estuary

8.4.3. The River Thames Estuary

8.4.4. The Maracaibo Estuary

8.4.5. The San Francisco Bay Estuary

8.4.6. Closing Remarks

8.4.7. Applications to Estuarine Modeling

8.5. Control of Environmental Pollution

Index

Copyright Page

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Preface

Cohesive sediments, which consist predominantly of silt and clay with size ranging from a few micrometers to a fraction of a micrometer, enter frequently in several areas within the civil engineering domain as, for example, in soil mechanics and foundations. In the field of water resources, the beds and banks of natural and artificial channels often consist of cohesive sediments subject to erosion. Such channels may also carry such sediments in suspension, and that sediment may eventually deposit in some areas of the canal or in reservoirs. In sanitary and environmental engineering, water purification and sewage treatment involve handling of cohesive sediments. In all the preceding cases and in some others, the physicochemical and colloidal properties of cohesive sediments are of primary importance in their response to external loads, as in the case of soil mechanics and foundations. The same is true in the control of erosion, deposition and transport in open channels, in the maintenance of reservoirs, in water and sewage treatment, and in the control of the environmental quality of natural water systems, such as lakes, rivers, and estuaries.

The primary difference between coarse and cohesive sediments lies in the capacity of the latter to form, under the influence of interparticle attractive forces, agglomerations with size, density, and strength much different than those of the original particles. Moreover, these properties are not even constant, but they are functions of the acting forces during their formation and can even vary with time and quality of pore or ambient water. Therefore, any simplified description and modeling of such sediments based only on some gross quantities without taking into consideration the effect of their intricate physicochemical, colloidal, and mineralogical properties may lead to erroneous results. The need to incorporate these properties into engineering design first became obvious in foundations and various soil mechanics problems. This need motivated extensive research, both fundamental and applied, since the early part of the 20th century, which led to significant interdisciplinary advances for a better and more reliable design of foundations and earth structures and to an estimate of their bearing capacity and settlement under external loading. There is indeed a large volume of publications on this subject.

Application of the same properties in hydraulic design and/or solution of sedimentation problems started much later, predominantly motivated by the design of open channels safe against scouring and deposition and for the control of shoaling in estuaries. Therefore, it is a relatively new field. Related fundamental research originated about 50 to 60 years ago. A large volume of knowledge has already been obtained while active research is still going on in a number of institutions in the world. The current existing knowledge, however, is sufficient for the formulation of a rigorous hydrodynamic framework for the overall hydraulic behavior of these kinds of sediments. This framework can be and should be used as the basic guideline for a rational approach to hydraulic problems involving cohesive sediments, for the planning of the necessary laboratory experiments and field measurements, and also for further research on these subjects.

The objective of this book is to present in a coherent form the entire spectrum of the behavior of cohesive sediments in a flow field, and more specifically in open channels, starting from the process of flocculation and proceeding to the processes of erosion, deposition, resuspension, and transport, always in relation to their physicochemical properties. The main subject is preceded by a brief treatment of the fundamentals of clay mineralogy and clay colloid chemistry for the sake of readers with inadequate background in these fields and as a starting point for those wishing to expend their knowledge with further studies. The subject matter was selected and arranged in a way to contribute to three objectives: first, as an introduction to undergraduate and graduate students of hydraulic, coastal, and environmental engineering; second, as a guideline to practicing engineers; and third, as a starting point and/or an aid for further research. It is hoped that the book will meet all these three goals.

Several people from various engineering and scientific areas have so far contributed to the present state of knowledge of cohesive sediment behavior. The subject matter of the book is based on a selection from work related in some way to its primary objective, which is the presentation of a rigorous framework with direct applications. The first major contribution started by Professor R. B. Krone at the University of California in Berkeley in the 1950s with his work on the effect of flow-induced shear stresses on the density and strength of flocs and floc aggregates and his laboratory and field studies on estuarial sediment transport processes. The early work of the author followed in 1960, also at the University of California in Berkeley, focused primarily on the processes of erosion of dense and deposited cohesive estuarine sediments. Work on the deposition phase continued at MIT by Professor J. F. Kennedy, the author and their graduate research assistant from 1963 to 1966. Their fundamental work was significantly enhanced by simultaneous field research on estuarine shoaling in the Bay of Maracaibo in Venezuela by the same people. A special research apparatus was developed and used in that research phase. The latter was furthermore developed and improved at the University of Florida in Gainesville, in 1968 and 1969, and was used from 1968 to 1983 for studies of deposition and resuspension by Mehta, the author, and a number of graduate assistants. Many researchers, mostly sanitary and coastal specialists, made important contributions to the hydrodynamics of floc formation and on the aggregate properties. The work of Watanabe, Hozumi, Tambo, and Kusuda and his colleagues in Japan is particularly noteworthy from the practical aspect, and their results are incorporated in this book as most directly related to its main theme and objectives. Additional important work of several other researchers is also mentioned and commented even with only indirect relation to the subject matter of the book.

Acknowledgments

The work by the author and his colleagues on the hydraulic behavior of cohesive sediments has been supported by the following agencies: Ford Foundation supported the author in his doctoral studies and his research at the University of California through a special predoctoral scholarship. The latter part of his research was also partly supported by the Corps of Engineers of the U.S. Army in 1961–1962. Ford Foundation also supported part of his research at the Massachusetts Institute of Technology (MIT) from 1963 to 1965 under a postdoctoral fellowship. The same fundamental research and field investigations in the Gulf of Maracaibo in Venezuela were supported by the U.S. Agency for International Development (AID) from 1963 to 1966.

The work at the University of Florida in Gainesville was first supported by the Environmental Protection Agency (EPA) from 1968 to 1970 and from then on by the National Science Foundation until about 1980. Substantial simultaneous support was also provided during that period by the Waterways Experiment Station of the Corps of Engineers in Vicksburg, Mississippi. Finally, the College of Engineering of the University of Florida provided funds for the building of a special room for the housing of the experimental apparatus used for all the fundamental research experiments on the deposition and resuspension of cohesive sediments. All this support, thanks to which the field of cohesive sediment hydrodynamics was brought to its present stage of development, is gratefully acknowledged.

Dedications

The author wishes to respectfully dedicate this book to the memory of the following outstanding professors with whom he had the privilege to be associated and who had a profound influence in his overall academic work.

First, to the memory of Professor H. A. Einstein, founder and one of the most important contributors to Sediment Transport Mechanics, and the PhD thesis supervisor of the author at the University of California at Berkeley.

Second, to the memory of Professor Arthur T. Ippen, director of the Hydraulics Laboratory at MIT during the author’s work there for his encouragement and support of his research and for introducing him to the Estuarine Hydrodynamics and shoaling in estuaries.

Third but not least, to the memory of Knox Millsaps, Chairman of the Department of Engineering Science at the University of Florida from 1974 to 1986 and in which the author has been a faculty member from 1974 to now, for inspirational leadership and his commitment to academic principles and values.

Chapter 1. Introduction

1.1. Importance and Distinction of Sediments

The importance of sediments in hydraulic engineering and, in general, in the technical development of water resources is well known. In rivers the total amount of sediment discharge is the most obvious and direct concern. Sediments also affect the roughness and the frictional resistance of natural waterways, thus raising the question of stage-discharge-sediment transport relationships. The stability of beds and banks against scouring and deposition is another important subject, particularly for manmade canals and waterways. The useful life of reservoirs depends on the sediment load of the contributing natural streams. The extent and frequency of maintenance of navigable waterways in estuaries are determined by the rates of deposition of sediments, particularly fine, by the discharging river or rivers into that specific estuary. Another serious problem in estuaries, bays, and lakes is sediment-induced pollution either by increasing the water turbidity or by depositing highly contaminated sediment on ecologically sensitive zones. There are indeed cases where the environmental damage is so severe that the restoration of the original quality becomes either impossible or extremely difficult and expensive. For all these reasons, sedimentation has been the subject of intensive fundamental, applied, and field research since the 19th century and many theories, empirical formulas, and semitheoretical equations have been developed for the prediction of sediment transport rates and the control of channel stability.

Sediments have been distinguished into two broad classes: coarse or cohesionless and fine or cohesive. The division has been arbitrarily placed on the grain size distribution. The first class refers to sediments ranging from fine sand to coarse gravel, whereas the second contains silt and clay. It has been observed that in river flows coarse sediments in transport are represented in appreciable quantities in the bed and that their rates of transport are functions of the flow conditions. In contrast, fine sediments are encountered in the bed in only very small quantities in proportion to their total load, and their transport rates appear to be unrelated to the flow parameters and to depend only on their supply rates [25, 26]. The boundary between the two sediment classes has been established at 50 μm. It should be noticed at this point that even the finest sediments eventually deposit under sufficiently low bed shear stresses and, therefore, at some stage their transport and deposition rates have to depend on the flow parameters.

The major difference between coarse and fine sediments lies not so much in their size but primarily in the mutual interaction of grains in a water environment. Coarse grains in suspension behave independently from each other, with the exception of mechanical interaction in highly dense suspensions, while as a bed material only forces of interlocking and friction enter into the picture. The settling unit in the sediment transported in suspension and the eroded unit from the coarse bed is the individual sediment unit; therefore, the sediment can be introduced in the relationships describing either the bed stability or sediment transport rates by a representative grain size distribution or by an equivalent distribution of settling velocities.

Cohesive sediment grains, which range in size from 50 μm to a small fraction of 1 μm, are subjected to a set of attractive and repulsive forces of an electrochemical and atomic nature acting on their surfaces and within their mass. These forces are the result of the mineralogical properties of the sediment and of the adsorption of ions on the particle surfaces. The fine sediment grains have in general a flat plate or a needle shape and a high specific area, which is a high surface to volume ratio, so that the total magnitude of the surface forces becomes dominant in comparison to the submerged weight of the particle. Dispersed particles have such a low settling velocity that the finer portion of them can stay in suspension almost indefinitely, whereas even a slight degree of agitation is sufficient to keep the coarser part of them in suspension. When, under certain conditions, the attractive forces exceed the repulsive ones, colliding particles stick together, forming agglomerations known as flocs with size and settling velocities much higher than those of the individual particles. Rapid deposition may then take place. This phenomenon is known as flocculation. In a flocculated cohesive sediment suspension, the settling unit is the floc rather than the individual particle. The same physicochemical forces are responsible for certain properties of consolidated cohesive sediment deposits, such as cohesive strength and plasticity. Flocs join together to form floc aggregates of various orders of magnitude. In a quiescent water environment, the Brownian motion of the water particles provides the only mechanism for interparticle and interaggregate collision. In flowing waters the shear rates and the turbulent velocity fluctuations affect the collision frequency to a much greater extent than the Brownian motion, so that a much higher rate of aggregate formation is expected. At the same time, however, the same forces induce disrupting stresses within the aggregates, thus limiting their maximum size and controlling their basic properties. A quasi steady-state aggregate size distribution is reached, which is a function of the flow parameters themselves. Settling units develop similar bonds with the cohesive bed that have to be broken for the units to be resuspended.

These surface forces constitute the enormous difference between coarse and fine cohesive sediments. The flow conditions, which control the degree and rates of deposition, also determine the size distribution and the important properties of the aggregates. The deposited cohesive bed is composed of flocs and/or higher order aggregates whose properties have been molded by the flow-induced stresses. Therefore, their erosional resistance and the rates of resuspension and erosion as well as their gross mechanical properties are expected to also be functions of the flow conditions.

The same physicochemical forces may attract other suspended matter in the water, such as organic and inorganic toxic substances, so that a highly contaminated bed is formed after deposition. Upon resuspension the polluted sediment may contaminate the entire water environ with detrimental consequences to the aquatic life. To make the situation more complicated, the surface interparticle forces are not even constant, but they may change drastically with small changes in the water quality, temperature, and time. At first glance this interdependence of flow, water quality, aggregate properties, deposition, and erosion gives the impression that a quantitative description of the hydraulic behavior of cohesive sediments constitutes an insurmountable problem. Fortunately, extensive fundamental and applied research, particularly since 1950, has led to a much better understanding of the dynamics of cohesive sediment behavior in a turbulent flow field. Quantitative equations have been developed for the initiation, degree and rates of deposition, erosion, and resuspension in terms of readily determinable flow variables and parameters representing the overall effect of the interparticle physicochemical forces. These relationships and an understanding of the dynamics of floc formation and of the processes of erosion, deposition, resuspension, and transport of cohesive sediments supplemented with some simple laboratory tests and field measurements may lead through mathematical and physical models to reasonable answers to problems involving cohesive sediments. The terms fines and cohesive sediments have been used in literature meaning essentially the same thing. Both terms will also be used alternatively in this book.

1.2. Outline of the Development of Cohesive Sediment Behavior

Because of the outlined importance of sedimentation, the hydraulic behavior of sediments has been a subject of concern and investigation since the inception of the field of hydraulic engineering. In fact studies on sedimentation followed closely the developments in that field and in fluid mechanics in general. A good summary of the historic development of sediment transport mechanics was given Graf [37]. Like many engineering disciplines, hydraulics in general and sedimentation in particular started from pure empiricism and gradually proceeded together with advances in fluid mechanics to more fundamental and universal relationships. This is particularly true for cohesive sediments, whose behavior is much more complicated than that of cohesionless soils.

The development of cohesive sediment hydraulics is summarized and discussed in Chapter 6, which deals with the mechanics of erosion of these sediments. An earlier analysis and discussion was presented by Paaswell and the author [92, 93, 116, 117]. Only a brief outline of representative examples of the evolution of the hydraulics of cohesive sediments will be given here.

The first and earliest phase of the subject consisted in establishing guidelines for the design of stable canals through empirical formulas and/or tables for limiting velocities as the only criterion. The soil properties were described by a mere classification or, at most, by some measure of their density. For instance, in the table of critical velocities recommended by Schoklitsch in 1914, the soil was described only by its type and the degree of compaction [125, Vol. I, p. 232]. Similar critical velocities were given by Etcheverry in 1916, also based on a very general soil classification [30]. The average velocity continued to be used as the stability criterion into the early part of the 20th century in spite of the fact that, as early as in 1816, Du Buat introduced for the first time as a criterion of sediment transport the concept of shear resistance, that is, essentially the force per unit area of the bed of the stream [19]. This was, in fact, the greatest contribution of Du Buat to the field of sediment transport. A similar concept of tractive force or bed shear stress was introduced in 1879 by Du Boys [18]. Still, however, in 1926 the Special Committee on Irrigation Hydraulics presented estimates of experienced irrigation engineers for critical design velocities reported by Fortier and Scobey again on the basis of soil classification [33]. It was only in 1955 that Lane reported data by Russian engineers giving both critical velocities and critical shear stresses for channels with cohesive boundaries and of various densities [70].

Another school of approach within that first phase is known as the regime theories. These theories aimed at the development of empirical formulas for velocities for the design of channels in specific areas on the basis of extensive and numerous field data on canals which exhibited various degrees of stability. No laws of mechanics were introduced in the derivation nor were any specific soil data reported. Kennedy’s work on irrigation canals in India is representative of this school of thought. It was presented in 1895, and the channel depth was the only parameter representative of the channel geometry [55]). In 1959 Leliavsky reported later investigations introducing, in addition to the depth of flow, some other variables describing the channel geometry and the boundary resistance [73]. Kennedy’s formulas for stable canal design do not specify whether they imply safety against scouring or shoaling. An analysis in Chapter 6, though, indicated that these formulas and rules really apply for safety against deposition or siltation. Like almost all empirical laws and formulas, these early representative results and criteria may be valid for soil types and soil properties and for general environmental conditions very similar to those they were based on. Otherwise, the results of their application may be erroneous. This is particularly true in the case of cohesive sediments in which even apparently minor changes in one or more aspects may drastically affect their erosional and depositional characteristics.

The realization that the flow-induced shear stresses on the bed rather than the average flow velocity is the controlling flow variable for channel stability and the recognition of the related importance of soil properties in addition to its composition and density led to the second phase of cohesive sediment research in both the field and in the laboratory. In all studies the objective was to relate the critical bed shear stresses, also referred to as critical tractive forces, to some soil mechanics parameters representative of the soil structure and the gross shear strength.

Field investigations conducted by the U.S. Bureau of Reclamation presented in 1953 revealed little correlation between critical tractive force for erosion and mean grain size. The same studies indicated a strong effect of some external factors, such as desiccation, on the critical tractive force [148]. At about the same time, the field studies by Sundborg on the Klarälven river suggested that the critical velocity decreases with decreasing particle size down to the silt range of 50 μm, but it increases as the sediment becomes finer [130]. This is the limit below which the interparticle physicochemical forces start causing flocculation.

These field investigations may supply valuable data on erosive and depositional trends, but seldom lead to a basic understanding of the particular process, much less to formulations of equations of general validity. The laboratory research of this second phase aimed at the derivation of experimental relationships linking a critical tractive force or boundary shear stress to some readily determinable soil parameters representative of the macroscopic shear strength and other mechanical properties of the soils. The mechanism of erosion and the details of bed structure were not considered. The work by Dunn in 1959 [20], of Smerdon and Beasley in the same year [128], of Moore and Mash in 1962 [90], of Espey in 1963 [29], of Flaxman in 1965 [32], of Berghager and Ladd in 1964 [5], and of Grissinger in 1966 [40] are representative examples of this kind of research effort. In 1966 the Task Committee on Cohesive Sediments of the ASCE published an annotated bibliography on the subject containing the results and conclusions of several other laboratory studies [136].

Most of these studies utilized small, improvised experimental setups, such as cylinders within which an interior cylindrical sample of the soil was subjected to shear by rotation, or jets impinging on a soil sample or relatively small samples of cohesive sediments placed over a section of an open flume. Some other researchers, like Smerdon and Beasley [128] and Abdel-Rahman in 1962 [1] did use a cohesive bed over the entire length of an open flume. The latter work, though, overlaps the third phase.

The experimental research of the second phase constitutes an important and indispensable part of the overall research effort in cohesive sediment dynamics. Its direct contribution, however, has been limited by two facts. First, because of their shape and small size, some of the experimental devices do not generate a flow field similar to that in open channels. As a result, the boundary stresses on the sample and the flow structure near the wall may deviate substantially from that in real conduits. In addition, the difference in the shape of the equipment with the associated disparity of boundary conditions makes any comparison between the results of various investigators even more difficult. Nevertheless, these early laboratory experiments on cohesive sediments led to some important conclusions. It was made clear that neither the shear strength nor the Atterberg limits could be used as a unique parameter for cohesive soil erodibility. Indeed, samples with comparable strength determined by any standard test used in soil mechanics, and/or comparable Atterberg limits, were found to display erosive resistance differing by orders of magnitude.

The third and more detailed phase started from about 1950 and was motivated by the need for a rational control of shoaling in navigable waterways. The erosion and deposition processes were studied in conjunction with flow structure, the dynamics of flocculation, and the interaction between settling suspended flocs and the bed. For the first time the erosion and deposition rates were introduced in addition to the critical limits. Equations have been derived for the initiation, degree, and rates of erosion and deposition of generalized validity, which, used with the appropriate evaluation of certain parameters through field measurements and/or laboratory tests, may lead to reliable estimates for erosion, deposition, and resuspension. This phase started with the pioneering laboratory and field work of Krone on the relationship of floc properties to the flow-induced stresses as well as on the shoaling processes in the San Francisco Bay [61–64]. Studies, primarily on the erosion of dense and deposited cohesive sediment beds, were initiated by the author in 1960 [102, 105]. This work was followed by fundamental studies on erosion, deposition, and resuspension by the author, Kennedy, Mehta, their associates and graduate students, as well as by others along similar lines of approach for several years and are still being continued [16, 31, 81–86, 94, 95, 99–101, 103, 104, 107–109, 112–115, 118, 154, 155]. Parallel research work was developed about the effect of flow-induced stresses on the properties of flocs and higher order aggregates. Many of these studies were conducted by sanitary engineers in their effort to improve the efficiency of water purification and sewage treatment. Their results, though, are equally applicable to any cohesive sediment suspension. A fundamental framework for cohesive sediment dynamics has been thus developed and formulated [101]. This framework can be used as a guideline for rational approaches to problems involving cohesive sediments. It can also serve as a basis for future research in the field of cohesive sediment dynamics.

The three outlined phases of cohesive sediment research do overlap, and in fact some of them interact. However, they define three distinct philosophies of approach and the various steps such a complicated subject has gone through to its present state of the art.

1.3. Objectives of the Book and Outline of its Contents

The objective of this book is to present in a unified framework current fundamental and applied knowledge on the hydraulic behavior of cohesive sediments in a turbulent flow field and specifically in open channels. It is based on extensive theoretical and laboratory research and on field investigations over the second half of the 20th century. A number of scientists and engineers from various specialties have contributed to the present state of knowledge on this subject. The main emphasis is focused on the processes of erosion, transport, deposition, and resuspension of cohesive sediments, and it addresses the following important and frequently encountered problems in water resources projects:

Erosion of natural and manmade canals with cohesive beds and/or banks and ways to stabilize them.

Control of deposition of fine sediments in suspension in canals and prevention of shoaling.

Fine sediment transport processes in tidal estuaries and bays and the relation of these processes to the salinity and the overall regimen of the estuary with the ultimate objective of control of shoaling in navigable waterways.

Control of sediment deposition in reservoirs.

Design of sedimentation basins in water purification and sewage treatment plants for optimum performance.

Prediction and control of turbidity in river and estuarine waters that may have undesirable effects on the marine life.

Proper planning of dredging and filling operations in estuaries and bays to avoid sediment pollution by spreading contaminated fine sediments in ecologically sensitive areas.

For a rational approach and a successful design of related operations and structures, the behavior of cohesive sediments in a flow field and the basic physicochemical properties of the sediments, which control and determine that behavior, have to be understood. Following are some typical, major questions involved in the problems and the kind of engineering operations listed previously:

Under which conditions and water quality do suspended fines flocculate?

What are the relevant properties of the flocs and of higher order aggregates, such as density, strength, and settling velocities, and how do they relate to the erosional and depositional processes and criteria?

Which hydraulic parameters determine the critical flow conditions for scouring and siltation in open channels?

Which sediment properties determine the erodibility and/or the zones of potential shoaling and what kind(s) of test(s) would be representative of these properties?

Is the erosive resistance of a cohesive bed related to the gross properties of the soil, such as the macroscopic shear strength and Atterberg limits, and how? One may recall from the brief outline of the second phase of development in the previous section that any correlation to such soil parameters may lead to erroneous results.

Are the sediment properties implied in question 4 constant, or do they change with time and/or environmental conditions and how?

The fundamental and applied research in the past 40 to 50 years was planned and conducted to provide guidelines, analytical principles, and suggestions for laboratory tests and field measurements as a basis for rational answers to these problems and questions. The model of the hydrodynamic behavior of cohesive sediments in a flow field to be presented in this book is based on the results of extensive fundamental and applied research as well as field investigations from 1950 to about the present time by Krone, the author, Mehta, their numerous collaborators and associates, and several others. This book is not meant to be a compilation of all the work related to cohesive sediment behavior. Work even remotely related to the main theme of the book has been analyzed, commented, and integrated in the overall picture. However, work either inconclusive or unrelated or far removed from the primary objectives outlined above is not included. This does not mean that the omitted work is unimportant by any means.