Fluvial Forms and Processes

A New Perspective

David Knighton

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781134666614
• Edition: 2
• Topic: Naturwissenschaften
• Subtopic: Geographie

David Knighton's best-selling book looks at the wide range of forms developed by natural rivers and the processes responsible for that development. The book combines empirical and theoretical approaches, and provides a critical assessment of the many schools of thought which have emerged for dealing with adjustment in the fluvial system. It is fully illustrated throughout by a superb range of figures, photographs and tables.Starting with the network scale, the book examines the interaction of hillslopes, drainage networks and channels, and goes on to considerations of catchment hydrology and catchment denudation. Fluvial processes are analysed in detail, from the mechanics of flow to sediment transport and deposition. Detailing the major components of river channels, the book examines the nature of river adjustment, particularly with respect to equilibrium concepts, and concludes with a look at channel changes through time, affected by flood discharges, climatic change and human activities.

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1

INTRODUCTION

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Overpopulation is the most serious problem facing the world today, with too many people making too many demands on a natural environment which is increasingly under stress. Rivers are a fundamental part of that environment. Their behaviour is of interest to a wide variety of concerns, ranging from water supply, navigation and power generation, to recreation and aesthetics. One of their main attractions to human populations is their unidirectional water flow, which provides a continuously renewable resource, a rapid removal system for unwanted substances, and a valuable source of energy. Such features are also of vital importance to the vast range of aquatic plants and animals inhabiting the fluvial ecosystem (Boon, 1992). In addition to their resource associations, rivers represent a potential threat to human populations and property through floods, drought, pollution and erosion. Whether as resource or hazard, rivers have political, social and economic as well as physical relevance. Most nations depend on sustainable river management.

Rivers are essentially agents of erosion and transportation, removing the water and sediment supplied to them from the land surface to the oceans. They provide the routeways that carry excess precipitation to the oceanic store, thereby completing the global hydrological cycle. Despite the fact that less than 0.005 per cent of continental water is stored in rivers at any one time, water flow is one of the most potent forces operating on the Earth’s surface, in terms of both the total energy expended and the total amount of debris transported. Rivers transport on average about 19 000 million tonnes of material each year, 80 per cent in solid and 20 per cent in dissolved form (Meybeck, 1979; Milliman and Meade, 1983; Walling, 1987). In performing their erosional and transportational work, rivers have developed and continue to develop a wide range of network and channel forms. The character of those forms relative to the underlying processes at work in rivers is the principal concern of this book.

Rivers usually have well-defined spatial boundaries and can usefully be regarded as open systems in which energy and matter are exchanged with an external environment. The character and behaviour of the fluvial system at any particular location reflect the integrated effect of a set of upstream controls, notably climate, geology, land use and basin physiography, which together determine the hydrologic regime and the quantity and type of sediment supplied (Fig. 1.1). Downstream controls such as the base-level are also important. Climate is of primary significance in that it provides the energy for the most important processes and, in combination with vegetation, directly influences basin hydrology and rates of erosion. Geology is less easily quantified but can have far-reaching effects at a variety of scales, particularly in constraining the nature and level of fluvial activity. In addition to these natural controls, human activities are becoming increasingly influential through river regulation and changing patterns of land use. There are already 36 000 dams worldwide and more than 200 large dams are completed each year (Gregory, 1995). Destruction of tropical forests is proceeding at a rate that could result in their virtual elimination within 40 years (Myers, 1989). Whether directly or indirectly, human interference with the physical environment commonly has an impact on rivers. They are, after all, the drains of the Earth’s surface.

Figure 1.1 Interrelationships in the fluvial system. Relationships are indicated as direct (+) or inverse (−). Arrows indicate the direction of influence.

Rivers are dynamic entities whose characteristics vary over time and space with changes in environmental controls. Large shifts in climatic conditions over the last 20 000 years have significantly affected levels of fluvial activity in most parts of the world, although, given the sensitivity of alluvial channels, relatively modest climatic changes can trigger major episodes of fluvial adjustment (Knox, 1993, 1995). Consequently, global warming scenarios that predict an increased frequency of heavy rainfall and rising sea-level could induce widespread channel instability. Rivers in different environments are likely to be differentially affected. A major distinction is traditionally drawn between humid-and arid-zone rivers (Knighton and Nanson, 1997; Fig. 3.6, p. 79), with the latter being potentially more sensitive to changing rainfall patterns. Channels also become appreciably more responsive as the boundary sediment decreases in size from boulders to sand.

Channelled flow occurs over a large range of spatial scales, from small headwater streams to major rivers. Within a moderate-size drainage basin, a hierarchy of fluvial forms can be visualized: from the drainage network (>10⁵ m) to the stream length (e.g. series of meanders), channel reach (e.g. single meander), channel unit (e.g. pool, riffle), subunit (e.g. point bar) and individual particle (<10⁻¹ m) scales. Perceptions of form-process relationships change, in proceeding from one scale to the next, even though ultimately all adjustment is the result of individual particle movements. All of these scales feature in this book.

The choice of an appropriate timescale has long been a source of debate among geomorphologists, as it influences our conception of equilibrium within streams, the relationship of cause and effect, and the significance attached to the magnitude-frequency characteristics of process action. In considering the conflict between short-term equilibrium and long-term evolution, Schumm and Lichty (1965) suggested a tripartite division into cyclic, graded and steady times, with corresponding periods of about 10⁴⁺, 10² and 10⁰ years. Progressive change over cyclic time is seen during the shorter span of graded time as a series of fluctuations about a mean state, underlying which are the day-to-day variations in streamflow when channel forms are essentially determined and independent. Over the intermediate or graded timescale and corresponding to that mean state, equilibrium channel forms may be expected to develop, adjusted to the average discharge of water and sediment delivered from the upstream catchment, and dependent on the valley characteristics inherited from the longer time period. Form-process relationships at this timescale are a primary consideration here.

Strong links are apparent between spatial and temporal scales of investigation. In many ways the approaches adopted by various groups differ in their choice of scale and in the significance given to change over time. At one end of the spectrum are mathematicians and physicists concerned with the development of rational theory with which to explain the detailed mechanics of flowing water at small spatial and temporal scales. The more pragmatic engineering approach has focused on stream behaviour over relatively short timespans of 10–100 years or less, with the sediment transport problem as a major concern, especially as it relates to the design of stable channels. To this end the Anglo-lndian school of engineers developed a set of semi-empirical equations, known collectively as ‘regime theory’, which were intended originally for use in the construction of stable irrigation canals.

Geomorphologists, at least in the first half of this century, have been more interested in long-term, large-scale landform development and in the reconstruction of events assumed to have led to the present forms, an approach owing much to the influence of W. M. Davis’s cycle of erosion. Dissatisfaction with the levels of explanation achieved by this approach in the period after 1945 led to a greater concern with the action of contemporary processes and their relationship to form. With the rapid expansion in the use of statistical techniques, functional relations were sought between form and process variables. The emphasis thus shifted from the broad temporal and spatial scales of the denudation chronologist to the short time and small space scales more familiar to the engineer. Although a specific concern with river channel form and process has been the major outcome of these changes in approach, the historical element remains an important part of the geomorphological perspective (Schumm, 1977).

No one approach could be successful in describing and explaining all aspects of natural river systems. A major distinction can be drawn between empirical and theoretical approaches. The first chiefly involves the collection and analysis of field data in order to establish relationships between form variables, or between a form variable and factors summarizing some aspect of process. Paramount among the large range of relationships which could be used has been the power function:

y = αx^β (1.1)

where y is a variable dependent on x, and α and β are coefficients to be determined (Fig. 1.2). Nowhere is this more apparent than in the ‘hydraulic geometry’ approach pioneered by Leopold and Maddock (1953) as a basis for analysing stream response to changing discharge, both at particular cross-sections and in the downstream direction. The equation is flexible and easily linearized, but whether in its simple bivariate (equation 1.1) or extended multivariate form, its application has not always been adequately justified. Although techniques of analysis have become more sophisticated, the procedure remains largely inductive, with the attendant problem of making generalizations from empirical results usually founded on a restricted database. A strong statistical relationship sometimes gives the illusion that explanation has been achieved, and that the effects of underlying processes have in some way been captured, when often the independent variable is merely a surrogate for some aspect of process. Despite these shortcomings, the empirical approach has been dominant in geomorphology and has provided valuable insights into the workings of the fluvial system. Results of this form are an important preliminary step but, to be of lasting value, need to be embodied within a theoretical structure, even if only qualitatively.

The more deductive theoretical approach has as its main aim the formulation and testing of specific statements based on established principles, and commonly involves the construction of models of varying complexity. With the relative lack of established theory, geomorphologists have drawn on the experience gained in allied fields, notably hydraulic engineering, and have frequently argued by analogy between geomorphic and other systems (e.g. Leopold and Langbein, 1962). Characterized by complex interactions of many variables (Fig. 1.1), the fluvial system is eminently suited to the adoption of modelling strategies in which some degree of abstraction or simplification is introduced. Again, a distinction can be made between deterministic and probabilistic approaches.

Deterministic reasoning is based on the belief that physical laws control the behaviour of natural systems and that, once the laws are known, the behaviour can be predicted exactly or to a satisfactory level of accuracy for a given set of conditions. The basic equations used in modelling are: (a) the continuity equations for water and sediment; (b) the flow momentum equation; (c) a flow resistance equation; and (d) a sediment transport equation. Whereas (a) and (b) are well defined theoretically, being based respectively on the principle of mass conservation and Newton’s Second Law of Motion, and (c) has a reasonably sound basis, many sediment transport equations include empirically derived coefficients. Deterministic modelling has been used in a wide range of contexts: from channel initiation (Smith and Bretherton, 1972) and drainage network development (Horton, 1945; Willgoose et al., 1991a, b, c) at one end of the scale spectrum, through meander development (Blondeaux and Seminara, 1985) and the prediction of bed topography in meander bends (Bridge, 1977), to particle entrainment (Parker et al., 1982; Andrews, 1983) at the other. It is possibly most useful when dealing with relatively small-scale problems. As situations become more complex, they are increasingly difficult to represent in terms of a set of closed equations, and consequently predictions become less reliable.

There appear to be two main arguments behind the adoption of an alternative, probabilistic strategy in model building. First, the natural world is so complex that a complete deterministic explanation can never be achieved even though each process may be deterministic, a view neatly summarized by Shreve (1975, p. 529): ‘Geomorphic systems are descendants of antecedent states that are generally unknown, and they are invariably parts of larger systems from which they cannot be isolated … a probabilistic theory that takes account of the apparent randomness is evidently a necessity, because if our theories are to succeed, they must reflect the world as it is, not as we would like it to be.’ The second argument goes one step further in not only recognizing an apparent randomness in physical systems, but claiming that inherent randomness is a fundamental property of such systems. Physical laws are regarded as not sufficient by themselves to determine the outcome of system interactions, however detailed are the observations. This view is a central theme in much of the theoretical work carried out by Leopold and Langbein, who intended that inherent randomness should be a basic principle governing behaviour in the fluvial system. Whatever view of randomness is taken, probabilistic methods have been widely used in the fluvial context, notably in drainage network analysis (Shreve, 1966, 1967) and network growth models (Dacey and Krumbein, 1976; Stark, 1991), and in the analysis of equilibrium channel geometry via minimum variance theory (Langbein, 1964a).

Inherent randomness is synonymous with deterministic complexity or chaos (Phillips, 1992). Chaos describes irregular behaviour in non-linear dynamical systems which, like the fluvial system, are often characterized by discontinuities or bifurcations during their evolution. Phillips (1992) argued that the potential for chaotic behaviour exists in many geomorphic systems, and that therefore non-linear dynamical systems theory could and should be applied to geomorphic problems. Thus far, applications of chaos theory have been limited, although the allied field of fractal geometry has been used to analyse drainage network structure (La Barbera and Rosso, 1989; Rinaldo et al., 1992) and meander form (Snow, 1989; Stolum, 1996).

Very real problems exist in modelling a complex physical system whatever framework, deterministic or probabilistic, is adopted, and most progress is likely to be made with a mixed approach. Considering the inherent variability of natural streams, physical modelling provides an opportunity for scaling down space, accelerating change over time, and holding certain conditions constant in order to identify detailed interactions. In this area the laboratory flume has occupied a pre-eminent position, even though it too has quite stringent limitations (Maddock, 1969). Hydraulic models have been used extensively to elucidate the sedimentary processes involved in the development of braiding (...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• List of symbols
• Acknowledgements
• Chapter 1 Introduction
• Chapter 2 Drainage networks
• Chapter 3 Catchment processes
• Chapter 4 Fluvial processes
• Chapter 5 Adjustment of channel form
• Chapter 6 Channel changes through time
• References
• Index