On the problem of indeterminacy in fluvial geomorphology

Abstract

This thesis is concerned with the indeterminacy of morphology in alluvial river channel cross-section and river channel planform. Indeterminacy herein, is defined as the inability to determine the individual case at a given scale of measurement. In order to assess current theoretical approaches in geomorphology to fluvial indeterminacy and to provide a framework for future investigations, an original historical review of the aims of geomorphology is presented. It is concluded that geomorphology has always considered itself as scientific, and in particular, that many geomorphologists view the discipline as a physical science. Given that geomorphology wants to produce scientific knowledge, and this is the framework within which indeterminacy is to be investigated, an original review of what characteristics scientific knowledge is presented. The primary conclusion of this review is that science is explanation by laws, of which there are three types: causal, aleatory statistical and chaos. Furthermore, it is concluded that a hallmark of scientific knowledge is its predictive capability, although the relationship between knowledge and prediction is problematic. In light of this conceptual framework, fluvial indeterminacy in geomorphology is approached by testing the three forms of law and their associated predictive statements.

The predictions that the three types of scientific law make are tested in experimental conditions using a flume, where the relationship between controls and morphology can be regulated. Causal laws which operate at the scale of the individual measurement fail predictive tests. It is not possible to predict the morphology of a specified point on a sediment surface, given values of controls, primarily because internal processes (which are as yet unknown) in the fluid-sediment system, obscure any relationship. The addition of variables does not make the system determinate. The proactive identification of chaos laws (which operate at the scale of the attractor and for which a new hypothesis is proposed in geomorphology, based on the chaos nature of turbulence) via fractal analysis also fails predictive tests.

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