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University of the Witwatersrand (1999)

Qualitative rule-based modelling of geomorphological change in semi-arid bedrock-influenced rivers

Nicolson CR

Titre : Qualitative rule-based modelling of geomorphological change in semi-arid bedrock-influenced rivers

Auteur : Nicolson CR

Université de soutenance : University of the Witwatersrand

Grade : PhD (Civil Engineering) 1999

This thesis deals with temporal change in bedrock-influenced river channels. Its emphasis is on developing computer models which relate geomorphological change in these rivers to their flow regime. Although simulation models of alluvial rivers were first developed over thirty years ago, bedrock- influenced channels have received relatively little modelling attention. Furthermore, these rivers have several properties which make them hard to model using existing simulation approaches. Firstly their morphology is often geometrically irregular, and unlike alluvial rivers their cross-sections can vary considerably within a short longitudinal distance. This makes them difficult to represent physically in numerical simulation models. Secondly sediment travels through these river systems in episodic pulses since high flow events which move sediment occur infrequently amid long periods of low flow. Thirdly these channels are more highly vegetated than most alluvial rivers and their vegetation affects geomorphic processes. Existing models generally ignore vegetation because its geomorphic effects are poorly understood and numerical models cannot easily simulate vegetation changes through time. A new modelling approach is needed for semi-arid bedrock-influenced rivers. This thesis presents several arguments for this view, then describes and demonstrates how qualitative rule-based modelling concepts can be used to develop models of sediment dynamics and morphological change in such rivers. The work presented here extends the discipline of fluvial geomorphology in three areas : (1) It introduces a new paradigm to river modelling, one which has already proved effective in other complex environmental systems ; (2) It extends the concepts of channel sediment budgeting by adding the ability to simulate temporal dynamics and relating temporal changes to flow regime ; and (3) It shows the geomorphological importance of vegetation-sediment interactions and demonstrates how vegetation dynamics can be modelled as an integral process in fluvial geomorphology. In addition to these contributions, the thesis also includes a detailed discussion of issues relating to model assessment in the environmental sciences. The thesis concludes that low-resolution rule-based models have significant potential for modelling and understanding fluvial dynamics. Such models are particularly useful for the spatio-temporal scales relevant to river managers, since these scales are not well-served by existing numerical models based on partial differential equations.

Présentation (NRF)

Page publiée le 20 septembre 2017, mise à jour le 3 janvier 2019