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Title: The Evolution of Composite Particles Within the Fluvial System
Author: Clark, Paul Alex
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2007
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Abstract:
The importance of composite particles within the fluvial environment has attracted increased attention in recent years, due to the need for improved understanding of the transport, fate and effects of sediment-associated contaminants and nutrients. As particle size exerts a fundamental influence upon the hydrodynamic behaviour and geochemical properties of sediment, the presence of composite particles must be seen as important for the mobilisation, transport and deposition of sediment, both in-channel and out-ofchannel. The development of composite particles, by aggregation and flocculation, is a complex process controlled by physical, chemical and biological factors, which act independently and in combination at different spatio-temporal scales. Current understanding of the processes involved is incomplete and limited to the consideration of individual components of the mobilisation-transport-deposition continuum, m· isolation ofone another. This thesis attempts to provide a more holistic approach to monitoring the changes in the degree of aggregation and flocculation, within the mobilisation-transport-deposition continuum by investigating the life histories of composite particles, as they move through this continuum. The thesis focuses upon the evolution (formation/destruction) ofcomposite particles as they pass from the hillslopes to the basin outlet and establishes the relative importance of composite versus discrete particle size at different points and times within the fluvial continuum. The specific aim of the research project is to develop an improved understanding of both the factors affecting the development, transport and deposition of composite particles and to identify the wider significance of composite particles. In order to meet the aims of this research project, a field-sampling network was established within the 46 km2 catchment of the Ri~~r Dart in Devon, UK. A programme of regular sampling of sediment mobilised from the catchment hillslopes, surficial fine-grained laminae (SFGL) and suspended sediment (SS), has provided information on the effective and absolute particle size (EPS, APS) of sediment moving throu~ the mobilisation-transport-deposition continuum. Spatio-temporal variability in EPS and degree of aggregation/flocculation was identified, which emphasized the diverse behaviour and complexity of the c<;mtrolling mech.anisms. Significant spatial variability was 'found to occur within the catchment m sediment mobilised from differt;nt land use types, and in SFGL and SS. Temporal variability in hilIslope mobilised sediment arid SFGL was found, with winter to summer trends showing an increase in EPS and degree of flocculation/aggregation. During dry periods the hilIslope sediment and SFGL evolve in a parallel way. This reflects the increasing availability of well-aggregated material upon catchment hillslopes and the development of SFGL floes, with longer residence time on the channel bed. During high flow events the two systems converge, the SFGL is flushed from the channel bed and replaced by sediment with characteristics tending towards that mobilised from catchment hillslopes. An assessment of the role of biostabilization in SFGL was conducted using DAPI epifluorescence counts. The degree of flocculation was shown to only weakly correlate with bacteria abundance; however, the sediment samples were observed to contain a large number ofattached non-viable cells. The results presented show that there is greater spatio-temporal variability in EPS than APS throughout the catchment and that contrasting trends exist between the two measures ofparticle size;1 The changes in EPS and the degree of flocculation relative to APS highlight the need to take account ofthe incidence of composite particles and their evolution when modelling the mobilisation, transport and deposition of cohesive sediment.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Exeter, 2007 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.487472  DOI: Not available
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