Introduction

Fine-grained (<62.5 ^m) sediment-associated transport dominates the land-ocean flux of many natural1 and anthropogenically derived nutrients and contaminants to the fluvial system.2 Globally, transport during storm events dominates the suspended sediment flux of most rivers.3-5 In addition to its role in the sediment-associated transport of nutrients, contaminants, and other geochemical materials, suspended sediment frequently represents a significant form of nonpoint source pollution in its own right, with both environmental, for example, ref. [6,7] and economic o costs.8

The hydrodynamic properties of suspended sediment particles are determined primarily by their size (diameter), density, and shape. Of these parameters, particle size represents the principal physical factor controlling both the hydrodynamic properties and chemical activity of fluvial suspended sediment particles. Particle size is a key variable in many numerical models of the transport and geochemical dynamics of fine sediment. A sound understanding of the in situ particle size characteristics of fluvial suspended sediment is, therefore, clearly essential to many areas of research, and particularly our ability to predict and manage the environmental impacts of fine-grained sediment transport.

It is increasingly being demonstrated that fluvial suspended sediment is typically composed of composite particles, comprising aggregates or flocs, as well as discrete particles, and that composite particles may comprise a significant proportion of the total sediment load, as well as being important determinants of the particle size characteristics of fluvial suspended sediment.9-14 In this chapter the following terminology is adopted; "composite particle" is a generic term used to describe a particle formed of smaller primary particles. "Degree of aggregation" (DOA) is the term used to describe the percentage reduction in volume median particle size of the in situ, or effective particle size distribution (EPSD) following laboratory treatment and measurement of the chemically dispersed mineral fraction, or absolute particle size distribution (APSD). These terms do not imply particular causal processes, as might be inferred from the use of "floc" or "flocculation" which are strongly associated in the literature with a specific set of physicochemical processes confined to the water column.15-17

Although the primary impact of aggregation upon the hydrodynamic properties of suspended sediment is one of coarsening the particle size distribution, an important secondary effect is the associated impact on the relationship between particle size and particle density. A growing number of studies have established that, in fluvial and other freshwater systems, composite particles may consist of a mixture of mineral grains and low density organic matter, as well as pores containing water or gas and thus regions of neutral buoyancy.16,18 Although the potentially unique structure of individual composite particles within a suspended sediment sample is likely to confound any simple relationship between particle size and settling velocity, an inverse relationship between composite particle size and density has been reported by numerous studies of particle density in the freshwater environment.11,19-21 The reduction in effective particle density (EPD) associated with an increased DOA has, however, been shown in the above studies to be typically of secondary importance compared to the increase in particle size, with the net impact of aggregation being an increase in sedimentation rates.

Characterization of the at-a-station in situ particle size regime is dependent upon a sound understanding of the temporal dynamics of the EPSD and associated DOA, at both the storm event and seasonal temporal scales. Furthermore, an appreciation of the temporal dynamics of these parameters of the suspended sediment load of river systems is essential for both spatial analysis of effective particle size data, and the calibration of reach specific sediment transport models. Although seasonal variations in the EPSD and DOA have been described by a number of studies from contrasting physiographic regions,9,22 there is a relative paucity of data concerned with the quantification and explanation of the degree of variation in the EPSD over shorter temporal scales, in particular at the storm event scale. Evidence of significant intra-storm variation in the EPSD of fluvial suspended sediment is provided by several recent studies,12,23,24 however, this phenomenon, and its relative importance compared to seasonal variations in the EPSD, is still poorly documented, and not yet fully understood.

A representative measure of the EPSD is dependent upon the use of a measurement technique that is conservative, that is, one that does not modify the EPSD by breaking up existing composite particles,25 or encouraging the formation of additional, or larger, composite particles.26 Furthermore, in view of the "operationally defined" nature of particle size data,27 the same measurement technique should, where possible, be used for derivation of both the EPSD and APSD in any comparative analysis. Although these uncertainties can be minimized by making measurements in situ, few devices currently exist that are capable of making nondisruptive in situ measurements of the EPSD of fluvial suspended sediment, especially during storm events when ambient channel conditions may be characterized by significant turbulence and high flow velocities (e.g., >2 ms-1), as well as elevated suspended sediment concentrations (e.g., >1000 mgl-1).

This chapter details the effective particle size characteristics and degree of aggregation of suspended sediment in eight rivers in the Exe Basin, Devon, United Kingdom Over a 13-month period, high-frequency nondisruptive in situ measurements of the EPSD were obtained during flood events using a laser backscatter probe,12,28 which was also used for measurement of the APSD of the suspended sediment. Temporal and spatial variations in the EPSD and degree of aggregation are examined, together with an assessment of effective particle density in the study area. The origin of the composite particles within the EPSD is also considered, with particular reference to the relative importance of "inherited" composite particles from in-channel and catchment slope sediment sources.

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