Conclusion

The effective particle size distributions of fluvial suspended sediment collected at eight study sites in the Exe Basin have been shown to be characterized by a significant degree of aggregation. Intra-storm variation in the effective particle size distribution was measured in all 37 of the sampled storm events and the magnitude of this variation was closely related to the degree of aggregation of the suspended sediment. The pattern of intra-storm variation was characterized by a relatively systematic reduction of both the median effective particle size, and degree of aggregation through the hydrograph. Inter-storm variation in the effective particle size distribution was not significant over periods of days, however seasonal variations in the effective particle size distribution and degree of aggregation dominate the effective particle size regime of the study area, with both parameters being at a maximum during the summer/autumn period.

The effective particle density of suspended sediment was reduced relative to that which could be expected from a single mineral grain, and evidenced an inverse relationship between effective particle size and effective particle density. Temporal variations in effective particle density have been considered and it is possible to infer that particle density typically increases through an individual runoff event, while seasonally EPD may also vary, being at a minimum during the summer/autumn period.

If more widely applicable, the dynamic nature of the effective particle size regimes identified in this study has important implications for the characterization of fluvial suspended sediment particle size regimes. In particular, and of particular significance for the modeling of suspended sediment transport, measurement strategies must be appropriate and detailed enough to encompass significant intra-storm and seasonal variation in the effective particle size distribution and degree of aggregation of fluvial suspended sediment. Finally, further research is needed to elucidate more fully the origin of the composite particles comprising the effective particle size distribution of storm-period suspended sediment. In particular, it is important to establish the extent to which aggregation reflects dynamic flocculation within the water column during the storm event or "inherited" aggregation from preexisting flocs/aggregates derived from in-channel and catchment slope sediment sources.

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