Results And Discussion

3.4.1 The Degree of Aggregation of Suspended Sediment in the Study Area

The influence of aggregation on the mean storm-period contribution of specific size fractions to the EPSD is summarized in Table 3.1, which presents the mean storm-period percentage of total effective volume for specific particle size fractions, together with the associated EPSD/APSD ratio for the storm event exhibiting the greatest event mean value of DOA. From these data, it is evident that at all sites the aggregated nature of the suspended sediment has caused a relative enrichment (ratio values of >1) of the >16 ¡xm (coarse silt-sand) fraction of the EPSD, while the <2 ¡xm to 16 ¡xm (clay-medium silt) fraction evidences relative depletion at all sites except that of the River Clyst. Furthermore, maximum enrichment and depletion at all sites are associated with the >62.5 ¡xm (very fine sand) and <2 ¡xm to 4 ¡xm (clays-very fine silt) fractions, respectively.

Median particle size is a key descriptive statistic for the characterization of particle size data, and in this study the DOA, as defined above, has been adopted as the main index of aggregation of suspended sediment at the study sites.

The importance of aggregation as a phenomenon within all of the 204 intra-storm effective particle size measurements made at the Exe Basin study sites is illustrated in Figure 3.2. A strong relationship is evident in Figure 3.2(a) between Ed50 and the DOA for all 204 intra-storm measurements made in the study area. The relationship between Ed50 and DOA is reasonably well described by an exponential trend with an r2 of 0.86. Figure 3.2(b) presents the relationship between Ed50 and the associated Ad50. From the scatter, and the limited range in the value of Ad50 (3.0 ¡xm to 9.6 ¡m) evident in Figure 3.2(b), it is clear that the APSD exerts little control upon Ed50,the magnitude of which is strongly related to the relative incidence of composite particles. This phenomenon is examined for the River Exe at Thorverton in Figures 3.2(c) and (d), this being the study site with the most complete data set. A strongly exponential relationship between Ed50 and DOA, similar to that established for the study areas as a whole (r2 of 0.87), is evident for the Exe data set in Figure 3.2(c). The limited range of the value of Ad50(4.9 ¡xm to 8.1 ¡m), combined with the significant degree of scatter in the relationship between Ed50 and Ad50 evident in Figure 3.2(d) further demonstrates that the principal determinant of Ed50 in the Exe Basin is the DOA, rather than temporal variations in the APSD of the sediment.

The above data emphasize that the in situ hydrodynamic properties of fluvial suspended sediment in the Exe Basin can only be meaningfully characterized by reference to the EPSD. Furthermore, the data presented in Figure 3.2 and Table 3.1, clearly indicate that the EPSD and associated DOA both vary at the inter-storm and

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