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and median effective particle size, as well as the degree of sorting. This analysis indicates that in the majority of cases inter-storm variation in the EPSD is a statistically significant phenomenon. The generally lower degree of significance in Table 3.6, relative to that in Table 3.5 can be explained by reference to the greater degree of variability within the intra-storm periodic data sets (Table 3.6), compared to the magnitude of variation within the replicate sets comprising the periodic measurements through an individual storm event (Table 3.5).

Seasonal variation in the factors controlling sediment mobilization and transport (e.g., rainfall intensity, variable source area, and land use), as well as the biological and physicochemical processes that may control aggregation (e.g., microorganism activity, water chemistry, water and soil temperature, suspended sediment concentration, and velocity and turbulence within the water column) can be expected to be important determinants of the inter-storm variations in EPSD and DOA summarized in Table 3.2. In this context, the measurements made on consecutive days in the Upper Exe and Exe study sites on the December 19 and December 20, 1993 provide an opportunity for investigating the magnitude of nonseasonally influenced inter-storm variation in the EPSD and DOA of suspended sediment in the study area. From Table 3.2 it can be seen that at both sites the median EPSD of the second event is characterized by a general reduction in Ed50, both on average, and in range. This trend is also exhibited by the DOA data for the Exe, although in the case of the Upper Exe the second event is characterized by an increase in aggregation. The general fining in the EPSD was also associated with a similar trend in the APSD, such that the silt/clay ratios of the EPSD for the two events were 24.7 and 21.4 for the Upper Exe, and 17.0 and 10.0 for the Exe, while the associated values for the APSD were 16.6 and 12.3 for the Upper Exe, and 10.7 and 8.4 for the Exe, respectively.

The inter-storm range in Ed50 at both sites, and also in the DOA in the case of the Exe is, however, relatively small and not statistically significant when assessed by Mann-Whitney U-test analysis (0.05 level). This phenomenon is illustrated more fully in Figure 3.4 which presents the event mean EPSDs for both sites (the mean

Upper Exe 19/12/93

Upper Exe 20/12/93

Exe 20/12/93 Exe 25/05/94

Particle size (um)

FIGURE 3.4 Inter-storm variation in the EPSD on the Upper Exe at Pixton, and the Exe at Thorverton.

Upper Exe 19/12/93

Upper Exe 20/12/93

Exe 20/12/93 Exe 25/05/94

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