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n = Number of samples within the storm event.

a Ed5o of last minus the first intra-storm measurement.

b Ratio of the DOA associated with last/first intra-storm measurement.

n = Number of samples within the storm event.

a Ed5o of last minus the first intra-storm measurement.

b Ratio of the DOA associated with last/first intra-storm measurement.

which also recorded two of the remaining three cases of a net increase in Ed50, and is indicative of a particle size regime in this upland catchment that is anomalous within the study area.

Table 3.3 presents the Spearmans Rank correlation coefficients for the relationship between Ed50 and sample time within the storm event. Sample time is defined as time (h) from the initial rise of the hydrograph. The majority of the correlation coefficients are negative, and, despite the small sample size of a number of the data sets, all study sites except the Upper Exe have at least one statistically significant negative correlation. The results of this analysis indicate that the net change in Ed50 is typically the product of a systematic change through the hydrograph, as was identified by the more detailed measurements presented in Figure 3.3. This conclusion is also supported by the finding that of the 29 storm events for which in situ measurements were made on both the rising and falling limbs of the hydrograph, rising limb samples accounted for the maximum value of Ed50 on 25 occasions, while falling limb measurements also accounted for the minimum value of Ed50 on 25 occasions.

The aggregation data presented in Table 3.2 display a similar pattern of intra-storm variability to that of the effective particle size data. There is a wide intra-storm range in the value of DOA, with values varying by between 1.2% (Exe 13/09/93) to 60.2% (Jackmoor Brook 08/04/94), with a study area mean value of 20.2%. On three occasions the value of DOA fell to zero, in every case a phenomenon associated with the final sample of a mid-winter storm event. This finding may reflect antecedent conditions in the sediment source areas, where biological processes leading to aggregation can be expected to be at a minimum as a result of low winter temperatures.

Perhaps most significantly, the DOA "Change" data are also indicative of a reduction in aggregation at the end of the measurement period, relative to the initial value. The only case of a net increase in the DOA is also associated with a net increase

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