Sources Of Aggregated Sediment In The Exe Basin

Detailed consideration of the origin of the composite particles comprising the EPSD in the study area is beyond the scope of this chapter. However, data obtained from Jackmoor Brook during the storm event of April 8, 1994 do provide an important insight into this issue. Measurements were made of the EPSD and DOA of four suspended sediment samples comprising runoff from slopes draining to the channel. The samples were collected from four cultivated fields immediately prior to the commencement of the detailed in situ measurements in the stream at this site, using 500 ml wide-mouthed bottles. EPSD measurements were made on-site using the Par-Tec 200, with the chemically dispersed APSD of each sample being determined subsequently in the laboratory. The EPSD of all four runoff samples evidenced aggregation, with a mean Ed50 of 13.5 ^m and DOA of 64.9%. The APSDs of the runoff and stream samples were virtually identical, with mean values of Ad50 of 4.2 jxm and 4.3 jxm, respectively. These results suggest that external slope processes are potentially important determinants of the EPSD and DOA of fluvial suspended sediment in the study area.

Phillips and Walling33 have described the aggregated nature of fine-grained bed sediment at the River Dart, Jackmoor Brook, River Exe, and River Culm study sites within the Exe Basin, with the EPSD of fine-grained bed sediments being consistently coarser than that of suspended sediment at these sites. Furthermore, when resuspended, the EPSD of the bed sediments was shown to become finer in response to the break up of composite particles. This phenomenon is likely to be of greatest importance to the EPSD of ambient suspended sediment during the rising limb of the hydrograph when the bed sediment is resuspended, as critical bed shear-stresses are first exceeded. In this context, the observed intra-storm trend of a fining of the EPSD through the hydrograph (Figure 3.3(a) and (b) and Table 3.2) may be hypothesized to result, in part, from the resuspension and subsequent break up of highly aggregated fine-grained bed sediments.

Phillips and Walling33 also identified a seasonal trend in the EPSD and associated DOA of resuspended fine-grained bed sediments that essentially mirrored that of the suspended sediment described in this study. Given their considerably coarser EPSD, it is reasonable to conclude that resuspended fine-grained bed sediments may be an important variable in determining the observed seasonal variation in the EPSD and DOA of fluvial suspended sediment in the study area.

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