The Study Sites

The location of the eight study sites within the Exe Basin, Devon, United Kingdom, is shown in Figure 3.1. Each site was associated with an existing discharge gauging station operated by the Environment Agency for England and Wales, or the Geography Department, University of Exeter, and was equipped with continuous turbidity

and suspended sediment monitoring equipment maintained by the Geography Department, University of Exeter. A number of previous studies have demonstrated that the contrasting physiographic characteristics of the study site drainage basins combine to produce a range of suspended sediment regimes.29'30

Typical annual sediment yields for the study area have been estimated as being greatest for the River Dart basin (58 tkm2yr-1), and at a minimum for the River Culm (20tkm2yr-1) and Upper Exe (19tkm2yr-1) basins. The mean annual sediment yields of the remaining study basins range from 26 t km2 yr-1 (River Clyst)

to 39 t km2 yr-1 (River Creedy).3 The maximum suspended sediment concentration sampled at each of the study sites during the study period follows a similar trend. Sampled suspended sediment concentrations were at a maximum in the River Dart at Bickleigh (2024 mgl-1) while the lowest recorded values of this parameter were associated with the Upper Exe (241.7 mg l-1) at Pixton and the River Culm at Rewe (406 mg l-1). Intermediate values of maximum sampled suspended sediment concentration were obtained for the River Clyst at Clyst Honiton (667 mg l-1), River Creedy at Cowley (887 mg l-1), and River Exe at Thorverton (805 mg l-1), while relatively high values of 1040 mg l-1 and 1275 mg l-1 were recorded for the Jackmoor Brook at Pynes Cottage and Yendacott, respectively. The APSD of suspended sediment has also been previously shown to differ between the study sites.29,30 While the precise values of the absolute particle size statistics reported depend on the measurement technique employed, the study period mean values of the median absolute particle size (Ad50) measured in this study are consistent with those of the previous studies in the Exe Basin. In particular, suspended sediment collected from the Upper Exe at Pixton had a characteristically coarser Ad50 (7.7 ¡m) than all the other study sites. In addition, suspended sediment samples collected from those basins underlain primarily by Permian strata (the Clyst, Yendacott, Jackmoor Brook, and Culm catchments) were characterized by a considerably finer median particle size (4.2 ¡m, 4.3 ¡m, 4.6 ¡m, and 5.0 ¡m, respectively) than those collected from the River Exe at Thorverton (6.5 ¡m), and River Dart at Bickleigh (6.1 ¡m), which are underlain primarily by Devonian and Carboniferous rocks, respectively.

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