Sediment transport processes in riffle-pool sequences and the effects of river regulation for hydro-electric power within the North Tyne

Abstract

This study examines the effects of 10 years of river regulation on the sediments and sediment transport processes within the gravel-bedded River North Tyne. The North Tyne was regulated following the closure of Kielder dam in 1981. Since 1984, the releases from Kielder reservoir have been dominated by the generation of hydro-electricpower. The work combines a long term review and re-survey of pre-regulation sediment and bathymetric databases, with measurements of contemporary sedimentological and sediment transport processes. This has involved the application of a range of techniques designed to characterise the bed morphology and sediments. These included two new techniques for determining the structure and strength of gravel-bed surfaces. The results of these surveys revealed subtle changes in the grainsize composition of riffle sediments, characterised by an increase in the frequency of coarse particles at the surface, and the accentuation of bed structure and particle compaction. This has resulted from a process of hydraulic winnowing sustained as a result of the high shear stresses experienced on riffles during the passage of the hydropower release wave. Direct measurements of sediment transport using a range of tracing and trapping techniques identified a sediment flux divergence between riffles and pools. During rising discharges, sediments are selectively restrained by bed structure on riffles, whilst pool sediments become competent in the order pool-head, mid-pool, pool-tail. This generates a queuing system for sediments culminating at high discharges in the evacuation of the pool-tail to the downstream riffle. The presence of bed structure on riffles presents a surface of higher particle entrapment probability; the net result of which is lower particle velocities over riffles than in pools, and a subsequent choking of riffles with pool sediments. The interaction of the regulated flood waves and the riffle-pool morphology produces riffle degradation and pool-tail aggradation, although at rates much lower than in a neighbouring regulated river. Hydropower releases retard the rates of aggradational channel change caused by the reduction of flood magnitude. However, historical evidence suggests that catchment sediment supply is variable through time, and should new supplies be accessed, major channel changes should be anticipated, particularly at the tributary junctions. The results of this study have direct implications for the management of game fisheries in regulated rivers, and for understanding the relationships between flood waves and sediment transport in morphologically diverse channels.