Flow separation in meander bends
Most investigations of river meanders have concentrated on classical bends that have a helical flow structure through the pool, which is carried through into the entrance of the next bend. However, it is known that flow separation can occur at the outer bank before the bend apex or at the inner bank after the apex and that this results in the breakdown of the classical flow model. Although some of the controls and the implications of flow separation are now known, the frequency of occurrence, threedimensional (3-D) flow structure, sediment dynamics, controls on separation presence, and full geomorphological implications are poorly understood. This thesis uses a combination of fieldwork and 3-D numerical flow modelling (computational fluid dynamics, CFD) to investigate meander bends where flow separation and recirculation zones are present. An initial reconnaissance survey of over 600 bends revealed that flow separation is common, being present in approximately 50% of bends. The survey also identified high bend angles of tum and the high angles of flow impingement onto the outer bank as important controls on the presence of separation. Investigations in a 22 bend sub-set indicated that expansions in channel width, breaks in bank-line curvature and the angle of inflow, as governed by upstream planform, were important in generating flow separation. Detailed fieldwork combined with validated and verified time-averaged CFD modelling in three bends with separation zones enabled identification and examination of the 3-D flow fields. The simulations reveal that the flow structure in bends with separation differs considerably from the classical flow model, with the helical motion found in classical bends being very intense in the bend entrance but dissipating at or just after the bend apex. The turbulent flow structures produced by the shear layer between the downstream and recirculating flow are also investigated in the field bends and attempts at modelling transient flow structures with large eddy simulation (LES) in a simple open channel flow expansion are detailed. The geomorphological, sedimentological, and ecological implications of both the time-averaged and the transient flow structures within the bends are examined and discussed.