Chapter 4 - Three-dimensional meander bend flow within the tidally influenced fluvial zone

Abstract

Abstract Although three-dimensional bend flow within fluvial meanders is well known and the linkages between the flow field, bend morphodynamics, and resultant floodplain sedimentology well connected, there is limited knowledge on the dynamics of flows in bends that are subject to tidal forcing or tidal influence. This chapter presents measurements of three-dimensional flow around a tightly curved meander bend in the tidally influenced fluvial zone (TIFZ) within the River Severn, UK. Repeat measurements were taken at two hydrological conditions: (i) high river flows and neap tides and (ii) lower fluvial discharge and spring tide. These two cases thus highlighted the end members of the flow forcing distributions experienced at the bend, with the former showing the maximum fluvial influence and the latter the maximum tidal influence on bend flow processes. Results show that during a period of higher river flow at neap tides, there was very little tidal influence experienced at the bend. During the second set of measurements, made during a period of low river flow and at high spring tides, a full flow reversal occurred, indicating that the tidal–fluvial transition had moved landward of the bend. In both cases, the maxima primary flow velocities measured were of a similar magnitude, even though flow was fully reversed during the spring tides. However, the secondary flow velocities increased notably during the flow reversal at spring tide. These flow patterns provide an explanation of the often observed stability of meander bends within the TIFZ, with the location and focus of maximum flow shear migrating around the bend during reversals, likely hindering bar push processes that can drive meander migration processes.