Turbulence Structure and Riffle-Pool Morphology in Coarse-Grained Channels: A Field Study of the Rouge River, Toronto, Ontario, Canada

Riffle-pool sequences play a crucial role in the fluid-sediment interactions that control bed scour, sediment transfer and deposition in rivers. Areas in rivers that experience changes in depth or width cause areas of differing flow acceleration and deceleration. Differing flow regimes strongly impact the turbulence structure and associated sediment transport processes. Past research has focused largely on the maintenance of riffle-pool sequences, specifically in the context of stage dependant velocity reversal hypotheses. However, little research has been completed on the three dimensional flow structures of these sequences and how this affects the depth of scour in pools and deposition height in riffles. Critical knowledge is lacking with respect to how these properties change or differ in straight versus meandering channels. Turbulence structure and sediment transport processes experience higher complexity in meandering reaches than straight reaches. Secondary circulations around meander bends create an asymmetrical profile that influences lateral sediment transport, thus leading to the occurrence of deep scour in pools.
The primary objective of this study was to assess the turbulence structure and scour in riffle-pool sequences in both straight and meandering channel sections of the Rouge River, Toronto, Ontario, Canada. The purpose was to assess the controlling factors that influence riffle height and depth of scour in pools and also how these factors differ in straight versus meandering reaches. The spatial and temporal variability of the turbulence structure and sediment transport will also be observed by comparing the responses to varying flow stages. This study was comprised of a field-based project using an Acoustic Doppler Velocimeter (ADV). Sediment sorting and transport patterns were observed by using a combination of painted tracer particles, permanent bedload traps and handheld bedload samplers. Results indicated that turbulent kinetic energy (TKE) and momentum exchange was typically observed 5 – 10 cm above the bed. Time series data indicated that the degree of curvature likely influenced secondary circulation and hence sediment transport and channel form. It was evident that riffle height and depth of scour in pools was strongly linked to channel morphology as well as micro scale bed variably.