Development of a combined Lagrangian / Eulerian approach to understand coherent flow structures in gravel-bed rivers

Sediment movement in rivers, which has wide-ranging consequences for fluvial landforms, ecology & management, is driven by fluid forces. Flow & sediment movement has traditionally been analysed & modelled on a time-averaged, continuum basis, but it is well known that large turbulent fluctuations exist in near-bed velocity, & therefore in the drag & lift forces that move sediment. Turbulence in gravel-bed rivers is not a simple random field: visualisation & multipoint measurements show it is possible to decompose complex, multi-scaled, quasi-random flow fields into elementary organized structures which posses both spatial & temporal coherence that are called either eddies or coherent flow structures. The aim of this research is to develop an enhanced quantitative understanding of the generation, evolution & dissipation of Coherent Flow Structures in Gravel Bed Rivers. This will be achieved by a series of proof-of-concept laboratory flume experiments using novel automated turbulence mapping techniques in the UK's best-equipped environmental fluid dynamics laboratory. The techniques give a combined Eulerian and Lagrangian view of Coherent Flow Structures revealed by injecting tracers into the flow that allows us to monitor and thus understand both the kinematic (size, scaling, shape, vorticity & energy) & dynamic properties (origin, stability, growth, genesis into new forms & contribution to the time average flow) of these Coherent Flow Structures as they are prduced and move over complex topographies, including gravel bed surfaces. The proposed methodology will allow us to both increase our phenomenological understanding of Coherent Flow Structures in Gravel bed rivers & develop & improve time dependent numerical models of flow over such complex surfaces.