The importance of riparian plant orientation in river flow: implications for flow structures and drag

In a series of high resolution numerical modelling experiments, we incorporate submerged riparian plants into a Computational Fluid Dynamics (CFD) model used to predict flow structures and drag in river flow. Individual plant point clouds were captured using Terrestrial Laser Scanning (TLS) and geometric characteristics quantified. In the first experiment, flow is modelled around three different plant specimens of the same species (Prunus laurocerasus). In the second experiment, the orientation of another specimen is incrementally rotated to modify the flow-facing structure when foliated and defoliated. Each plant introduces a unique disturbance pattern to the normalised downstream velocity field, resulting in spatially heterogeneous and irregularly shaped velocity profiles. The results question the extent to which generalised velocity profiles can be quantified for morphologically complex plants. Incremental changes in plant orientation introduce gradual changes to the downstream velocity field and cause a substantial range in the quantified drag response. Form drag forces are up to an order of magnitude greater for foliated plants compared to defoliated plants, although the mean drag coefficient for defoliated plants is higher (1.52 defoliated; 1.03 foliated). Variation in the drag coefficients are greatest when the plant is defoliated (up to ~210% variation when defoliated, ~80% when foliated).