Vegetation is known to have a significant influence on the hydraulic, geomorphological, and ecological functioning of river systems.  Vegetation acts as a blockage to flow, thereby causing additional flow resistance and influencing flow dynamics.  For characteristic riparian vegetation, the flow resistance is primarily introduced through form drag, resulting in flow separation and diversion about the blockage.  The plant morphology, i.e. the vertical and horizontal distribution of branch and leaf elements, therefore acts as a first order control on the drag response observed.

This paper reports a novel method for the measurement and incorporation of natural vegetation morphologies into a computational fluid dynamics (CFD) model, allowing the numerical prediction of flows around individual plants.  A physically-based characterisation of the plant morphology is measured through terrestrial laser scanning (TLS).  The morphological complexity of the plant is maintained in the numerical prediction of flow fields by incorporating a voxelised representation into the CFD scheme through a mass flux scaling algorithm (MFSA).  To account for plant motion, the force balance acting on the plant is calculated and its movement is predicted through a time-varying MFSA to enable a dynamic treatment of complex plant morphologies, providing more physically realistic results.  Pressure fields are used to back-calculate the drag force and drag coefficient for individual plants in a given flow.  The results enable an actual plant drag coefficient to be calculated and demonstrate how this drag coefficient varies dependent on plant posture.