Wave Interaction with Emergent Vegetation: A Stem Scale Large Eddy Simulation Investigation

Between 1932-2012, Louisiana has lost 1883 sq. miles of wetland primarily due to wave erosion, subsidence, lack of sediment supply and anthropogenic activities. Coastal vegetation attenuates waves and surge, traps sediments and has thus received a lot of attention in recent years. Past sttempts at modeling vegetation drag in wave flows have been confined at approaches that use a quadratic drag law based sink term in the momentum equation and suffers from the need to calibrate against flume and field experiments. Further, these models do not offer any physical insights into the turbulent flow structures within the canopy. Our present study, employing the Large Eddy Simulation (LES) based turbulence closure in the Navier Stokes Equation, is used to invesitgate the highly resolved three-dimensional flow structures within an array of emergent vegetation stems, idealised as rigid cylinders under different wave conditions. Our past research has shown that for emergent cylinders the drag force on the cylinder is predominantly in the wave direction for the zone between the crest and the trough due to the shoreward wave orbital velocity associated with the crest only. This phenomenon creates a different hydrodynamic behaviour between the trough and the crest from the rest of the cylinder and will be the focus of our investigation here. Relationships between the bulk drag and the individual cylinder drag will be obtained and the effect of stem sheltering will be studied by varying the stem spacings. In this work we will present drag force, characteristic flow structures, phase averaged mean velocity profiles and turbulence kinetic energy profiles within the array of cylinders. The free surface is handled by a Volume of Fluid (VOF) model. The simulations are being carried out using the open-source CFD software OpenFOAM on massively parallel computers, employing computational resources provided by the Center for Computation and Technology at Louisiana State University.