High-Resolution Numerical Modeling of Short-Crested Waves through Mangrove Pneumatophores

The Sonneratia caseolaris mangrove system is an intricate system of tree trunks, roots, and pneumatophores that can impact hydrodynamics and sediment transport patterns. A recent field program collected point measurements of waves, velocities, and water levels within a fringing mangrove system located along Cu Lao Dung Island within the Mekong Delta system, Vietnam. Although these field results provide detailed temporal hydrodynamics, they have limited spatial resolution. Therefore, a high-resolution modeling study may be helpful towards elucidating basic physics of velocity, shear, wakes, and vorticity by extending the spatial details. An approximately unit cubic domain is discretized that is identical to the field program's deployment, which is composed of a muddy sea bottom and vertical pneumatophores. The model domain is idealized: the pneumatophore geometries are specified as rigid Gaussian bumps that are fixed to a nonerodable seafloor. The three-dimensional, nonlinear, and nonhydrostatic RANS equations of motion are used to compute the momentum while a VOF formulation is used to capture the free-surface. The modeling is driven with wave data at a particular tidal stage from the field along with slip lateral boundaries and an absorbing far end boundary condition. The simulation is run for several waves, and the results are computed that rely upon computing velocity gradients. Results show a complex progressive wave behavior that includes run-up along the mangrove pneumatophore, oscillating wakes, and downward propagating vortex cores adjacent to the pneumatophores. Although the bottom is nonerodable, bed shear stress is computed as a proxy for sediment bedload transport.