Mixing Processes at the Mangrove Forest Edge under Coastal Squeeze: Insights from PIV and 2DH Modelling for Nature-Based Restoration

Mixing processes at the edge of mangrove forests primarily control the exchange of momentum and suspended materials between adjacent channels and the vegetated interior. When a mangrove forest is blocked by sea dikes and fish farms, known as squeeze conditions, the extent of this exchange can change due to stronger velocity gradients and less space for shear-layer development. However, our quantitative understanding of mixing under these squeeze conditions is limited. In this study, particle image velocimetry (PIV) observations and a 2DH model were used to explore mixing at the edge of a squeezed mangrove forest. The goal is to investigate near-edge flow structures that drive lateral exchange and to integrate these processes into eddy viscosity parameters for practical 2DH simulations. A physical experiment in the Delft laboratory flume was performed to examine velocity fields and mixing dynamics at the mangrove edge. PIV was used to measure instantaneous free-surface velocities along the interface, using 2 mm floating tracer particles at 10 Hz for 300 seconds. Results reveal that large horizontal coherent structures (LHCSs), which propagate along the forest edge with cycloidal-like motions, create alternating sweep and ejection events, along with stagnation and reverse-flow phenomena. To simulate the coastal squeeze, the width of the vegetated floodplain (forest) was gradually reduced. PIV data show that LHCSs can influence a larger area inside the vegetation (about 0.40 m) than the mean mixing-layer penetration (about 0.1 m). Reducing the mangrove forest width from 50 cm to 10 cm prevents the mean streamwise velocity from reaching transverse equilibrium, causes peaks in edge Reynolds stress, and shifts dominant quasi 2D structures toward higher-frequency, smaller, and less regular LHCSs (reducing the period from 11.5 to 8.5 seconds and normalised energy from roughly 80% to 65%). These changes hinder lateral exchange and limit conditions for sediment deposition within the forest. Additionally, a depth-averaged 2DH numerical model was created in Delft3D-FLOW to replicate the physical experiment. The simulations successfully captured vortex structures and demonstrated that LHCSs, along with sweep, ejection, stagnation, and reverse-flow events, can be modelled effectively. However, matching the observed magnitude of lateral momentum exchange required employing a hybrid eddy-viscosity model that enhances edge-intensified mixing, which conventional models do not capture. These findings suggest that constructing continuous shore-parallel breakwaters to fully enclose eroding and squeezed mangrove patches in estuarine and open-coast areas could suppress edge-flow events and coherent structures responsible for lateral exchange. This would impede mixing processes essential for mangrove survival and growth.