Mangroves are an interesting species of vegetation, surviving and thriving at the interface of land and water, in the inter tidal brackish coastal waters between the mean sea level and mean high water. Mangroves are a highly productive and complex ecosystem, providing numerous services and goods to people and marine environment. Mangroves are home to a large variety of underwater animals. Mangrove wood is valuable because it is resistant to rot and insects and can be harvested for pulp or charcoal production. Most importantly, complex roots, stems and canopies of mangroves provide effective protective means for coastal and estuarine regions. Waves and tidal flows are significantly slowed down as they make their way into and through the roots, stems and canopies of the mangrove forest. Nutrients and sediments can be deposited, providing necessary conditions for a sustainable development of the mangrove ecological system in particular and a stable coastal area in general. However, despite the important role of mangroves along the Mekong delta estuary, a large part of the mangrove forests has been converted into fish farms. In many regions, only a narrow strip of mangroves remained and, as a consequence, mangrove forests degraded and banks and shorelines experience severe erosion. Although numerous attempts were implemented to restore the mangroves and to enhance the river bank stability, these were not really successful. A possible explanation may be that knowledge about the hydrodynamics and exchange processes in and around the mangrove forest vegetation area is still not yet rigorously researched. In order to understand the dynamics of mangroves, the remaining width of mangrove forests and erosion (accretion) rate were observed, collected and analysed in terms of a morphological perspective. It is found that the river bank erosion appears to relate to the width of mangrove forest. The larger the width of the mangroves, the less erosion of the river bank and vice versa. In this context, the concept “Squeeze Phenomenon,” explaining the degradation of mangroves together with the erosion of the river bank, is introduced. Based on a schematised numerical model, changes in hydrodynamics and exchange processes caused by the limited width of the forest are proposed to be the fundamental reason for the “Squeeze Phenomenon.”… @en

The significant defensive role of vegetation in general and mangroves in particular for coastal and estuarine regions has been increasingly recognized. However, understanding the shallow flow field in and around the region of vegetation is still limited. In order to gain more insight, a laboratory experiment of a vegetated compound channel was conducted. The emerged circular cylinders are a representative model for the emergent mangrove forest. Scenarios of different widths and densities of vegetation were considered. Data acquired from Acoustic Doppler Velocimetry (ADV) and force sensors have been analysed. The influences of vegetation on the shallow flow field were clarified by comparing different scenarios with and without vegetation. Furthermore, the results confirm the presence of the large horizontal coherent structures (LHCSs). The large coherent structures formed in the mixing layer at the vegetation interface emerge, promoting the transverse momentum exchange toward the forests. The LHCSs has not only boosted flow penetration into the vegetated floodplain area but also strongly disturbed the flow inside the forest. As the LHCSs move, they cause the fluctuation of the force on the cylinders. The fluctuations are largest at the vegetation edge. Negative values of stream wise forces were also recorded.@en

Although the protective role of mangroves for coasts has been increasingly recognized, that of estuarine mangroves is less well acknowledged. The complex root, stem, and canopy system of healthy estuarine mangroves efficiently reduces the impact of a strong, along-bank flow during high tides and high river discharge, protecting the riverbank from eroding. If a sediment source is available, a healthy mangrove forest also offers a higher potential for sedimentation to compensate for sea-level rise. Unfortunately, along the Mekong, Vietnam, estuaries, mangroves have been rapidly destroyed. In many regions, estuarine mangroves have degraded into narrow strips of <50 m. Riverbanks at those locations are eroding at a rate of 2-4 m y-1. The main reason for this "estuarine mangrove squeeze" phenomenon is due to the increasing demand to create more space for local fish farming. Hence, squeeze is used in a broader sense than in the context of sea-level rise impact alone. The hypothesis is that there is a critical minimum width for an estuarine mangrove forest strip to maintain its ability to survive. The analysis of available data, both from literature and from satellite observations, supports the hypothesis: An average critical width for Mekong estuaries was found to be approximately 80 m. To obtain insight into the efficiency of a mangrove forest in reducing the impacts of alongshore flow, the state-of-The-Art Delft3D model was applied to the data. The model showed that the penetration-length scale of the shear layer into a mangrove forest requires a certain minimum space to develop fully. It is hypothesized that the minimum width of a mangrove forest, which equals this maximum penetration-length scale, has a crucial role for the health of a mangrove system.@en

Vegetation in general and mangrove, in particular, plays a significant role in the protection of the coastal and estuarine regions from erosion. In particular, estuarine mangrove forests can effciently reduce the impact of a strong along-bank flow during high tides and high river discharge, creating shelter regions for the development of the ecological system. As estuarine mangrove is usually inundated during high tides and exposed to strong tidal flows, the hydrodynamic of estuarine mangrove forest is similar to that of a vegetated compound channel. In order to gain more insight into this field of the research, a hydraulic laboratory experiment of a shallow flow field in a vegetated compound channel has been conducted. Experimental results confirm a pronounced existence of large horizontal coherent structures (LHCSs). Although the presence of the large horizontal coherent structures in the vegetated channel has been well recognized, it is still unclear how they affect the flow field, mass and momentum exchange in the vegetated compound channel. Detailed analyses of the experimental data reveal more about the role of the large horizontal coherent structures. The flow field under the effect of the large coherent structures shows a spatially and temporally cycloid motion, contributing a large part to the momentum and mass transfer. The quadrant analysis of the Reynolds shear stresses has been applied to examine the connection between the motion of the large horizontal coherent structures and their associated cycloid flow events.@en

Mangrove forests along the Mekong delta estuaries are usually observed to degrade together with the increasing extension of fish farms. In this limited-width condition, the formation of coherent structures, their interactions with mangroves, the role of the width of the mangrove forest, and their effects on the exchange processes between the open channel and the adjacent floodplain are still not well examined. In order to obtain more insight, a unique laboratory experiment of a vegetated compound channel mimicking estuarine mangroves has been conducted. The results show that in a compound vegetated channel with a very gentle transverse slope, the vegetated shear layer dynamics resembled that associated with vegetation rather than that associated with a depth differential. Furthermore, the flow field under the effect of large horizontal coherent structures (LHCSs) shows a spatially and temporally cycloid motion with associated flow events, namely, sweep, ejection, stagnant, and reverse flows. It is also suggested that the coherent structures can have an influence on a broader area than the vegetation area into which the eddy structures can penetrate. In terms of the exchange processes, the momentum transfer and the intensity of transverse fluctuations induced by the LHCSs can be related to this phenomenon. Consequently, decreasing the mangrove width can significantly affect the pattern of the LHCSs, disturb the transverse exchange processes induced by these structures, and thereby changing the shear layer, creating unfavorable conditions for sedimentation inside forests and for river bank stability.@en