Vegetation provides practical protective tools for estuarine and coastal regions. The roots, stems, and canopy systems of mangroves can divert and retard the flow field within and surrounding vegetation regions (Truong et al., 2019) and also absorb external forces from waves (Phan et al., 2015). The area within the vegetation is usually calmer compared to the unprotected region outside. Consequently, sediment tends to be deposited inside the vegetation region (Vargas Luna et al., 2015). The sediment deposited then may have feedback on the wave and flow field and the growth conditions of the vegetation (Truong et al., 2017). These mutual interactions between ecological area (vegetation), hydrodynamic conditsions (wave and flow field), and morphological conditions (sediment transport) are the crux of any proposed nature-based solutions (NbS). From a hydraulic engineering perspective, these dynamic interactions can translate into the momentum and mass exchange processes between vegetation and nearby areas. By observing the evolution of mangrove forests and associated with the rate of erosion/accretion of the shoreline, Phan, 2015 and Truong., 2017 proposed a hypothesis of “squeezed mangrove forest”, in which “the mangrove width” is considered a crucial length-scales that is related to the sustainable development of the mangroves. This length scale was physically interpreted and connected to the penetration of the mixing layer into the vegetation region (Truong., 2017; Truong et al., 2019). It is noted that whereas the characteristic of the incoming waves mainly controls the penetration of the mixing layer into the coastal mangroves, that of the estuarine mangroves is mainly governed by the characteristic of lateral flow. The latter is the primary focus of this study. Large vortex structures caused by the Kelvin-Helmholtz instability at the vegetation’s edge play an essential role in the transverse exchange of mass and momentum (White & Nepf, 2007; Truong et al., 2019). These structures are usually large compared to the water depths and are termed large horizontal coherent structures (LHCSs). The Reynolds Shear stresses (RSs) induced by LHCSs contribute more than 90% to the total turbulent shear stress at the edge of the floodplain vegetated region (Truong & Uijttewaal 2019).

Nevertheless, our understanding of this topic often stems from small-scale laboratory experiments. Whether the presence and characteristic of vortex structures at the interface of the low flow and fast flow region obtained from small-scale physical models remain true for estuaries and coasts has not yet been determined. In order to obtain more insight into the physics of the exchange processes occurring at the vegetation interface at different scales, two unique physical models of vegetated channels have been conducted. One small-scale and another large-scale experiment, both with and without vegetation, were conducted at TU Delft Water Lab and the Korea Institute of Civil Engineering and Building Technology - River Experiment Center (KICT-REC), respectively. Two digital twin models of this flume were subsequently constructed using Delft3D, which were calibrated and validated using the collected datasets. In this study, recent findings pertaining to these experiments are presented

Cua Dai inlet is a typical microtidal, mixed energy-wave dominated inlet in a tropical monsoon regime in central Vietnam. Both the river flow regime and coastal processes such as induced by waves and tides influence Cua Dai Inlet and its adjacent coasts. Cua Dai Beach, the northern adjacent coast of Cua Dai inlet, has experienced severe erosion since 1995 due to an apparent non-periodic cyclic process, a decrease of sediment supply from the river, estuary and squeeze by coastal developments (Do et al. in J Coast Res 34(1):6–25, 2018). The inlet channel has shifted from North to South which served as an important controlling mechanism for the creation of a new ebb shoal. However, the role of the ebb-tidal delta in relation to the channel shifting and seasonal varying hydrodynamic conditions (river discharge and wave climate) remains poorly understood. Most studies have only considered the impact of waves and tides on the development of the ebb tidal delta. No study has included the impact of a varying river discharge on ebb shoal development and inlet migration. This chapter investigates the seasonal varying hydrodynamics and sediment transport of the inlet and adjacent coasts due to the seasonal varying river discharge and wave climate. The 2DH process-based morphodynamic numerical model (Delft3D) is applied using schematized wave conditions and river discharge. Six simulations with varying dominant wave conditions for the winter and for the summer are executed in combination with varying river discharge classes that corresponding to the dry, wet and flood seasons. There exists an East North East monsoon with a flood season from September to December, an East North East monsoon with a wet season from January to March, and a dry bidirectional South East/East North East monsoon from April to August. We investigate the effect of the seasonal wave climate and seasonal river discharges at Cua Dai inlet by analyzing the effects on the resulting hydrodynamics, sediment transports and potential morphological changes through the inlet and at the adjacent coasts. Primary results indicate that the seasonal variation in the wave climate has a strong influence on the sediment transport patterns in the adjacent coasts. The variation in the river flow dominates the magnitude of sediment transport through the inlet. The results of the simulations show that the inlet generally imports sediment into the estuary except in the case of the flood season. During the flood season the estimated sediment export is significant. Interestingly, the wave direction that varies during summer also influences the magnitude of sediment import into the estuary. Waves coming from the ENE contributes to larger sediment import than waves coming from the SE.