Hydrodynamic effects of mangrove in different vegetation settings

        Coastal flooding hazard has intensified in past few decades, induced by the effect of climate change. The presence of mangroves can mitigate wave effect on levees and seawalls thus reducing flood risk at the inshore regions. Meanwhile, mangroves can facilitate the tidal flat that they have encroached with having a relative accretion by trapping sediment, which enable these regions to counteract the drowning associated with sea level rise. Previous studies have provided a solid theorical base for understanding the hydrodynamic process within mangroves through numerical modeling and prototype experiments, which allow for an in-depth comprehension also on the geomorphological changes associated with different mangrove settings by field observation. In this study, a transect characterized by changing mangrove condition (plant size and density) was set up at the Nanliu delta, the largest delta in the southwest part of China, colonized by Aegiceras Corniculatum (AC). A series of hydrodynamic, turbidity and bio-morphodynamic data were acquired during both normal and storm weather conditions, which revealed the capability of native AC in both attenuating wave height and capturing suspended sediment in relation to vegetation dimension and densities. The results showed that the wave damping coefficient of AC was three times larger during the storm period than during normal weather conditions. Moreover, wave height was linearly attenuated with landward wave propagation. Our work further indicated that the slopes and intercepts of the linear fits between wave height and landward wave propagation distance under storm and normal conditions are closely related to incident wave height, water level and submerged vegetation volume. A vegetation parameter used to evaluate the vegetation occupied volume in a water column was calculated as an indicator of different settings, which proved to be highly correlated with the sediment transport process. These findings highlight how mangrove forests can positively act in reducing coastal flooding hazards suggesting the possibility of designing naturally based interventions exploiting the mitigation capacity of this vegetation type.