Tradeoffs among hydrodynamics, sediment fluxes and vegetation community in the Virginia Coast Reserve, USA

Both submerged and emergent vegetation plays a fundamental role in coastal bays. Vegetation stabilizes the substrate, increasing resilience to storms. Vegetation also traps sediments favoring accretion and therefore counteracting sea level rise. Previous modeling studies on flow-vegetation-sediment interactions have focused on one specific vegetated community, but we lack a general understanding of the synergistic effects of multiple vegetation species. We focus our study on the Virginia Coast Reserve Long Term Ecological Research (LTER) site, USA, where we apply numerical modeling (Delft3D-SWAN) to investigate the independent and synergistic effects of salt marsh vegetation and seagrass. Our numerical results show that salt marshes and seagrass beds reduce the volume of water entering and exiting the shallow coastal bays up to 15% during each tidal cycle. Vegetation also reduces bed shear stress and hence increases sediment deposition in the bay and marshes up to 10% compared to the no-vegetated case. Our study shows the double benefits of seagrass as an ally of salt marsh in promoting bays resilience. On the one hand, seagrass helps the salt marsh to survive during storms by reducing wave energy; on the other hand, seagrass generates more friction in subtidal parts of the bay where salt marsh cannot survive.

沉水植被和挺水植被在沿海海湾都起着至关重要的作用。植被稳定了基质，增强了对风暴的抵御能力。植被还截留沉积物，有利于淤积，从而抵消海平面上升的影响。先前关于水流 - 植被 - 沉积物相互作用的模拟研究集中在一个特定的植被群落上，但我们缺乏对多种植被物种协同效应的全面了解。我们的研究聚焦于美国弗吉尼亚海岸保护区长期生态研究（LTER）站点，在那里我们应用数值模拟（Delft3D - SWAN）来研究盐沼植被和海草的独立作用以及协同效应。我们的数值结果显示，在每个潮汐周期中，盐沼和海草床使进出浅海沿海海湾的水量减少多达15%。与无植被的情况相比，植被还降低了床面剪切应力，从而使海湾和盐沼中的沉积物沉积增加多达10%。我们的研究表明了海草作为盐沼的盟友在提高海湾适应力方面的双重益处。一方面，海草通过降低波浪能帮助盐沼在风暴期间存活；另一方面，海草在盐沼无法生存的海湾潮下带产生更多的摩擦力。