Measurement of the Drag Coefficient for Seagrass at Tidal, Storm, and Seasonal Scales

海草能够减弱波浪和水流，为许多海洋和河口环境提供生态稳定性，使这些地区成为最具生产力的沿海生态系统。 人们还认为，大而密集的海草床可以保护海岸线免受风暴波浪的影响，否则会造成严重的侵蚀。 植被衰减波场的能力通常用阻力系数来描述，阻力系数在不同程度上取决于当地的水动力作用力（波高、水位和准稳流）、植被区的特征（密度、空间配置、位置和大小）以及叶片形态（几何形状、相对水深高度、浮力和刚度）。 虽然计算刚性（挺水）植被的阻力系数很简单，但对于往往具有柔性并随波浪移动的沉水植被来说，计算阻力系数相对困难。 沉水植物"随波逐流"的能力减少了阻力，因此需要很大的距离才能显著减少波浪能。 一般来说，沉水植被似乎是一个有效的衰减器，当植被：1）举行了一个稳定的电流刚性; 2）刚性拍摄在生长季节早期;或3）摇摆在一个不同的频率比当地波场。 换句话说，海草衰减波浪的能力非常有限，特别是在风暴事件期间，海岸线侵蚀的可能性最大。 因此，在潮汐、风暴和季节尺度上，沉水植被衰减波浪能量的能力会随潮流和水深的变化、波高和周期的分布以及植被的演变特征而变化。 这项为期一年的实地研究将量化与频率有关的阻力系数以及海草在波浪、海流和水位作用力范围内衰减波高和能量的能力。 2005年卡特里娜飓风过后，人们对植被在减少风暴潮和海浪造成的侵蚀方面的作用进行了大量辩论。 对此，美国国家工程院指出，"迫切"需要在一系列空间和时间尺度上描述植被与近岸流体动力学的相互作用。 虽然有显着的进步，我们的理解波衰减控制实验室设置，这些结果主要是基于人工植被在单色波场。 单一的阻力系数并不能解释海草在风暴条件下和生长季节的行为变化。 这项研究将提供海草在大范围水位和波浪强迫下的阻力系数的估计。 由于海岸管理在很大程度上依赖于波浪模型，使用摩擦系数来描述植被对波浪衰减的潜力，因此海岸管理人员对现实的海草阻力系数非常感兴趣。 具体而言，从这项研究中得出的阻力系数将使管理人员能够评估现有海草床对海岸线保护的潜在好处，并在设计涉及海草的海岸线保护或恢复项目时加以利用。

The ability of seagrass to attenuate waves and currents provides ecological stability to many marine and estuarine environments and makes these areas some of the most productive coastal ecosystems. It is also believed that large and dense seagrass beds can protect the shoreline from storm waves that would otherwise cause dramatic erosion. The ability of vegetation to attenuate the wave field is often described in terms of the drag coefficient, which is dependent, to varying degrees, on the local hydrodynamic forcing (wave heights, water levels and quasi-steady currents), the characteristics of the vegetated area (density, spatial configuration, location and size) and the blade morphology (geometry, relative height to water depth, buoyancy and rigidity). While it is straightforward to calculate the drag coefficient for rigid (emergent) vegetation, it is relatively difficult for submerged vegetation that tends to be flexible and moves with the waves. The ability of submerged vegetation to "go with the flow" reduces drag, such that large distances are required for there to be a significant reduction in wave energy. In general, submerged vegetation appears only to be an effective attenuator when the vegetation is: 1) held rigid by a steady current; 2) a rigid shoot early in the growing season; or 3) swaying at a different frequency than the local wave field. In other words, the ability of seagrass to attenuate waves is quite limited, particularly during storm events when there is the greatest potential for shoreline erosion. As a consequence, the ability of submerged vegetation to attenuate wave energy will vary at tidal, storm and seasonal scales in response to changes in tidal current and water depth, the distribution of wave height and period, and the evolving characteristics of the vegetation. This year-long field-based study will quantify the frequency-dependent drag coefficient and the ability of seagrass to attenuate wave height and energy over a range of wave, current and water level forcing. Following Hurricane Katrina in 2005, there has been considerable debate about the role of vegetation in reducing storm surge and wave-caused erosion. In response, the National Academy of Engineering identified the "urgent" need to describe the interaction of vegetation and nearshore hydrodynamics at a range of spatial and temporal scales. While there have been significant advancements in our understanding of wave attenuation from controlled laboratory settings, these results are largely based on artificial vegetation in a monochromatic wave field. The single drag coefficient does not account for changes in the behavior of the seagrass during storm conditions and over the growing season. This study will provide estimates of the drag coefficients for seagrass over a wide range of water levels and wave forcing. Because coastal management largely depends on wave models that use friction factors to describe the potential for wave attenuation by vegetation, realistic seagrass drag coefficients is of great interest to coastal managers. Specifically, the drag coefficients derived from this study will allow managers to assess the potential benefits of existing seagrass beds to shoreline protection and in designing shoreline protection or restoration projects involving seagrass.