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

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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.