The purpose of this study is to conduct a set of large-scale experiments, guided by a combination of hydraulic engineering, plant ecology, and coastal wetlands restoration practices, to develop an understanding of the dynamics of wave-sediment-vegetation interaction. The project goals are to (1) model the wave characteristics of two plant types common to high energy wave environments (Schoenoplectus pungens or 'bulrush' and Spartina alteriflora or 'cord grass'), (2) compare changes in sediment volume and bed form resulting from wave action in vegetated and non-vegetated areas, (3) determine the wave thresholds of plant survivability, and (4) understand the post-storm response of plants, particularly their root structure. The first goal will parameterize the wave attenuation through an empirical drag coefficient that can be integrated into regional-scale wave models currently used in wetland restoration practice. The second goal will directly impact decisions in land use planning since the comparisons will provide quantitative information on value of existing vegetated shorelines (or proposed restoration projects) to reduce shoreline loss. This study will document the thresholds for bulrush and cord grass survival in terms of wave energy (height and period), looking at both loss of stem due to breakage and loss of the entire root system. This information can be used, for example, to determine the likelihood of plant loss for a given extreme event (e.g., hurricane). The project will integrate research and education through an established Education and Outreach program, including summer Research Experience for Undergraduates (REU) and Teacher (RET) activities, an annual open house to increase ecological engineering literacy among the general public, and weekly K-12 tours and demonstrations.
As trends in global climate change and coastal population projections are realized, the nation's coastal margins including our wetlands will experience significant development pressures. The need to develop strategies for maintenance of sustainable ecological systems has been recognized for over a decade, but the services provided by coastal wetlands, including wave-energy reduction and erosion control, sediment and nutrient accumulation are not well understood. Researchers have observed that earthen levees fronted by extensive wetlands escaped extensive damage, suggesting that the ecological engineering of wetlands can provide a self-sustaining complement to the built infrastructure in reducing the risk to coastal hazards. The purpose of this project was to understand the role that wetland vegetation provides in reducing (or attenuating) wave heights, since wave height is a good measure of the amount of energy that affects the transport of sediments or mixing of nutrients by waves. We conducted two experiments: one at full scale using a uniform stand of live vegetation in a controlled hydraulic laboratory and another using artificial vegetation (at one-quarter scale) in a smaller flume to look at heterogenous or mixed vegetation types. Both sets of laboratory experiments were compared to analytical equations and numerical models that engineers and coastal managers use to predict wave height attenuation. For the large-scale experiment with live vegetation (threesquare bulrush or Schoenoplectus pungengs), we found that vegetation could be represented reasonably well with a few basic parameters -- the mean diameter and mean height of the plant stem and the number of stems per unit area -- with one tunable coefficient to represent the fluid drag on the stems. This drag coefficient could be determined by either the Reynolds Number or Keulegan-Carpenter, but it need to be calibrated for each condition. This research shows the overall limitation of this approach and suggests that the vertical variation of the plant should be taken into consideration as well as the wavelength to water depth ratio of the waves. For the quarter scale experiments, we used two different types of artifical plants to represent two different plant species. We found the attenuation equations from the large-scale were applicable to each plant type and that a linear combination of the decay coeffients for each plant type can be used to determine the wave height decay for the cases with a mix of the two plant types. These results were also supported by a numerical simulation.
