ABSTRACT

Sand dunes are often the primary and sometimes only 'line of defense' for coastal infrastructure, and are increasingly constructed and actively managed to protect against extreme events. Coastal managers require knowledge of how dunes will respond under these events so assets can be pre-positioned. Both natural and constructed dunes dissipate energy by modifying breaking waves and runup to limit overwash, thereby minimizing coastal flooding during extreme waves and storm-surge events. However, because extreme physical forces only interact with the dune for a relatively short, yet critical time when the water level rises, there is limited understanding on how dune sediments and vegetation can modify hydrodynamic forces and alter beach-dune profile evolution. This research focuses on dune response to a range of water level and forcing conditions that mimic the passage of an extreme storm event. A near prototype-scale laboratory experiment will be conducted over a mobile bed in the large wave flume at Oregon State University. Physical model studies will occur over a bare dune, a rapidly constructed (loosely compacted) dune following wave-induced erosion, and a dune with live vegetation. Data related to processes ranging from short-term (turbulence) to longer time scales (individual events) will be collected and analyzed to develop a fundamental understanding of the fluid-sediment-vegetation dynamics affecting dune stability, as well as damage mitigation strategies for extreme events. The collected data will be used to validate numerical models.

A multiphase flow model sedwaveFoam (created in the open-source OpenFOAM framework), capable of simulating the full profiles of sediment transport under realistic waves, will be extended for dune erosion with or without vegetation. Detailed simulations will further inform the creation of improved parameterizations of turbulence- and wave-scale processes in the event-scale morphodynamic model XBeach. A fragility framework, consistent with risk-based decision support tools, will be created to predict the probability of damage states (e.g., dune volume loss) for a given level and duration of hydrodynamic forcing. The collected data and extensive XBeach simulations will provide required input parameters for the fragility analysis. The data and modeling for different dune archetypes will be used to: (i) identify the fundamental processes (including waves, turbulence, and sediment transport) that drive dune evolution during extreme events; (ii) define the conditions by which dune vulnerability increases as function of berm erosion; (iii) investigate the interaction between the different processes and identify the threshold forcing conditions and time scales beyond which vegetation no longer enhances dune resilience; and (iv) examine the extent a fragility modeling framework can be used to improve risk-based decision for dune erosion during extreme surge and wave events. Natural resource managers and practicing engineers with on-the-ground experience, from Federal and State (Delaware, Texas) levels will contribute to this project through a stakeholder workshop planned for year 3. The fragility framework will be developed in collaboration with managers from Delaware and Texas, allowing prediction of dune damage based on commonly used measures of storm intensity. The project will support PhD and undergraduate students.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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