Coupling of Deterministic and Probabilistic Models for Prediction of Storm-Driven Erosion on Barrier Islands

Coastal areas are subjected to storms and subsequent erosion and flooding. Waves and storm surge can cause damage to infrastructure and short- and long-term changes to coastal morphology. These morphodynamic changes can range from mild beach erosion to severe dune removal, overwash, and breaching. In this dissertation, a combination of models are used to explore the storm-driven hydro- and morphodynamic processes on different scales and their interactions over time. Additionally, their application is extended for prediction of beach response to sea-storms by coupling deterministic and probabilistic models.

In this dissertation, first a morphodynamic model is used to explore the effects of Isabel (2003) on the NC Outer Banks, with the focus on a large domain that covers 30 km of the barrier island from Rodanthe to Avon. It is hypothesized that the model can be coarsened and expanded to a large domain while preserving accuracy. Model predictions for dune erosion and overwash are in good agreement with post-storm observations. Sensitivity studies show that the model accuracy is less sensitive to the alongshore resolution of the mesh. Then, the topographic elevation changes are upscaled to a region-scale flooding model to allow overwash and inundation behind the dunes. The loose coupling of these process-based models improves the flooding predictions in region-scale model significantly.

Then, a more complex case of beaching and its impacts on larger-scale circulations are explored. Isabel (2003) breached the barrier island near the town of Hatteras and formed three channels connecting the ocean to the sound. Two-way coupling of high-resolution numerical models for coastal erosion and flooding is implemented to study the temporal and spatial evolution of the breach and its contribution to the hydrodynamics in the sound. It is hypothesized that the channels were formed due to the combined effects of ocean-side dune erosion and lagoon-side elevated water levels. The model shows that the flow from the sound to the ocean has an important role in deepening the breached channels. The morphodynamic model can predict the initiation and approximate location of the breach. However, it failed to accurately capture the channels’ depths. Several flooding scenarios are considered to implement the ground surface changes in the flooding model. The evolving breach can affect the timing and extent of flow into the lagoon. The model results show that the breach has region-scale effects on flooding that extend about 10 km into the lagoon.

Finally, the erosion of nourished beaches subjected to multiple storms is investigated. Beach nourishment provides a buffer during extreme events in the short-termbut has a finite lifespan as the beach responds to subsequent storms. Numerical models are widely used to predict the beach morphodynamics, however, they are computationally expensive. In this research, a surrogate model is developed by coupling deterministic and probabilistic models to improve the computational efficiency and to include the randomness of possible future scenarios. A large data set of storm data and beach profiles is used to create a library of thousands of hypothetical scenarios to train the surrogate model. It is hypothesized that adding the beach profile variability in the analysis can improve the model in the sense that it can be applied to any beach state. The results show that predicted erosion volume by the surrogate model is very close to the numerical model predictions. The model produced the results in a few seconds which shows a significant improvement in computational time compared to numerical models.

This research has the potential to improve the prediction of storm-driven erosion, overwash, inundation and breaching. The loose coupling of hydro- and morphodynamic models allows for better predictions of storm-driven flooding into previously protected areas, such as coastal communities and back-barrier regions. The use of observed beach profiles in the development of a surrogate model can improve its predictions of nourishment response to single and successive storms. These better predictions can enable better planning and design for mitigation of future hazards.

A Gharagozlou (2021). “Coupling of Deterministic and Probabilistic Models for Prediction of Storm-Driven Erosion on Barrier Islands,” North Carolina State University.

Alireza Gharagozlou defends PhD Dissertation

We gathered again for a successful defense! Alireza Gharagozlou defended his PhD dissertation to a mix of virtual and in-person attendees, who asked a lot of great questions about his research in modeling of storm-driven erosion in the Outer Banks. Congrats to Alireza!

Alireza starts his oral defense presentation.

Virtual Conference: ASBPA Coastal Conference 2020

Virtual Conference: ADCIRC 2020

Seminar: UNC Wilmington

Sustainability of Barrier Island Protection Policies under Changing Climates

This project will address methods to adapt beach and dune nourishment to improve resilience in a changing climate. As storms become more powerful and seas continue to rise, major erosion events will occur more frequently. However, coastal communities do not yet understand how to evaluate their increasing vulnerabilities and adapt their long-term planning. In this project, we will identify the climate patterns that most often trigger the need to nourish, the variability of the time interval between such nourishments, and the economic costs and sediment volumes necessary to maintain this coastal protection policy into the 21st century.

A stochastic climate emulator will first be developed to simulate 1000s of realizations of chronological climate patterns (forced by satellite and GCM products) to create future storm events coupled with sea level rise scenarios. A library of high fidelity, open source, hydrodynamic and morphodynamic simulations (ADCIRC+SWAN and XBeach) will then be used to develop a surrogate model to predict erosion and flooding for each future realization. Triggers like beach width, dune height, and community preferences will be used to identify how often communities will need to re-nourish, contingent on future climate and sea level rise scenario.

JC Dietrich, DL Anderson. “Sustainability of Barrier Island Protection Policies under Changing Climates.” U.S. Coastal Research Program, 2019 Academic Research Opportunities, 2019/10/18 to 2021/10/17, $226,624 (Dietrich: $226,624).