Modelling of Beach State Variability
Authors:   , Nwanosike Blessing 
Published: July 2024

Beaches change their shape in response to the incoming wave conditions. They may exhibit changes both in their longshore configuration and their cross-section. Beach cross-sections have been classified into different categories, broadly corresponding to different states that represent different balances between the physical forces arising between the waves, currents, and sediments. In this approach, beach evolution over time can be considered as a sequence of changes between different beach states. It is important to forecast impending morphodynamic states of a given beach. The main goal of this thesis focuses on deriving an empirical relationship between different beach states and incident wave conditions, using synthetic beach state simulations from a numerical coastal model, which would provide coastal managers with a simple and effective means of predicting how beaches will evolve over different timescales and therefore assist with the management of the beach. To create a database from which an empirical beach state variability model was developed, a process-based morphodynamic numerical model (XBeach :1D) was employed to generate different beach profiles based on over 300 wave simulations. The XBeach model was validated against a set of large-scale experimental data in order to develop a practical empirical model for evaluating different beach states. This numerical model was able to simulate morphodynamic changes of beaches with widely varying sediment characteristics and slopes from a large number of incident wave conditions. Different parameters were examined, but particular focus was put on describing the response of the nearshore berm formation and shoreline change with respect to the beach states. Hence, the Dean parameter, was used to represent the beach state. Berm crest, berm height, berm length and shoreline change were extracted from the numerical simulations. An empirical formulation was derived to predict each of these parameters, thus determining the beach state variability for incident wave conditions. The formulation takes into account several factors, such as water depth, wave steepness, the slope of the beach, and a derived parameter from the data, which can all contribute to determining beach state variability. The empirical formulation was also validated against beach profile change at a number of field sites with a wide range of characteristics (Narrabeen Beach, NSW, Australia, Hasaki Beach, Japan, Duck Beach, NC,United States, and an experimental data by Andrea Polidoro). The validation was based on selected storm profile on each field analysed. In most cases, the empirical model captured underwater berm development satisfactorily, but shoreline change was not adequately captured. As a result of these findings, the results demonstrate that the shape of the antecedent beach profile is crucial, as all parameters are calculated based on the profile. Thus, the empirical model functions better for intermediate to dissipative beaches, whereas it underestimates all quantities for highly dissipative beaches. The model’s performance is particularly notable in accurately predicting various parameters for these types of beaches, enhancing our understanding of the complex interplay between beach states and wave conditions. Despite limitations, the model’s practical implications are noteworthy for coastal management. It offers a valuable tool for informing strategies such as infrastructure planning. Additionally, its applicability to different coastal systems holds promise for broader insights into coastal morphodynamics, benefiting coastal communities globally. The empirical model will enhance understanding of the relationship between beach state and wave conditions, facilitating more effective coastal management and decision-making processes.