No abstract

The present study focuses on the long-term multi-year evolution of the shoreline position of the Nha Trang sandy beach. To this end an empirical model which is a combination of longshore and cross-shore models, is used. The Nha Trang beach morphology is driven by a tropical wave climate dominated by seasonal variations and winter monsoon intra-seasonal pulses. The combined model accounts for seasonal shoreline evolution, which is primarily attributed to cross-shore dynamics but fails to represent accretion that occurs during the height of summer under low energy conditions. The reason is in the single equilibrium Dean number Ωeq of the ShoreFor model, one of the components of the combined model. This equilibrium Dean number cannot simultaneously account for the evolution of strong intra-seasonal events (i.e., winter monsoon pulses) and the annual recovery mechanisms associated with swash transport. By assigning a constant value to Ωeq, when the surf similarity parameter is higher than 3.3 (occurrence of small surging breakers in summer), we strongly improve the shoreline position prediction. This clearly points to the relevance of a multi-scale approach, although our modified Ωeq retains the advantage of simplicity.

Empirical data-driven models are increasingly being used to simulate shoreline evolution over time scales ranging from days to decades. For the empirical shoreline evolution modeling, the combined longshore and cross-shore modeling demonstrate the ability to improve the prediction skills for shoreline changes. This study focuses on the feasibility of the combined model with a case study of the embayed beach shoreline of Nha Trang, Vietnam. Nha Trang’s climate is dominated by the tropical monsoon climate, so the beach changes here are strongly affected by this typical climate. The combined model applied in this study is a coupling of the cross-shore model and a longshore model. The combined model can provide the seasonal shoreline position fluctuations induced by the longshore contribution. Besides, it is realized that the very mild waves in summer induce strong accretion which the model cannot simulate. Therefore, a small modification of the equilibrium Dean number (Ω0) in the ShoreFor cross-shore model is suggested to improve the accretion simulation.