UAV-Based Analysis of Beach Cusp Morphology and its Relationship to Hydrodynamic Forcing: A Case Study at the Central Coast of Vietnam

The central coast of Vietnam, characterized by diverse terrain and climate, provides a good natural laboratory for studying beach cusps' complex formation and evolution. This study investigates the key hydrodynamic and morphological parameters that strongly influence beach cusps' formation and evolution mechanism. First, to quantify the morphological characteristics of beach cusps, high-resolution Unmanned Aerial Vehicle (UAV) imagery was captured weekly at multiple sites along the My Khe coast. Key 2D and 3D features of the beach cusp, including cusp spacing, depth, amplitude, and elevation, were extracted from these images and validated using detailed one-meter digital elevation models (DEMs). Concurrently, incident wave characteristics, including wave heights, wave periods, and wave directions, were recorded at each location. Moreover, important hydrodynamic and morphological parameters were collected, such as water level, tidal components, and sediment characteristics. The primary factors driving beach cusp formation and evolution mechanisms are better clarified by examining the spatial and temporal variability of cusp features in relation to hydrodynamic and morphodynamic conditions. A notable finding is the strong relationship between cusp spacing and changes in incident wave direction and energy induced by headlands. The curved coastline sheltered by headlands experiences reduced incident wave angle and energy, resulting in smaller and shallower cusps than exposed areas. The most pronounced beach cusp is recorded in the transition from the protected to the exposed coastline. A conceptual model of the beach cusp was then constructed in a coupled XBeach and Delft3D model. The model integrates field observations and theoretical models that can be used to explore the relationship between incident wave patterns, sediment characteristics, and coastal geomorphology, which contribute to the development of rhythmic shoreline patterns.