Laboratory Investigation of Hydrodynamic and Sand Dune Morphology Changes Under Wave Overwash

Since the field measurement of wave overwash is difficult to obtain during severe storms, a laboratory experiment is conducted to measure the time and space evolution of the overwash and beach profile over the sand dune continuously using high‐speed video image analysis for different wave conditions of irregular waves and solitary waves. Characteristics of the cross‐shore sediment transport and morphology change are discussed from the laboratory experiment data. The inflection point, which marks the shifting turn in dune crest movement during overwash, is determined by analyzing feature points in the variation of dune geometry. It is confirmed that the overwash water volume rate reaches a maximum value when the inflection point occurs. Additionally, the overwash water volume is found to be linearly related to the deposited sediment volume, and the dune erosion volume is proportional to wave energy flux during the overwash process. The presence of secondary dunes is found to significantly alleviate coast erosion during overwash and reduce shoreline retreat by more than 40%, regardless of the height of the secondary dune crest. Furthermore, the advances and limitations in overwash hydrodynamics are discussed. It is recommended that this continuous morphological change monitoring data be combined with a wave resolution model for detailed hydrodynamic simulations to further understand the overwash process. Plain Language Summary Beach erosion and land inundation occur when wave runup exceeds the crest of dunes during extreme natural hazards. Dunes play a crucial role in coastal defense by reducing sediment loss and maintaining beach stability. To understand the hydrodynamic conditions and morphology change during the overwash process, we set up a dune‐beach experiment in a laboratory flume. With the support of high‐speed camera technology, this study examines the response of dunes during the overwash process and the changes in dune feature points. It is pointed out that dune erosion shows a relationship with wave energy flux and is consistent with shoreline recession. Moreover, secondary dunes can effectively reduce coastal erosion during overwash. Key Points The experiment reveals that the inflection point in dune crest movement during overwash corresponds to the maximum overwash water rate Overwash water volume is linearly related to deposited sediment volume, while dune erosion volume is proportional to wave energy flux The presence of secondary dunes signi