Research on Beach Erosion Risk and Shoreline Stability: A Case Study of the Zhongjieshan Islands Marine Reserve

In this paper, taking the key sandy shoreline of Zhongjieshan Islands Marine Reserve as an example, considering Shapu Beach and Ercunxiao beach on Qingbang Island, and Dongjihou Beach on Miaozihu Island, the coupling mathematical model of wave, tidal current and sediment is established to study the erosion risk in key shoreline sections and understand the evolution of the shoreline. During the research, the model is verified by using the measured tide level and tidal current observation data in the sea area where the study area is located to ensure the suitability of the model. The data include tidal level data at one tidal station and complete flow velocity and direction observations of six temporary tidal current stations during the period from December 3 to December 11, 2019, which includes both spring tides and neap tides. The coastal erosion risk of the key sandy section is analyzed by numerical simulations of the evolution trend of the underwater terrain under the extreme wind and wave conditions with the return period of once every ten years. The dynamic equilibrium shoreline morphology of Shapu Beach, Ercunxiao Beach and Dongjihou Beach is calculated by using the parabolic model of the shoreline, and the long-term erosion and deposition trend of the sandy beaches is predicted. The results show that under the simulated wave conditions, all three beaches are eroded, and the size of erosion is 30 m3 at Shapu Beach, 39 m3 at Ercunxiao Beach and 86 m3 at Dongjihou Beach, respectively. It is suggested that sand recruitment should be carried out manually after such dynamic processes (such as typhoon wave) occur. The prediction results of Shapu Beach shoreline and parabolic model are basically consistent, thus the beach is in a relatively dynamic equilibrium state, while the parabolic models of Ercunxiao Beach and Dongjihou Beach predict the shoreline to be at the land-side of the current shoreline, thus the beach shoreline is in a dynamic adjustment and is likely to be eroded, so it is recommended to carry out targeted monitoring regularly.