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The study of coastline changes is crucial for the monitoring, development, and management of coastal environments. This article specifically focuses on the coastline of Yancheng. To analyze the changes of coastline, latest remote sensing images of Landsat series from 2002 to 2021 were selected. Threshold segmentation was employed to extract the coastline from these images. Subsequently, tidal level correction was applied to rectify the instantaneous water edge in the binary image of the target coastal area based on tidal channels, resulting in the acquisition of the target coastline. Finally, the obtained target coastline from different time periods were inputted into the Digital Shoreline Analysis System (DSAS). DSAS can be used to assess coastline changes using the end point change rate (EPR) and linear regression change rate (LRR). The research findings reveal that the average siltation rate of various sections along the Yancheng coastline is 105.62 m/y, with the maximum siltation length measuring 12,048 m, located in the Tiaozini reclamation area. Conversely, the maximum erosion length is 1820 m, situated near the mouth of the Shuangyang River. Taking the Sheyang River mouth as the boundary, the northern coastline of the river mouth experiences predominantly erosion, while the southern coastline is primarily affected by sedimentation. Moreover, the rate of coastline change gradually increases from north to south. The transformation of artificial coastline is the outcome of a combination of natural and human factors. Agricultural reclamation, port construction, and coastal protection engineering are identified as the main reasons for the artificial and seaward advancement of coastline. This study provides a foundation for the construction and protection of Yancheng's coastal zone and offshore engineering. It holds significant practical implications for urban planning and development. [ABSTRACT FROM AUTHOR]

AbstractWith the development of offshore wind power projects worldwide, the stability of deep and shallow sea wind power sites has become a crucial concern. Internal solitary waves (ISWs) are nonlinear and large amplitude waves in the stratified ocean. Their strong vertical and horizontal motion and vorticity and turbulence caused by fragmentation have an important impact on the marine environment, seabed sediments, and marine engineering. This study focuses on the interaction between ISWs and the seabed of wind power sites in deep and shallow seas. Specifically, we investigate the interaction between ISWs and monopile foundations or floating fan plate anchor structures in the seabed. Through the simulation of CFD software and Comsol software, a law of seabed pore pressure variation during the effect of ISWs on the structure can be well demonstrated. Our results demonstrate that ISWs induce the accumulation of excess pore water pressure around the structures. The monopile foundation experiences negative excess pore water pressure at its bottom and the plate anchor at its top. The pore water pressure at the bottom of the monopile foundation initially decreases and then increases. In contrast, the pore water pressure at the right side of the monopile foundation increases, decreases, and then increases. This phenomenon promotes the settlement of the structure. At the bottom of the plate anchor, there is a sharp increase in positive excess pore water pressure by ISW. The pore water pressure above the plate anchor structure initially decreases and then increases, while below the plate anchor, it increases and then decreases. This phenomenon leads to the movement of the anchorage structure and instability of the surrounding soil. The influence of ISWs on the seabed bottom intensifies with the obstruction of the structure, resulting in significant positive pressure at the bottom. Between the two anchoring structures, there is increased seepage of pore water, making the seabed more prone to instability. Through the study of this article, we know that ISW has different effects on the force of monopile foundation and plate anchor structure, which provides a favorable basis for its design and maintenance. [ABSTRACT FROM AUTHOR]

Recently, the interactions between internal solitary waves (ISWs) and the seabed have directed increasing attention to ocean engineering and offshore energy. In particular, ISWs induce bottom currents and pressure fluctuations in deep water. In this paper, we propose a method for predicting the shear stress induced by shoaling ISWs based on machine learning, and the developed approach can be used to quickly determine the safety and stability of ocean engineering. First, we provided a basic dataset for model training. Four machine learning models were selected to predict the shear stress induced by shoaling ISWs under different trim conditions. The results indicated that the performance of the convolutional neural network-long short-term memory (CNN-LSTM) forest prediction model was significantly better than the three other tested models, including long short-term memory (LSTM), support vector regression (SVR) and deep neural network (DNN) models. Therefore, the CNN-LSTM forest prediction model was the optimal model for predicting the shear stress induced by shoaling ISWs. Specifically, each metric of the CNN-LSTM model was smaller than that of the other three, and the root mean squared error to the standard deviation ratio was closest to 0.7. In addition, the CNN-LSTM model significantly outperformed the SVR and DNN models in terms of the length of prediction time. The predicted values by the CNN-LSTM model were consistent with the experimental values. The method for predicting shear stress based on machine learning in this paper can be used to predict the shear stress induced by shoaling ISWs, guide future field experiment designs, reduce damage to the seabed caused by ISWs, and promote the development of ocean engineering in deep water. [ABSTRACT FROM AUTHOR]

We have identified large pockmarks in an area of approximately 0.3 km2 in the subaqueous Yellow River Delta in the Chengdao Sea. Gas eruption channels not been identified in the sediment layers in this area, and the formation mechanism of these large pockmarks remains unknown. To study the formation mechanism of these large pockmarks, we constructed a layered silty sediment model composed of appropriate geological materials. Then, we calculated the stress, displacement, and excess pore pressure in the layered silty sediment from the surface to a depth of 10 m using the Biot theory. A comparative analysis of the calculated results and the data measured in the field was then performed. Based on these results, we established a new formation mechanism for the large pockmarks. With the occurrence of storm waves, two extreme areas of displacement and excess pore pressure appeared in the layered silty sediment. These extreme values increased quickly in the seabed during the continuous action of storm waves. When the excess pore pressure surpassed the effective stress, the top silty layer instantly liquefied and then reconsolidated. Then, when the pore pressure of the interface position exceeded the effective stress produced by the overlying sediment, the sediments experienced "sand boil" damage. With the repeated action of strong waves, the boundary of the pockmark continued to expand, forming a large and stable pockmark. This work is of great value for further understanding and mitigating marine geologic hazards, such as coastal erosion, silt deposition, and unstable sediment, in the subaqueous Yellow River Delta. [ABSTRACT FROM AUTHOR]

Long-term, continuous in-situ observation of seabed deformation plays an important role in studying the mechanisms of sand wave migration and engineering early warning methods. Research on pressure sensing techniques has examined the possibility of using the temporal characteristics of the vertical deformation of the seafloor to identify important factors (e.g., wave height and migration rate) of submarine sand wave migration. Two pressure sensing tools were developed in this study to observe the seabed deformation caused by submarine sand wave migration (a fixed-depth total pressure recorder (TPRFD) and a surface synchronous bottom pressure recorder (BPRSS)), based on the principle that as a sand wave migrates under hydrodynamic forcing, the near-bottom water pressure, bottom pressure and total fixed pressure synchronously change with time. Laboratory flume experiments were performed, using natural sandy sediments taken from the beach of Qingdao, China, to better present and discuss the feasibility and limitations of using these two pressure sensing methods to acquire continuous observations of seabed deformation. The results illustrate that the proposed pressure sensor techniques can be effectively applied in reflecting elevation caused by submarine sand wave migration (the accuracy of the two methods in observing the experimental bed morphology was more than 90%). However, an unexpected step-like process of the change in sand wave height observed by BPRSS is presented to show that the sensor states can be easily disturbed by submarine environments, and thus throw the validity of BPRSS into question. Therefore, the TPRFD technique is more worthy of further study for observing submarine sand wave migration continuously and in real-time. [ABSTRACT FROM AUTHOR]