The dynamic coastal zones, marked by rich biodiversity and rapid transformations due to human activities, present a challenging environment for monitoring and management. Tidal flats, a key natural feature of these zones, are increasingly subjected to anthropogenic stress, rising sea levels, and various environmental pressures. This study addresses the critical need for a robust, large-scale remote sensing approach to monitor these changes over time, particularly under fluctuating tides and evolving coastal landscapes. Using the three decades of Landsat 5 and Landsat 8 data (1990-2020), we developed an innovative approach for the automatic acquisition of low-tide imagery. Our method, which incorporates knowledge-based of tidal flat extraction, achieved a classification accuracy of over 95%. This technique effectively mitigates the impact of clouds, fog, and waves on image analysis, enabling precise and rapid delineation of large-scale intertidal zones. As a result, we produced the most extensive dataset on tidal flat areas in China's coastal zone, updated at three-year intervals. The spatial analysis results showed the primary distribution of tidal flats in Liaoning, Shandong, Jiangsu, Zhejiang, and Guangxi, which collectively account for over 70% of China's tidal flat areas. We observed distinct patterns of tidal flat evolution, with rapid changes in regions like the Liaohe River Delta, Yellow River Delta, Yangtze River Delta, and the Jiangsu Coast. These changes are closely linked to increased reclamation activities and salt marsh vegetation expansion. In contrast, coastal areas like Tianjin and Zhejiang showed a swift expansion of intertidal zones initially, followed by a stabilization post-2010, constrained by limited development space. Our study's approach to rapid tidal flat extraction has shown promising applications in other global river deltas. The comprehensive tidal flat mapping and data generation presented here offer valuable insights and support for the monitoring, management, and sustainable development of coastal wetlands.

Tidal flats play an important role in the geomorphological and biological dynamics of coasts. Research on the morphological evolution of tidal flats constitutes one of the key research issues pertaining to the sustainability of coastal ecosystems and related coastal defense issues. In this work, a novel indicator, the surface tidal trail (STT), was extracted from a near infrared terrestrial laser scanner and studied. The results show that the area intensity and size of STTs decline yearly. Meanwhile, the position shift of the peak value on the STT curves presents a similar pattern of hydrodynamic force in response to the seawall, which has been studied in previous works. Although no direct correlation between the STT intensity and the deposition rate was found, the corresponding hydrodynamic force data were not available in this work. The change process of STTs still provides a possible speculation that hydrodynamic force and the softness of tidal surfaces are two main factors that form and influence STTs. For future research, establishing the direct quantitative relationships among hydrodynamic force, topography, and STTs on different temporal and spatial scales would help to better understand this novel indicator.

Research on the expansion pattern of pioneer vegetation in tidal flats is important, because this pattern affects the development of both topography and ecology. This study aimed to determine the monthly expansion patterns of seedlings and tussocks by using terrestrial laser scanning (TLS). Overall change process research and spatial analyses of both seedling recruitment and tussock development were carried out in this work. The results show that the overall change in pioneer vegetation reflected an expansion toward the shoreline in the first year and then a northward colonization in the second year. Moreover, positive feedback effects were observed between vegetation colonization and sedimentation accretion. Colonization accelerated the depositional process at the study site. Moreover, sedimentation accretion in the northern subarea, which is located close to the seawall, promoted the colonization of vegetation in this region. A strong spatial relationship was observed between seedling recruitment and tussock development. Tussocks tended to force seedlings to expand outward by squeezing the established space of the seedlings. Moreover, seedlings were densely concentrated within a certain distance from tussocks. The distances between seedlings and tussocks tended to shorten annually as the entire vegetation area underwent the expansion process. The average distance between seedlings was found to be concentrated from 10 to 20 m. The monthly vegetation expansion process was studied on small and medium scales in this work, revealing the advantages of TLS technology in rapidly acquiring data with high resolution and high precision.

The natural geomorphology of muddy coastal zones can easily change in a short period of time, especially under the influences of coastal engineering. However, although short-term morphological change patterns can affect the establishment of salt marsh vegetation on bare tidal flats, these patterns are rarely studied. This work presents the results of an investigation of the daily variation pattern of tidal flats in response to anthropogenic activities by using microtopographic units (areas of local uplift) as indicators. The changes exhibited by these microtopographic units (microunits) were monitored by a coastal video system with two high-definition cameras for 29 days. The results show that microunits initially tended to form near the sluice gate, spread throughout the study area in the alongshore direction, and then gradually disappeared with increase in time. Finally, the geomorphology of the tidal flat recovered to the state before the scouring event in only a few days. A statistical analysis of the areas of individual microunits showed that they tended to become miniaturized and fragmented. The number of small microunits (area between 1 and 10 m2) increased significantly during the recovery period under the influence of the tidal current. This dynamic equilibrium was also observed through an analysis of one region of interest, thereby illustrating an iterative equilibrium by shifting between fragmentation and flattening during the recovery period. As a result, this work presents the first usage of a coastal video system to monitor tidal flats and gives several contributions to prove that microtopographic units can be regarded as useful indicators for studying the daily change patterns of tidal flats in response to anthropogenic activities, thereby providing an alternative method for analyzing the morphological changes corresponding to other short-term events, such as storms.