## 2 Materials and Methods  
### Shoreline Data Sources and Processing  
The shoreline data of the southwest part of the bay are derived from remote sensing image interpretation, and the shoreline data of other areas remain unchanged. The shoreline data in 2000 are based on the extraction results of Landsat-5 TM satellite images with a spatial resolution of 30 m, and the shoreline data in 2020 are based on the extraction results of GF-6 satellite images with a resolution of 2.0 m. When interpreting the coastline of the study area, the maximum high tide line method is used, and the impermeable structures exposed above the sea surface at the maximum tide level are regarded as the coastline. Impermeable structures are long and narrow structures that are perpendicular or oblique to the coastline. They are impermeably constructed structures for sea use such as docks, piers, dikes, and breakwaters. The water depth data are sourced from digitized nautical charts and collected water depth measurement data. The digitized nautical charts are taken from Nautical Chart No. 10011 published by the Ministry of Navigation and Marine Safety. Affected by factors such as tides and storm surges, the coastline has dynamic characteristics. Therefore, it is necessary to adopt corresponding quantitative indicators to extract the coastline as a fixed linear element according to the research purpose [19]. Based on the extraction of this linear element, methods such as the maximum high tide method, the average tidal line method, and the general tidal line method have been proposed successively [20]. The rapid development of remote sensing technology has made it possible to quickly obtain coastlines over a large area. However, the boundary lines directly extracted from remote sensing images typically represent the instantaneous water edges when the satellite passes over the area, and further processing is required to obtain the standard coastline in a practical sense [20]. Among the numerous methods available, the maximum tide method stands out for its core principle of directly extracting the water-land boundary precisely at the high tide level over multiple years. Conceptually, in terms of the definition, the standard coastline is intrinsically related to the water-land boundary at the high tide level. The maximum tide method naturally conforms to the definitional requirements of the standard coastline. As a result, this method has the advantages of rapid interpretation and easy access, which have led to its wide application in many studies [21]. In the process of screening satellite data, the appropriate extraction time is determined in line with the tidal law and the existing tidal data. Additionally, the image data during the high-tide period without cloud cover is selected. This selection procedure reduces the complexity and uncertainty of data acquisition, thereby enhancing the availability and effectiveness of the data. In this paper, the maximum tide method is employed to extract the coastline information of the southwestern Laizhou Bay in 2000 and 2020 for subsequent analysis and calculation. The remote sensing images are selected from high-tide period data of the corresponding year, ensuring no cloud cover. Over the past two decades, advancements in satellite technology have led to significant improvements in spatial resolution, resulting in differences between data from two periods. This may introduce shoreline scale effects during the shoreline extraction process. Specifically, high-resolution data can capture more details, whereas low-resolution data may smooth out some small-scale features. Consequently, the quantitative description of the coastline cannot overlook the corresponding measurement scale, and the corresponding spatial scale conversion is an essential task. To eliminate this error, we resample the satellite data from 2020 to unify the spatial resolution of the two data sets.