Semi-enclosed bays are significantly influenced by the interactions between land and sea, as well as human activities. One notable human activity, land reclamation, impacts water exchange within these bays. However, the variability of sediment transport and cross-bay transport following reclamation remains poorly understood. This study aims to enhance the understanding of sediment dynamics and the responses of cross-bay transport to reclamation. A well-validated three-dimensional numerical model was developed in the Laizhou Bay (LB). Following reclamation, tidal currents suspended sediment concentration, and erosion increased seaward, while these factors decreased landward. In LB, surface and bottom subtidal currents flowed in opposite directions, with the direction of volume transport primarily determined by bottom currents. In the western LB, volume and sediment transport exhibited an anticyclonic pattern, with pronounced seasonal variations observed elsewhere. During summer, volume and sediment transport predominantly occurred from the northeast to the southwest. In winter, volume transport in northern LB was directed westward, while it was eastward in the southern part; sediment transport was primarily eastward. Advection played a significant role in sediment transport dynamics. The pathway of cross-bay sediment transport was primarily located in the central part of the bay. Notably, the cross-bay sediment transport flux in winter was approximately 3.5 times greater than in summer, with reclamation resulting in a reduction in cross-bay transport flux by about 22.17%.

Using remote sensing imagery of the Yellow River Delta (YRD) from 1984 to 2024, the Digital Shoreline Analysis System (DSAS) model was employed to analyze the coastline position, migration rate, and characteristics of four typical coastal sections. The response of the coastline changes in the study area to global climate change and human activities was quantitatively assessed. Over the past 40 years, the modern YRD coastline has generally advanced seaward at an average rate of 109.64 m/a. This progression can be divided into three distinct phases: (i) The rapid transition period from 1984 to 2000, during which the total coastline length reached its maximum of nearly 440.65 km in the last 40 years. In 1986, the proportion of artificial coastline surpassed that of natural coastline for the first time. (ii) A decreasing trend in total coastline length characterized the slow transition period from 2000 to 2015. The transition in coastline types continued the trend of the previous period, with artificial coastlines exceeding 90% for the first time in 2015, marking the highest proportion in the past 40 years. (iii) The stable period from 2015 to the present, during which the total coastline length has shown an increasing trend. The artificial coastline has stabilized, while the growth of the natural coastline has been concentrated around the Yellow River estuary. However, the increase in the natural coastline has gradually slowed due to water and sediment regulation projects from 2001. The evolution of the coastline in the YRD has shifted from early control by river diversions to a current primary influence from human-driven land reclamation projects. Coastal changes in the present estuarine sections are mainly controlled by Yellow River water and sediment inflows, while abandoned northern channels experience pronounced effects from extreme weather, such as cold wave-induced winds. Additionally, factors such as sea-level rise due to global climate change and delta subsidence caused by sediment compaction have lowered the relative elevation of the coastline, further accelerating its erosion and retreat. However, these natural factors have had a lesser impact on coastline evolution than river diversions and human activities.

This study constructs a digital elevation model of the coastal waters of the Shandong Peninsula using GIS-based interpolation methods to investigate the 15-year characteristics of seabed scouring and siltation. It utilizes depth data of the Shandong Peninsula’s offshore area between 2004 and 2006 and 2017 and 2019. The results indicated that the Shandong Peninsula’s coastal-water seabed is characterized by integral siltation and local point scouring. In addition, the northwest Shandong Peninsula is dominated by siltation, mainly distributed between 2 and 5 m isobaths, with the largest siltation area located in Laizhou Bay. The net volume of siltation is 2.13 × 10⁸ m³, the siltation rate is 0.152 × 10⁸ m³.a⁻¹, and the scouring is concentrated in the estuary and the west coast of Laizhou Bay. Scouring and siltation balance generally characterize the seabed in the northern coastal waters of the Shandong Peninsula, and the degree of scouring and siltation is weak. The siltation occurs primarily near the land area, while scouring occurs as the sea deepens. In the northeast of the Shandong Peninsula, the degree of scouring and siltation is extreme, and their distribution range is small, whereas in the south, siltation is distributed in large bays and straight coastal offshore waters, and its degree is relatively weak. At the southernmost end of the study area, the scouring and siltation in Jiaozhou Bay are spatially distributed and balanced. In reality, the key factors affecting the distribution of coastal-seabed scouring and siltation are materials transported by river sediment, coastal scouring, and human activities.