Effects of shore-normal coastal structure on medium- to long-term embayed shoreline evolution

Based on high-tide shoreline data extracted from 87 Landsat satellite images from 1986 to 2019 as well as using the linear regression rate and performing a Mann-Kendall (M–K) trend test, this study analyzes the linear characteristics and nonlinear behavior of the medium- to long-term shoreline evolution of Jinghai Bay, eastern Guangdong Province. In particular, shoreline rotation caused by a shore-normal coastal structure is emphasized. The results show that the overall shoreline evolution over the past 30 years is characterized by erosion on the southwest beach, with an average erosion rate of 3.1 m/a, and significant accretion on the northeast beach, with an average accretion rate of 5.6 m/a. Results of the M–K trend test indicate that significant shoreline changes occurred in early 2006, which can be attributed to shore-normal engineering. Prior to that engineering construction, the shorelines are slightly eroded, where the average erosion rate is 0.7 m/a. However, after shore-normal engineering is performed, the shoreline is characterized by significant erosion (3.2 m/a) on the southwest beach and significant accretion (8.5 m/a) on the northeast beach, thus indicating that the shore-normal engineering at the updrift headland contributes to clockwise shoreline rotation. Further analysis shows that the clockwise shoreline rotation is promoted not only by longshore sediment transport processes from southwest to northeast, but also by cross-shore sediment transport processes. These findings are crucial for beach erosion risk management, coastal disaster zoning, regional sediment budget assessments, and further observations and predictions of beach morphodynamics.