Long-Term Impacts of Runoff and Coastal Reclamation on Tidal Bore Variations in the Qiantang River Estuary, China

Tidal bore dynamics in estuarine environments are influenced by both natural hydrological changes and human activities, such as coastal reclamation. This study focuses on the Qiantang River estuary, assessing the impacts of runoff variations and reclamation on tidal bores over the past five decades. By employing statistical and time-frequency methods, including the Mann–Kendall test, ordered clustering, and wavelet analysis, the relationships between tidal bore height, river discharge, and reclamation activities are examined. The results indicate that increased freshwater discharge reduces bore intensity over short timescales of 0.3 to 1.2 years, while decreased runoff amplifies it. Over longer periods of 4.1 to 8.3 years, a positive correlation emerges, with changes in runoff preceding variations in tidal bore height. Coastal reclamation, particularly the narrowing of channels, has significantly reduced the bore height at Yanguan, especially in the years following the 2000s. Additionally, the long-term interactions of other factors influencing tidal bores are explored. These findings reveal a delayed estuarine response to human modifications, highlighting the necessity of long-term monitoring and adaptive management strategies to mitigate these impacts. The study provides valuable insights into the complex interplay of natural and human factors, offering guidance for future estuarine management and conservation efforts.