Spring-neap variations in tidal duration asymmetry in the Pearl River Estuary: Dominant tidal combinations and long-term evolution

Tidal duration asymmetry (TDA), defined as the inequality in ebb and flood tide durations, significantly impacts sediment transport, estuarine morphology, and water resource management. This study investigates spring-neap variations in TDA at 21 stations within the Pearl River Estuary, focusing on their spatiotemporal variability, dominant tidal constituent combinations, and long-term trends under anthropogenic influences. Using spectral analysis, Pearson correlation, and relative change rate methods to tidal skewness (γ_N) time series, we identify dominant tidal constituent combinations (K₁-O₁-M₂ and M₂-M₄) driving spring-neap TDA variations. Our findings demonstrate a pronounced semi-monthly periodicity, characterized by enhanced ebb dominance during neap tides, with γ_N values generally 0.1–0.2 higher than during spring tides. Spatially, TDA shifts from river-dominated upstream zones (ebb dominance) to tide-dominated downstream areas (flood dominance), with the transition regions strongly modulated by shallow-water constituents (M₂-M₄). Over the 1966–2016 period, spring-neap TDA variability experienced substantial long-term changes (up to 0.003/year), clearly associated with major human interventions, such as dam construction, sand mining, and reclamation, altering tidal constituent amplitudes and phases.. These interventions primarily amplified non-linear interactions during neap tides, with sluice construction and reclamation reducing spring-neap TDA differences while reservoir regulation and sand mining increasing this difference. These findings underscore the importance of fluvial-tidal interactions and human impacts in shaping tidal asymmetry, providing critical insights for sustainable estuarine management.