Unravelling the causes of tidal asymmetry in deltas

Tidal asymmetry in deltas is caused by both the intrinsic asymmetry, resulting from the combination of astronomical tides, and by nonlinear tidal interactions that occur in shallow water. The relative importance of these sources of tidal asymmetry in delta channel networks have remained poorly studied, partly due to the limitations of classical harmonic analysis (HA) in hindcasting nonstationary tides. The Pearl River Delta (PRD) is a multichannel system with rapidly changing bathymetry, subject to severe human interventions. This work applies a nonstationary harmonic model (NS_TIDE) to hydrological data from 15 stations in the PRD spanning the period 1961–2012. The spatiotemporal variation of multiple sources of tidal asymmetry is quantified by a skewness metric, revealing how similar or dissimilar the development of alternative sources of tidal asymmetry develop in the delta subject to study. A relative sensitivity coefficient (RSC) is introduced to decompose the contribution of tidal amplitude variations to multiple tidal asymmetries. Analytical results show the development of tides becoming gradually more asymmetric as they propagate into delta channel networks, and how this depends on the river flow. Variation in the orientation of tidal asymmetries induced by different combinations of tidal constituents leads to a complex pattern of the overall tidal asymmetry in the PRD. Our results show that tidal asymmetry in deltas can be significantly dependent on the river flow.