Impacts of secondary and quarter-diurnal tidal species on backwater hydrodynamics in tidal rivers

As river flow debouches into the sea, it is affected by the tidal fluctuation at the estuary mouth, which results in a backwater zone, where the rising residual water level (averaged over a lunar day) in the landward direction is changing periodically. This is known as backwater hydrodynamics, especially the variation of the residual water levels that are controlled by the interplay between tide and river flows, while these hydrodynamics follow the traditional stage-river discharge relation in the upstream river-dominated region. However, the tidal asymmetry contributions to the residual water level that are caused by secondary tidal species (e.g., diurnal tide D₁), overtide generation (e.g., the quarter-diurnal tide D₄) and their interplay with river flow are poorly understood. In this study, we aim to understand the impacts of secondary and quarter-diurnal tidal species on the increase in residual water level in tidal rivers. To quantify the contributions made by different components, we decomposed the numerically computed subtidal friction into different components representing the contributions made by the river flow alone, the tide-river interaction, and tidal asymmetry due to the nonlinear interaction between different tidal species. The results show that the contribution by tidal asymmetry to the residual water level is minor, while the contributions by river flow and tide-river interaction are dominant, accounting for more than 90% and 80% at the upstream and downstream end of the estuary, respectively. The contribution made by river flow increases, while the tide-river interaction decreases in the landward direction. The semidiurnal tidal species (D₂) predominantly control the residual water level induced by the tide-river interaction along the Yangtze estuary, which accounts for 70%, while the contributions made by the diurnal and quarter-diurnal tidal species are minor and comparable in magnitude in the seaward reach where tide-river interactions are important. The method proposed in this study to quantify the contributions of river flow and different tidal species to the residual water level will enhance our understanding of riverine and tidal impacts on estuarine backwater hydrodynamics and can be used to guide effective and sustainable water management in the Yangtze estuary and other tidal rivers with substantial river flow.