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. As a tide propagates into the estuary, river discharge affectstidal damping, primarily via a friction term, attenuating tidal motion byincreasing the quadratic velocity in the numerator, while reducing theeffective friction by increasing the water depth in the denominator. For thefirst time, we demonstrate a third effect of river discharge that maylead to the weakening of the channel convergence (i.e. landward reduction ofchannel width and/or depth). In this study, monthly averaged tidal waterlevels (2003–2014) at six gauging stations along the Yangtze River estuaryare used to understand the seasonal behaviour of tidal damping and residualwater level slope. Observations show that there is a critical value of riverdischarge, beyond which the tidal damping is reduced with increasing riverdischarge. This phenomenon is clearly observed in the upstream part of theYangtze River estuary (between the Maanshan and Wuhu reaches), which suggestsan important cumulative effect of residual water level on tide–riverdynamics. To understand the underlying mechanism, an analytical model hasbeen used to quantify the seasonal behaviour of tide–river dynamics and thecorresponding residual water level slope under various external forcingconditions. It is shown that a critical position along the estuary is wherethere is maximum tidal damping (approximately corresponding to a maximumresidual water level slope), upstream of which tidal damping is reduced inthe landward direction. Moreover, contrary to the common assumption thatlarger river discharge leads to heavier damping, we demonstrate that beyond acritical value tidal damping is slightly reduced with increasing riverdischarge, owing to the cumulative effect of the residual water level on theeffective friction and channel convergence. Our contribution describes theseasonal patterns of tide–river dynamics in detail, which will, hopefully,enhance our understanding of the nonlinear tide–river interplay and guideeffective and sustainable water management in the Yangtze River estuary andother estuaries with substantial freshwater discharge.

hydrology and earth system sciences

Seasonal behaviour of tidal damping and residual water level slope in the Yangtze River estuary: identifying the critical position and river discharge for maximum tidal damping