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
Cai H, Savenije HHG, Erwan Garel, Zhang X, Guo L, Zhang M, Liu F, Yang Q.
Hydrol. Earth Syst. Sci. 23: 2779-2794.
As a tide propagates into the estuary, river dis-charge affects tidal damping, primarily via a friction term,attenuating tidal motion by increasing the quadratic veloc-ity in the numerator, while reducing the effective frictionby increasing the water depth in the denominator. For thefirst time, we demonstrate a third effect of river dischargethat may lead to the weakening of the channel convergence(i.e. landward reduction of channel width and/or depth). Inthis study, monthly averaged tidal water levels (2003–2014)at six gauging stations along the Yangtze River estuary areused to understand the seasonal behaviour of tidal damp-ing and residual water level slope. Observations show thatthere is a critical value of river discharge, beyond whichthe tidal damping is reduced with increasing river discharge.This phenomenon is clearly observed in the upstream partof the Yangtze River estuary (between the Maanshan andWuhu reaches), which suggests an important cumulative ef-fect of residual water level on tide–river dynamics. To un-derstand the underlying mechanism, an analytical model hasbeen used to quantify the seasonal behaviour of tide–riverdynamics and the corresponding residual water level slopeunder various external forcing conditions. It is shown that acritical position along the estuary is where there is maximumtidal damping (approximately corresponding to a maximumresidual water level slope), upstream of which tidal damp-ing is reduced in the landward direction. Moreover, contraryto the common assumption that larger river discharge leadsto heavier damping, we demonstrate that beyond a criticalvalue tidal damping is slightly reduced with increasing riverdischarge, owing to the cumulative effect of the residual wa-ter level on the effective friction and channel convergence.Our contribution describes the seasonal patterns of tide–riverdynamics in detail, which will, hopefully, enhance our un-derstanding of the nonlinear tide–river interplay and guideeffective and sustainable water management in the YangtzeRiver estuary and other estuaries with substantial freshwaterdischarge.