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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.

 

https://www.hydrol-earth-syst-sci.net/23/2779/2019/hess-23-2779-2019.pdf