Abstract. As a tide propagates into the estuary, river discharge affects
tidal damping, primarily via a friction term, attenuating tidal motion by
increasing the quadratic velocity in the numerator, while reducing the
effective friction by increasing the water depth in the denominator. For the
first time, we demonstrate a third effect of river discharge that may
lead to the weakening of the channel convergence (i.e. landward reduction of
channel width and/or depth). In this study, monthly averaged tidal water
levels (2003–2014) at six gauging stations along the Yangtze River estuary
are used to understand the seasonal behaviour of tidal damping and residual
water level slope. Observations show that there is a critical value of river
discharge, beyond which the tidal damping is reduced with increasing river
discharge. This phenomenon is clearly observed in the upstream part of the
Yangtze River estuary (between the Maanshan and Wuhu reaches), which suggests
an important cumulative effect of residual water level on tide–river
dynamics. To understand the underlying mechanism, an analytical model has
been used to quantify the seasonal behaviour of tide–river dynamics and the
corresponding residual water level slope under various external forcing
conditions. It is shown that a critical position along the estuary is where
there is maximum tidal damping (approximately corresponding to a maximum
residual water level slope), upstream of which tidal damping is reduced in
the landward direction. Moreover, contrary to the common assumption that
larger river discharge leads to heavier damping, we demonstrate that beyond a
critical value tidal damping is slightly reduced with increasing river
discharge, owing to the cumulative effect of the residual water level on the
effective friction and channel convergence. Our contribution describes the
seasonal patterns of tide–river dynamics in detail, which will, hopefully,
enhance our understanding of the nonlinear tide–river interplay and guide
effective and sustainable water management in the Yangtze River estuary and
other estuaries with substantial freshwater discharge.
Evolution characteristics and drivers of the water level at an identical discharge in the Jingjiang reaches of the Yangtze River