Seasonal variation of river and tide energy in the Yangtze estuary, China

Abstract

In many large estuaries there are significant variations in flow conditions due to the interaction between tide (with spring–neap changes) and river discharge (with wet–dry seasons), which is key to understanding the evolution of the morphology and the resultant equilibrium state. To explore whether there exists an equilibrium state, and what might control such a state in such a dynamic environment, both numerical and analytical methods have been used to investigate the relative importance of tide and river contributions to the work done locally and globally over a wide range of discharge conditions in the Yangtze estuary. In particular, we have quantified the contributions from the tidal flow, the river flow and the tide–river interaction in terms of energy and its dissipation under different river discharge conditions. Model results suggest that there is a state of minimum tidal work for the case representing the wet season, when river and tide are doing uniform work locally and minimum work globally, within the bi‐directional tidal reach for tide and along the whole estuary for river. We also observe that the system is not optimized for other conditions (peak discharge and low flows during the dry season), but the system would tend to do the minimum work possible given the constraints on the system (e.g. imposed forcing conditions and available sediment supply). Results, therefore, are consistent with the use of these two energetic optimization principles, and the proposed method could be applicable to other alluvial estuaries. Copyright © 2015 John Wiley & Sons, Ltd.

Quantification of Tidal Asymmetry and Its Nonstationary Variations

“…Analyses from a tidal energy perspective also confirm the above finding. Work by Zhang et al () suggests that the tidal asymmetry is one of the degrees of freedom used by the estuary to maintain a state of minimum work by adjusting tidal wave deformation and tidal asymmetry along the estuary under varying river discharges.…”

Is “Morphodynamic Equilibrium” an oxymoron?

“…Townend and Pethick (2002) used similar entropy based arguments to consider the most probable distribution of energy flux in an estuary. The possible existence of general geomorphic relations has also been explored in a "virtual world" either explicitly (Nield et al, 2005), or by examining the resultant properties from idealised long-term morphological simulations (van der Wegen and Roelvink, 2008), and in the "real world" based on measurements (Huang et al, 2014;Ensign et al, 2013), model analysis (Zhang et al, 2016) and combining both measurements and model results (D'Alpaos et al, 2010).…”

Multi‐decadal morpho‐sedimentary dynamics of the largest Changjiang estuarine marginal shoal: Causes and implications

“…The wave dynamics around the NHS were modelled using the TELEMAC2D‐TOMAWAC modelling system, first established by Zhang, Townend, Zhou, and Cai (). The typical summer and winter scenarios were considered based on wind data from the Quick Scatterometer (http://www.remss.com/missions) and daily water discharge data (at Datong gauge station) from the Changjiang Water Resources Commission of the Ministry of Water Resources between 2002 and 2012.…”