Quantification of tidal asymmetry in varying tidal environments

Abstract Tidal wave deformation and tidal asymmetry widely occur in tidal estuaries and lagoons. Tidal
asymmetry has been intensively studied because of its controlling role on residual sediment transport and
large‐scale morphological evolution. There are several methods available to characterize tidal asymmetry
prompting the need for an overview of their applicability and shortcomings. In this work we provide a brief
review and evaluation of two methods, namely, the harmonic method and the statistical method. The latter
comprises several statistical measures that estimate the probability density function and various forms of
skewness. We find that both the harmonic and statistical methods are effective and have complementary
advantages. The harmonic method is applicable to predominantly semidiurnal or diurnal regimes, while the
statistical methods can be used in mixed tidal regimes. Assisted by harmonic data, a modified skewness
measure can isolate the contribution of different tidal interactions on net tidal asymmetry and also reveal its
subtidal variations. The application of the skewness measure to nonstationary river tides reveals stronger
tidal asymmetry during spring tides than neap tides, and the nonlinear effects of river discharges on tidal
asymmetry in the upper and lower regions of long estuaries.
Plain Language Summary Astronomical tide is the primary forcing that drives water motion
and subsequent sediment transport and morphological changes in coastal and estuaries waters. Tidal
waves propagating from open oceans into tidal estuaries and lagoons often experience changes in wave
amplitude, speed, and shape, displaying tidal wave deformation and associated tidal asymmetry that is
featured by unequal rising and falling tidal periods. This work first provides a brief review of the methods
available for the quantification of tidal asymmetry in varying tidal environments, and discusses their
applicability based on constructed data. The application of these two methods to measured nonstationary
tides in a long estuary under significant time‐varying river discharges reveal strongly nonlinear and
nonuniform features of tidal asymmetry. The findings of this work have implications for the interpretation of
high water levels in flood management and large‐scale estuarine morphological evolution.
Publication Name: 
Journal of Geographical Research-Oceans
Page Number: 
Leicheng Guo*, Zheng Bing Wang, Ian Townend , and Qing He