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Observations of water elevation in the short and small tidal junctions of the Ota River, Japan, showed an increase in tidal nonlinearity at the apex of the junction. To quantitatively estimate the increase in nonlinearity, the barotropic hydrodynamic model was applied in an idealized junction domain, inspired by the Ota River Estuary junction. Even though the model was simplified, it successfully reproduced the increase in nonlinearity at the junction apex. A sensitivity analysis of tidal nonlinearity to the width of the upstream channel at the junction was performed by varying the upstream channel width from the same width as the branch channel width to three times the branch channel width. The relationship between the upstream channel width at the apex and tidal nonlinearity was not linear. Tidal nonlinearity was maximized when the apex width was twice the branch channel width. The convergence of the tides in the small width junction induced an increase of some positions of quarter-diurnal tidal constituent that raised the tidal nonlinearity. In the case of a wider channel, the flushing from river runoff dampen the tidal constituents, making it decrease tidal nonlinearity

Investigating subtidal friction and mass transport is pivotal for examining subtidal dynamics in tidal rivers. Although the behavior of subtidal friction and transport has been discussed in recent years, most studies have been conducted on tidal rivers that are affected by high amounts of river runoff. The aim of this study is to offer an initial understanding of the spatial and temporal behaviors of subtidal friction and subtidal flux in a tidal river channel with limited river runoff. This study utilized the frequency domain and theoretical decomposition analyses to determine the dominant tidal and subtidal mechanisms. Frequency domain analysis indicated the dominance of semidiurnal and diurnal tides in the observed tidal river channel. The rate of energy transfer owing to shallow water interaction was found to be stronger for the current velocity than for the water elevation. Decomposition analysis showed that subtidal friction and flux in a low-discharge tidal river channel were largely influenced by subtidal flow-induced subtidal friction and Eulerian return flux, respectively. The key findings of this study are as follows: (i) the limited amount of river runoff (4–20 m3/s) leads to the vertical variability of subtidal friction contributions from subtidal flow and subtidal-tidal interaction, as well as Eulerian return flux, and (ii) the vertical variability of the aforementioned terms can be associated with the existence of influential longitudinal subtidal density gradients along the tidal river. We believe that these findings advance our understanding of subtidal dynamics in tidal river systems, particularly those with limited discharge.

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