Abstract
Abstract. Observations indicate that the fortnightly fluctuations in the mean amplitude of water level increase in the upstream direction along the lower half of a tide-dominated estuary (the Guadiana Estuary), with negligible river discharge, but remain constant upstream. Analytical solutions reproducing the semi-diurnal wave propagation shows that this pattern results from reflection effects at the estuary head. The phase difference between velocity and elevation increases from the mouth to the head (where the wave has a standing nature) as the timing of high and low water levels come progressively closer to slack water. Thus, the tidal (flood–ebb) asymmetry in discharge is reduced in the upstream direction. It becomes negligible along the upper estuary half as the mean sea level remains constant despite increased friction due to wave shoaling. Observations of a flat mean water level along a significant portion of an upper estuary suggest a standing wave character and, thus, indicate significant reflection of the propagating semi-diurnal wave at the head. Details of the analytical model show that changes in the mean depth or length of semi-arid estuaries, in particular for macrotidal locations, affect the fortnightly tide amplitude and, thus, the upstream mass transport and inundation regime. This has significant potential impacts on the estuarine environment in terms of ecosystem management.
Highlights
When averaged over a tidal cycle, the slope of the free surface elevation is generally not flat everywhere in an estuary
Several factors operating at distinct frequencies may be responsible for mean water level variations along the channel, such as the tide, freshwater inputs, local or remote atmospheric conditions, and various coastal ocean processes acting at the mouth (e.g. Aubrey and Speer, 1985; Gallo and Vinzon, 2005; Henrie and Valle-Levinson, 2014; Jay et al, 2015; Laurel-Castillo and Valle-Levinson, 2020; Matte et al, 2013; Ross and Sottolichio, 2016; Shetye and Vijith, 2013; Wong et al, 2009)
To examine how the fortnightly signal is produced, the difference in Zf between St3 and St0 ( Zf, hereafter) is represented along with the difference in high water level (HWL) and low water level (LWL) at the same stations ( HWL and LWL, respectively) for each tidal cycle (Fig. 5, with Zf on the right axis that shows the maximum on springs and minimum on neaps)
Summary
When averaged over a tidal cycle, the slope of the free surface elevation is generally not flat everywhere in an estuary. At a fortnightly timescale, tide-dominated estuaries commonly feature relatively high and low mean water levels (MWLs) on spring and neap tides, respectively, in relation to the tidal forcing variability produced by the interaction of the semi-diurnal M2 and S2 tidal constituents (Aubrey and Speer, 1985). Fluctuations in the subtidal friction generated by tidal contributions alone may produce a fortnightly tide (Vignoli et al, 2003) This case concerns many worldwide estuaries, typically in semi-arid regions, where the river flow influence is not relevant compared to the tidal forcing – during a large part of the year, at least Subtidal water level observations in a semiarid estuary with negligible freshwater discharge (the Guadiana Estuary) are compared with the outputs of analytical solutions considering a semi-diurnal tide of variable forcing amplitude propagating along a convergent channel.
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