Estuaries are often modified by human activities. Adjustments in the morphology of an estuary have a potential impact on the hydrodynamics and on the reflection behavior of the tide. The influence of such system changes on the complex tidal regime with a large number of superimposed tidal constituents is not fully understood yet. The reflection properties of estuaries that are characterized by abrupt changes in geometry are systematically investigated on the basis of simplified estuary model approaches to improve the understanding of the oscillation and reflection behavior of tidal waves in estuaries. The reflection coefficients at abrupt cross-sectional changes are determined by two different methods, i.e., an analytical energy-based approach and a hydrodynamic numerical (HN) model. Comparisons indicate a high agreement of the results of the different methods when evaluating the reflection coefficient. The tidal constituents are reflected at partial and total reflectors and amplified by shoaling depending on the water depths, the height of the bottom step and the horizontal constriction. A harmonic analysis of simulated water level data partly shows the formation and amplification of higher harmonic components as a result of shallow water effects. The interaction with reflectors results in an increasing amplification of the tidal constituents and the tide.

Many tidal influenced estuaries and coastal basins feature tidal amplification because of, e.g., convergence and reflection. Increasing amplification rates were observed in the Elbe estuary, with consequences for construction measures, nautical manoeuvring, flood protection, riverbed morphology and ecosystems. Although many studies were conducted investigating the tidal wave transformation in estuaries, studies based on spatially well-distributed empirical data covering periods over more than a year are rare. To fill this gap, a self-developed adapted harmonic analysis method of least squares was applied to hydrographs from 25 gauges, distributed over the tidal influenced estuary from the river mouth to the tidal border which is given by the weir 160 km upstream of the river mouth. The investigation period for the harmonic analyses covers a whole nodal cycle of 18.613 a beginning in the year 2000. The tidal constituents’ oscillatory behaviour including the appearance of compound tides, generated by nonlinear shallow water processes, and the formation of reflection induced partially standing waves are determined. The tidal constituents show shared frequency-group specific partial clapotis, but also have significant differences in amplification within those groups. The latter fact contributes to the detected inverse proportionality of tidal range amplification inside the estuary to incoming tidal wave height. As reflection can cause resonance in tidal influenced rivers, tests are developed to analyse whether criteria for resonance are met. To determine the system’s specific resonance frequency, a new method was introduced with the three-parameter Lorentzian curve-fitting. As the detected resonance frequency is not close to tidal frequencies, full-established resonance of the tidal wave and of the tidal constituents is not observed in the Elbe estuary. Migrating nodes of the partially standing tidal wave hint at increasing latent resonance.

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