MORPHOLOGICAL RESPONSE OF ESTUARIES TO NODAL TIDE VARIATION

Abstract: Nodal tide variation causes a periodic change of the mean tidal range with a period of 18.6 years and amplitude of about 4%. As the cross-sectional area of the channels in estuaries is related to the tidal prism, such a variation of the tidal range can significantly influence the morphological development of tide-dominated estuaries. Field observations from the Westerschelde estuary in The Netherlands and the Humber estuary in England confirm that there is indeed an 18.6 year cycle in the evolution of the total water volume of the estuaries. Especially in the Humber estuary, with a relatively small morphological time scale, the response to the nodal tide variation appears to dominate the long-term large-scale sediment budget changes of the estuary. Even in the Westerschelde, with a much larger morphological time scale, the response of the estuary to the nodal tide variation can be clearly observed in the historical record of estuarine volume changes. The semi-empirical model ESTMORF can explain these observations. Numerical results from the models for the two estuaries reproduce the observed amplitude and phase lag (relative to the variation of the tidal amplitude) of the morphological response well. The amplitude of the morphological response strongly depends on the morphological time scale of the estuary. The phase lag hardly depends on this time-scale. This remarkable phenomenon is analytically explained using a simplified model based on the ESTMORF theory