Temporal development of the tidal range in the southern North Sea

Abstract

The North Sea is a shallow sea on the Eurasian continental shelf. This study focuses on the temporal development of the tidal range in the southern part of the North Sea. Records of annual tidal range data from gauging stations along the Dutch coast show in general an increasing trend perturbed by random fluctuations. Changes in tidal ranges are caused by natural (periodic) processes and anthropogenic effects. To examine the impact of human measures on the tidal range, the natural components are separated from the recorded tidal range. The only natural cycle significantly present in annual mean tidal range records is the nodal tide with a period of 18.6 yr and amplitudes of 0.2-2.0%. The short-term (few years) fluctuations are qualitatively similar for different stations with individual peaks occurring at the same time. This indicates that the noisy signal is caused by regional processes and not by measurement errors (as is not expected using annual mean values). The residual tidal range shows that apart from noise the tidal range remained constant form the early 19th century till circa 1955. During the period 1955-1980 a gradual increase in tidal range is observed at all stations. Although differences exist between the stations, their trends are very similar. Because the short-term and the long-term developments are similar, it is meaningful to construct an artificial average of the tidal range records from different stations. In this study the average of the Dutch stations and the average of the German stations are compared. The analysis shows that a similar gradually increasing trend is observed at the Dutch and German coast. The hypothesis that this trend is caused by coastal engineering in the River Rhine delta, is tested. A conceptual model is applied in which the North Sea is schematised to a rectangular basin. Estuaries with tidal prisms corresponding to Zeeland estuaries are included in this model. The model shows that the tidal range increases at the mouth of the estuary when it is closed. This increase decays along the Dutch coast with a typical length with an order of magnitude of 150 km. The results of the conceptual model are supported by a detailed model study incorporating the shape of the coast line and the bathymetry [LANGENDOEN, E. J. (1987) Onderzoek naar de vergroting van het tijverschil te Vlissingen. Faculteit der Civiele Techniek. Delft, TU Delft]. Both analyses suggest strong evidence that the tidal range is not significantly affected north of Den Helder after a closure of an estuary in the Zeeland delta. This implies that the observed trend at the Dutch and German coast does not have as common cause coastal engineering in the River Rhine delta. A satisfactory explanation for the trend since 1955 is not given in this report. However, our results suggest that the most likely cause is complex changes in the oceanic and the shelf sea system due to meteorological and astronomic changes. Analyses of tidal spectra based on hourly observations can give more insight into the processes involved.