Analysis of observed water level data in and around the Dutch Wadden Sea

The objective of the present study is to improve insights into the tidal dynamics along the Dutch coast and in the Wadden Sea by analysing water level data measured at tidal gauges. The analysis focuses on the amplification, propagation and deformation of the tidal wave on various timescales. The time series of water level measured at 10-minute intervals at each station (since 1987; the exact starting date varies per station) are divided into periods of 24 hour and 50 minutes to carry out Fourier series analysis for determining the mean water level (D0), the diurnal (D1), the semi-diurnal (D2) and the quarter-diurnal (D4) tides for each lunar day. Tidal amplification is investigated by comparing the amplitudes of D2 at two pairing stations (e.g. inside and outside a tidal basin), and tidal propagation is investigated by comparing the phases of D2 at the two stations. Tidal asymmetry is investigated using various indicating parameters. In addition to the well-known relative phase difference between D4 and D2, and the ratio between periods of rising and falling tide, two new parameters have been defined. These new parameters are proportional to the integral of the fifth power of time derivative of the measured water level, providing a better indication of the residual sediment transport due to peak velocity asymmetry.