The influence of sampling frequency, non-linear interaction, and frictional effects upon the accuracy of the harmonic analysis of tidal simulations

A two dimensional tidal model of the northwest European shelf is used to examine the influence of sampling rate, number of harmonic constituents analysed for, and length of data upon the accuracy of tidal constituents. Calculations show that in shallow water, where non-linear interactions give rise to higher harmonics, an accurate analysis can be obtained from a short span of data provided the higher harmonics are included in the analysis. In very shallow water where the tidal range is comparable to the water depth, asymmetry in the tidal signal due to substantial differences in friction at times of high and low water produces a number of semi-diurnal constituents in particular ν2 and L2 that must be included in the harmonic analysis. When these constituents together with the “classical” shallow water constituents are used in the harmonic analysis then an accurate analysis can be performed on a short span of data. The significant saving in computer time, particularly for a fine grid three dimensional model of using frequent sampling and analysing for a full set of constituents is stressed.