Besides the wave characteristics, the tidal characteristics also vary globally. The two main variables on the basis of which tidal environments can be classified are:
- Magnitude of the tide, which can be characterized by the tidal range, i.e. the vertical distance covered by the tide;
- Tidal character, which can be determined by the importance of diurnal versus semi-diurnal components.
As we have seen in Sect. 3.8 the tidal wave is distorted by local differences in water depth (and thus influenced by the slope and the width of the continental shelf) and by the location and shape of land masses and large embayments. This results in a global variation in tidal range controlled by the large-scale coastal configuration and indicated in Fig. 4.10. The categories that are the basis of this figure are:
- Micro-tidal regime: mean spring tidal range \(< 2m\);
- Meso-tidal regime: mean spring tidal range \(2m\) to \(4m\);
- Macro-tidal regime: mean spring tidal range \(> 4m\).
Figure 4.10 shows that semi-enclosed seas possibly enhance tidal amplification (see Sect. 3.8.2) and therefore often experience a macro-tidal range. For open coasts and fully enclosed seas generally a micro-tidal regime can be observed.
The tidal character is defined by the form factor \(F\). The form factor is determined as the ratio of the amplitudes of the sum of the two main diurnal components \(K_1\) and \(O_1\) and the sum of the two main semi-diurnal components \(M_2\) and \(S_2\):
\[F = (K_1 + O_1)/(M_2 + S_2)\]
with the symbols of the constituents in this case indicating their respective amplitudes.
Based on the form factor, four categories are distinguished (see Table 4.1). Examples of tidal curves per category are given in Fig. 4.11.
|Category||Value of \(F\)|
|Semidiurnal||0 - 0.25|
|Mixed, mainly semidiurnal||0.25 - 1.5|
|Mixed, mainly diurnal||1.5 - 3|
Global variations in the form factor arise due to a combination of geography and latitude. As explained in Sect. 3.7.5, diurnal components are introduced due to the declination of the earth axis. The combination of diurnal and semi-diurnal components manifests itself as daily inequality (one high water that is higher than the other). The daily inequality increases with latitude and is further influenced by the presence of land masses which can locally magnify the larger tide. The latter may occur when the diurnal tidal component excites one of the resonance modes of the basin or bay.
Due to the latter resonance phenomenon, quite a few areas around the equator (e.g. Vietnam) experience a mainly diurnal tidal regime, in spite of their low latitude.
The world’s distribution of the tidal character is shown in Fig. 4.12. The figure shows that most of the world’s coastlines experience either semi-diurnal or mainly semidiurnal mixed tides. Nevertheless the extent of the areas with diurnal and mainly diurnal mixed tides is still significant. When comparing Fig. 4.10 and Fig. 4.12, it can be seen that many of the areas where diurnal tides dominate have a micro-tidal regime and none a macro-tidal regime. Apparently, areas with diurnal and mainly diurnal mixed tides tend to have smaller tidal ranges compared to semi-diurnal systems.