There is always a difference in height between high and low tide of one tide; the so-called
decline. At spring tide the decline is greater than at neap tide. (see figure 28)
The next drawing (Figure 29 on the next page) seems very complicated but not that bad. It represents a random quay somewhere in a tidal zone in the Netherlands.
- The horizontal lines represent the water level. This is always different: at low tide the water level is the lowest. And at high tide the water level is the highest.
Chapter 1 also mentions decline. Decline meant the difference in height between two points of a river. The term ‘decline’ can therefore be used in different meanings. It always has to do with height differences!
Figure 30 shows the course of the tidal movement, from New Moon to New Moon, for Vlissingen. The gradual progression from spring tide to neap tide and back is clearly visible. The time difference between full moon and spring tide can also be ‘read’ here. There is also a difference in level between two consecutive high and low tides. This is known as ‘daily inequality in height’. – The time between neap tide and spring tide is indicated by fourteen amplitudes. Two amplitudes or results form one day (24 hours).
After all, in Vlissingen we have high water and low water twice a day. From neap tide to spring tide, the HW gets a little higher every day. And the LW gets a little lower every day. Those pieces a day aren’t the same. We’ll come back to this later.
The difference in altitude between HW and LW, the decline, is very different per place within the Netherlands. For example:
- at Hook of Holland the average decline is 1.80 m;
- at Vlissingen, the average decline is 3.70 m;
- at IJmuiden the average decline is 1.50 m;
- at Delfzijl, the average decline is 2.60 m.
These differences are caused by, among other things, pushing up the water at that location. A good example is the Western Scheldt. The further the flood on the Western Scheldt flows in, the greater the decline. Vlissingen has an average decline of 3.70 metres and in Bath the decline has increased to an average of 4.5 metres.
In the vicinity of Antwerp, the decline is about 6 m. This is due to the funnel-shaped impoundment with which the Western Scheldt can be compared.
Very large decline can be found at the west coast of France, at Le Mont Saint-Michel the decline is locally even 15 m.