High water and low water at Nikolaifleet (Deichstraße) in Hamburg.
The tides are periodically repeating movements of the sea water. These movements are caused by the gravitational forces of the Moon and the Sun. The various tidal phenomena (e.g. high and low water or spring and neap time) are determined by the positions of the Earth, Moon and Sun relative to each other and by the rotation of the Earth. In addition to astronomy, the exact course of the tides at a given location also depends to a large extent on the shape and depth of the sea. The tides near the coast are often difficult to describe using theoretical models. Therefore, good tidal predictions are also based on water level measurements taken in recent years.
In the narrower sense, only the alternating rise and fall of water levels is referred to as ocean tides. The corresponding horizontal back and forth flow of the water particles, on the other hand, is called tidal current.
Tidal prediction and water level forecasting
The tide prediction gives the water level as it can be expected due to the astronomical constellation and under average meteorological conditions. BSH publishes tide forecasts for one year in the tide tables and in the tide calendar. On the BSH website, the predictions can also be viewed under the online forecast.
The actual water level can deviate significantly from the tide forecast due to weather conditions (e.g. onshore or offshore wind). For this reason, the BSH also makes a water level forecast four times a day, for the North Sea and the Baltic Sea respectively.
The tides on the German North Sea coast
If you observe the sea at the German North Sea coast for a long time, you can see how the water rises twice a day (ebb) and falls again twice a day (flood). The highest water level at which the change from rising to falling takes place is called high water. The lowest water level at which the change from falling to rising takes place is called low water. Large areas of the Wadden Sea dry out around the time of low water (Figure 1).
The average time between two successive high waters (or two successive low waters) in one location is 12 hours and 25 minutes. We speak of semidiurnal tides because this interval corresponds to half a (mean) lunar day. A lunar day is the duration between two points in time at which the moon stands at a certain place in exactly southern direction in the sky. In addition to the semidiurnal tidal form that occurs on the entire North Sea coast, there are also diurnal tides or mixed tidal forms on other coasts around the world.
The times of high and low water vary from place to place along the coast. The tides in the North Sea are essentially controlled by the tides in the Atlantic Ocean and the tidal signal from the Atlantic Ocean reaches the different locations on the German North Sea coast at different times. Therefore, a high water occurs approximately two hours earlier in Borkum than in Cuxhaven.
The difference in altitude between high and low tide is called the tidal range. On the German North Sea coast, the tidal range lies between about one meter and over four meters. In total, the water levels are measured at over 130 tide gaugesalong the coast and the tidally influenced rivers. BSH analyses these water levels and makes predictions for future tides. On your next trip to the coast, pay attention to whether you discover a tide gauge (Figure 2).
Terms, abbreviations and reference heights
Figure 3 illustrates some of the most important terms and abbreviations used in tidology. Behind each term there is a single value. If the word "medium", "medium", or "M" precedes the term, the values are derived from a longer time series using a special procedure.
Figure 3: German terms used in tidology.
A tide refers to the interval from one low water to the following low water.
Important designations for the heights:
|HW||High water||the highest water level of a tide|
|NW||Low water||the lowest water level of a tide|
|SpHW||Spring high water||the high water during spring time (higher high water)|
|NpHW||Neap high water||the high water during neap time (lower high water)|
|SpNW||Spring low water||the low water at spring time (lower low water)|
|NpNW||Neap low water||the low water at neap time (higher low water)|
|TF||Tidal fall||the difference between the high water height and the following low water height|
|TS||Tidal rise||the difference between the low water height and the following high water height|
|TH||Tidal range||the arithmetic mean of the tidal rise and fall of a tide|
Terms relating to time:
|HWI||High water interval|
(not shown in Figure 3)
|the period from the (upper or lower) lunar transit at the Greenwich meridian to the occurrence of high water|
|NWI||Low water interval|
(not shown in Figure 3)
|the period from the (upper or lower) lunar transit at the Greenwich meridian to the occurrence of low water|
|FD||Fall duration||the period from a high water to the occurrence of the following low water|
|SD||Rise duration||the period from a low water to the occurrence of the following high water|
The tidal or water level heights can be specified using different reference heights. The choice of a reference altitude depends on the intended use. In spite of different heights relative to the chosen reference height, the actual water depth does not change of course. The most common reference heights are (see also Figure 4):
- Gauge zero (PNP): The zero point of a tide gauge. When a tide gauge is set up for the first time, the zero point is set so that the water levels always occur above it. This gives all heights the same sign. The zero point in the tidal areas is usually 5 m below normal height zero (NHN).
- Chart datum (SKN): The zero surface to which the depths in a sea chart and the height of the tides in the tide tables refer. The chart datum is usually set so low that the indicated water depth is almost always available. In the open North Sea, the SKN is based on the lowest astronomical tide (LAT). In rivers influenced by tides, the SKN is determined separately.
- Normal Height Zero (NHN): Zero surface to which the heights on the map refer. It corresponds approximately to the mean sea level.
- Mean high water (MHW): Mean height of the high waters, derived from a sufficiently long time series of observations.
Figure 4: Reference heights for tidal heights and water levels.