What are the elements of tides?

Tidal phenomenon refers to the periodic movement of seawater under the tidal force of celestial bodies (mainly the moon and the sun). Traditionally, the vertical fluctuation of sea surface is called tide, while the horizontal flow of sea water is called tide.

Tide is the first seawater movement phenomenon that attracts people's attention among all ocean phenomena, and it has a very close relationship with human beings. Port engineering, shipping and traffic, military activities, fisheries, salt industry and aquaculture, offshore environmental research and pollution control are all closely related to tidal phenomena. In particular, the endless vertical fluctuation of the sea surface contains huge energy, and the development and utilization of this energy has also aroused people's interest.

1, tidal element

Figure 7- 1 shows the process curve of tidal level fluctuation (i.e. the vertical height of sea surface relative to a certain datum). In the figure, the ordinate is the height of tidal level, and the abscissa is time. When the tide rises, the tide level keeps rising. After reaching a certain height, the tide level does not rise or retreat for a short time, which is called flat tide, and the middle time of flat tide is called high tide. The duration of flat tide varies from place to place, ranging from several minutes to dozens of minutes. After the flat tide, the tide level began to fall. When the tide level recedes to the lowest level, similar to the normal tide level, there is also a phenomenon that the tide level does not retreat or rise, which is called tide stop, and the middle time is low tide. After the tide stopped, the tide level began to rise again, so it moved again and again. The interval from low tide to high tide is called high tide, and the interval from high tide to low tide is called low tide. Generally speaking, in many places, the high tide is not as long as the low tide. The height when the sea surface rises to the highest position is called high tide, the height when it falls to the lowest position is called low tide, and the difference between adjacent high tides and low tides is called tidal range.

2. Tidal inequality and tidal types

I. Types of tides

It can be seen from the tidal observation curves in various places that there are periodic changes in high tide and low tide, high tide and tidal range. According to the period and tidal range of tidal fluctuations, tides can be roughly divided into the following four types:

1. A regular semidiurnal tide has two high tides and two low tides in a lunar day (about 24: 50). The tidal range from high tide to low tide and from low tide to high tide is almost equal. This tide is called regular semidiurnal tide (Figure 7-2a).

2. Irregular semidiurnal tides In most days of a new moon, each lunar day generally has two high tides and two low tides; However, for a few days (when the declination is large in the month), the second high tide is very small and the characteristics of semidiurnal tide are not obvious. This tide is called irregular semidiurnal tide (Figure 7-2b).

3. A normal diurnal tide has only one high tide and one low tide in lunar day. Tides like this are called regular diurnal tides, or regular diurnal tides (Figure 7-2c).

4. Irregular diurnal tide Figure 7-2D is the tidal process curve of irregular diurnal tide. Obviously, this tide has the characteristics of diurnal tide in most days of a new moon, but it has the characteristics of semidiurnal tide in a few days (when the declination of the month is close to zero).

Second, the inequality of tides.

Generally, the tidal range between two tides in a day is not equal, and so is the tide rising and ebbing. This irregular phenomenon is called the diurnal variation of tides. The higher climax is called high climax, and the lower climax is called low climax; Low tide is called low tide and high tide is called high tide.

It can also be seen from the tidal process curve (Figure 7-2) that the tidal range is different every day. The tidal range is the largest in two or three days after the first month of the lunar calendar and the second month of the lunar calendar, and the tidal range at this time is called the tidal range of the spring tide. On the contrary, after the upper and lower chords, the tidal range is the smallest, and the tidal range at this time is called the tidal range of small tides.

References:

Introduction to marine science