Joint accounting for drift and current with manual graphical reckoning. Tidal currents. Ebb and flow See what "tidal currents" are in other dictionaries

Tidal or tidal currents are called periodic horizontal movements of water masses that occur when a tide occurs under the influence of the forces of the Moon and the Sun. They constitute the second integral side of a single complex phenomenon of the tide.

These currents differ from all other currents that arise in the oceans and seas in that they capture the entire thickness of the water masses from the surface to the bottom, only slightly reducing their speed in the bottom layers, where friction against the bottom already affects

It must be noted that the nature of the movement of tidal currents near the coast and in the open sea is different.

Near the coast, especially in narrow straits, bays, bays, river mouths, etc., tidal currents are reversible (reversible) in nature, since tidal and ebb currents are reversed in direction.

For a semidiurnal flow, the movement proceeds at an increasing speed for about 3 hours, then at a decreasing speed for the next 3 hours, after which its direction changes to the opposite, and the cycle repeats.

For a daily current, movement in one direction occurs for 12 hours. In the first half of this period, the current goes at an increasing speed, and in the second - with a decreasing one. A change in the direction of reverse currents occurs either near the moment of high or low water, or at an average level. It depends on whether the tidal wave will have the character of a standing or forward wave.

When changing reverse currents, there are moments when no current is observed. During these hours, the water is at rest. In our northern seas, this phenomenon is called short water.

In the open sea or in the middle parts of fairly wide bays, tidal currents have a slightly different character. There is no so-called change of currents. Tidal currents never stop, but their direction, along with the change in the speed of the currents, continuously changes clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere. The currents in such places “bypass” the entire compass card for either 12 hours and 25 minutes (with a semi-diurnal tide) or 24 hours and 50 minutes (with a diurnal tide). Currents of this nature have received the special name of rotating.

The orbits of water particles described by rotating currents can be simple, almost circular, and can be complex, closed, curved figures.

Information about tidal currents is given in sailing directions, on navigation charts, in special tables and atlases. Tidal currents in different places vary differently relative to high and low water, but in navigational aids (tables and atlases) they are usually given for every full hour relative to high water at some point taken as the main one.

The difference between the zero depth and the high water level is called the high water height hHW. The difference between the zero depth and the low water level is called the low water height hMB. The difference between the heights of high water and the low one following it is called the magnitude of the tide B \u003d hPV hMV. The time between two adjacent moments of high or low water is called the high tide period.

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rocky classic navigation. Location.

18. Tides

and their accounting in navigation.

The surface of the oceans is not at rest, but periodically changes its position fluctuates. This happens under the influence of various processes and forces, which can be combined into the following main groups:

Geodynamic and geothermal phenomena in the earth's crust earthquakes and seaquakes, volcanic eruptions (tsunamis), uplift and subsidence of land (tectonics), heat transfer through the ocean floor.

Mechanical and physico-chemical effects on the surface of the ocean - solar radiation, changes in atmospheric pressure, wind, which causes surge oscillations, precipitation, coastal runoff, etc.

Cosmic (astronomical) tide-forming forces, which are the main ones in tidal phenomena.

The concept of tides and terminology

Tidal phenomena are complex wave movements of ocean water masses. The consequence of these movements are periodic changes in the level and currents.

Tidal phenomena arise as a result of the action between the Earth, the Moon and the Sun of tide-forming forces. The tide-forming force of the Moon is 2.17 times greater than the tide-forming force of the Sun (due to remoteness), therefore, the main features of tidal phenomena are determined mainly by the mutual position of the Earth and the Moon.

A significant influence on the magnitude and nature of tidal phenomena in each particular place is exerted by physical and geographical conditions: depths, coastline outlines, the presence of islands, and others. Due to the influence of physical and geographical conditions, the nature of the tides can vary over a very wide range. So, in the Baltic Sea they are practically absent, in the Bay of Fundy, located at approximately the same latitude, level fluctuations reach 18 meters.

Tidal phenomena are characterized by two main factors:

level changes;

Tidal currents.

Both sides of this process are interconnected, however, due to the lack of a unified theory, tidal fluctuations in the level and tidal currents are studied separately.

Tidal phenomena have a great impact on navigation and safety of navigation, therefore information about them is regularly published in special manuals. In order to use them correctly for solving various navigational tasks, navigators must have a good understanding of the nature of this phenomenon.

Tidal fluctuations can be depicted graphically.

On the graph of the daily tide along the abscissa, the time t , and along the y-axis, the height of the tide, h , above the conventionally accepted level depth zero, 0gl.

The process of sea level rise is called high tide , low low tide .

The highest level at high tide is called full water PV, low tide low water MW .

The difference between zero depth and high water level is calledfull water height h PV.

The difference between the zero depth and the low water level is calledlow water height h MV.

The difference between the heights of full water and the next low water is calledtide

B \u003d h PV - h MV.

For zero depth on Russian sea charts in seas with tides, the lowest theoretical level (LTL) is adopted the lowest level possible according to astronomical conditions, that is, according to the relative position of the Earth, Moon and Sun.

The time between two adjacent moments of high or low water is calledtide period.

Depending on the length of the period, tides are divided into diurnal, semidiurnal, mixed, irregular semidiurnal, irregular diurnal, and anomalous.

per diem tides (C) those in which the average period is equal to a lunar day (24 hours 50 minutes). Diurnal tides occur most often in the Pacific Ocean.

Semi-daily tides (P) those whose period is equal to half a lunar day (12 hours 25 minutes). Semidiurnal tides are observed along the Murmansk coast of the Barents Sea, in most of the White Sea and almost throughout the Atlantic Ocean.

At semidiurnal tides, high water occurs twice a day, MW, and twice low water, MW. Since both PVs and both MVs have different heights, they are designated as follows:

ERW high high water;

LEL low high water;

WWII high low water;

LMW low low water.

The heights of SH and MW of semidiurnal tides above zero depth are denoted as follows:

h ERW height of high full water;

h LEL high low water height;

h WWII height of high low water;

h NMV Height of low low water.

mixed tides are those in which during the lunar month the period changes from semi-diurnal to diurnal. Mixed tides are divided into irregular diurnal (IC), in which the diurnal period predominates, and irregular semidiurnal (ID), in which the semidiurnal period predominates.

anomalous tides are those in which the nature of the rise and fall of water is complicated by shallow water, these are diurnal shallow (SM) and semidiurnal shallow (PM). Abnormal tides are observed in some ports of the English Channel and in the White Sea.

The magnitude of the tide B varies during the month, and on some days it reaches its maximum value, and on others it reaches its minimum. The magnitude of the tide varies according to the phase of the moon, that is, it depends on the relative position of the Earth, the Moon and the Sun.

The highest high water and the lowest low water, that is, the maximum tide (B) is observed after full moons and new moons, that is, when the Earth, Moon and Sun are approximately in the same straight line, and the tide-forming forces of the Moon and the Sun add up. Such periods are called syzygy (gr. sizigia connection).

The lowest high water and the highest low water, that is, the minimum tide, is observed after I and after IV quarters in the phases of the moon. At this time, the Moon and Sun are approximately at right angles to the Earth, and the tidal forces of the Sun weaken the tide-generating forces of the Moon. Such periods are called quadrature (lat. quadratura fourth part, quarter).

The tides are also influenced by the declination of the moon. At high declinations of the moon, tides are called tropical , and when the Moon passes through the equatorequatorial.

The time interval between the moment of the upper or lower culmination of the Moon and the moment of the onset of full water on a given meridian is calledlunar interval Tl.

The average of the lunar intervals in the days of syzygy, calculated from a large number of observations, is calledport application hour FC.

The following terms are used to characterize tides in time:

t SW high water moment;

t MW moment of low water;

T p level rise time time from the moment of low water to the moment of high water:

T p \u003d t PV t MV;

T p level drop time time from the moment of high water to the moment of low water:

T p \u003d t MV t PV;

T st level standing time time during which the level, having reached a certain height, remains unchanged.

Russian tide tables

Tidal phenomena in different regions of the world's oceans have not been studied in the same way. Depending on the degree of knowledge, all items are divided into three groups:

Major points (ports) for which detailed tide data are available.

Additional points linked to the main ones, for which the calculation of tides is made through the main point.

Points for which an applied clock is given, from which the moments of MW and MW and their heights can be calculated, based on the moments of the Moon's climax.

The Oceanographic Institute annually publishes Tables from which it is possible to predict the moments and heights of the tides. Tide tables are published in four volumes:

Volume I . Waters of the European part of Russia.

Volume II . Waters of the Asian part of Russia.

Volume III . foreign waters. Atlantic, Indian and Arctic Oceans.

Volume IV . foreign waters. Pacific Ocean.

Volume I and Volume II Each consists of three parts:

Part I - Tides at major points.

Part II - Amendments for additional items.

Part III - Tidal currents.

Volume III and Volume IV Each consists of two parts:

Part I - Main points.

Part II Additional items.

At the beginning of each volume, general information about the tides is given, and at the end, auxiliary tables and an alphabetical index of items.

The General Information section provides the following information:

Influence of hydrometeorological conditions on tides;

Basic terms and designations;

Information about the inequality of tides;

Criteria that determine the nature of the tides;

Examples of using tide tables.

There may be differences in general information in tide tables from different years of publication, so it is necessary to get acquainted with them each time when using new tables.

In I the part "Tides at the main points" shows the moments and heights of high and low water for each day of the given calendar year for the main points, the list of which is given in alphabetical order on the back of the cover of the table.

In II of the part "Corrections for additional points" the corrections of the moments and heights are given, introducing which into the selected from the part I information about tides in the main port, it is possible to obtain data on the moments and heights of MW and MW at additional points.

In the "Auxiliary tables" are given:

Interpolation table for calculating the level for intermediate moments between MV and PV;

Mean heights of spring and quadrature SH and SH and mean sea level (MSL) for some locations;

Tables of corrections for mean sea level for seasonal changes and atmospheric pressure;

Tables for converting standard time into local time;

Feet to meters conversion tables;

Astronomical data (phases, declination, perigee and apogee of the Moon).

Tasks solved by tables

Determination of the moment and height of high and low waters at the main point.

Determination of the height of the tide level at any intermediate moment between the MW and MW at the main point.

Determination of the moment and height of high and low waters in an additional paragraph.

Determination of the height of the tide level at any intermediate moment between the MW and MW at an additional point.

Teacher of the highest category Kisenkov Vladimir Ilyich

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Tidal fluctuations in the level of the ocean are accompanied by a horizontal movement of water masses, which is called the tidal current. Therefore, the navigator must take into account not only the change in depths, but also the tidal current, which can reach a significant speed. In areas where there are tides, the boatmaster must always be aware of the height of the tide and the elements of the tidal current.

The tides allow ships with deep draft to enter some ports located in shallow bays and estuaries.

In some places, the tides are intensified by surge phenomena, which leads to a significant increase or decrease in the level, and this, in turn, can lead to accidents for ships that are under cargo operations at the berths or in the road.

The nature and magnitude of the tides in the oceans are very diverse and complex. The magnitude of the tide in the ocean does not exceed 1 m. In coastal areas, due to the decrease in depth and the complexity of the bottom topography, the nature of the tides changes significantly in comparison with tides in the open ocean. Near straight coasts and headlands protruding into the ocean, the tide ranges from 2-3 m; in the coastal part of the bays and with a strongly indented coastline, it reaches 16 m or more.

For example, in Penzhina Bay (Sea of ​​Okhotsk), the tide reaches 13 m. At the Soviet coast of the Sea of ​​Japan, its height does not exceed 2.5 m.

In the seas, the height of the tide depends on what connection the given sea has with the ocean. If the sea protrudes far into the land and has a narrow and shallow strait with an ocean, then the tides in it are usually small.

In the Baltic Sea, the tides are so small that they are measured in centimeters. The height of the tide in Calais is 7 cm, in the Gulf of Finland and Bothnia about 14 cm, and in Leningrad about 5 cm.

In the Black and Caspian Seas, the tides are almost imperceptible.

Tides in the Barents Sea are semidiurnal.

In the Kola Bay, they reach 4 m, and near the Iokan Islands - up to 6 m.

In the White Sea, the tides are semidiurnal. The highest tide height is observed on the Tersky coast in the throat of the sea, where at the Orlovsky lighthouse it reaches 8.5 meters, and in the Mezen Bay - up to 12 m. In other areas of this sea, the tides are much less; so, in Arkhangelsk about 1 m, Kemi - 1.5 m, and Kandalaksha - 2.3 m.

The tidal wave, penetrating into the mouth of the rivers, contributes to fluctuations in their level, and also significantly affects the speed of water flow in the mouths. So, often the speed of the tidal current, prevailing over the speed of the river, changes the course of the river to the opposite.

Winds have a significant effect on tidal phenomena.

A comprehensive study and accounting of tidal phenomena is of great importance for the safety of navigation.

The current that is directed in the direction of the movement of the tidal wave is called tidal, the opposite is called ebb.

The speed of the tidal currents is directly proportional to the magnitude of the tide. Consequently, for a certain point, the speed of the tidal currents in syzygy will be much greater than the speed in quadrature.

With an increase in the declination of the Moon, as well as when the Moon moves from apogee to perigee, the speed of the tidal currents increases.

Tidal currents differ from all other currents in that they capture the entire thickness of water masses from the surface to the bottom, only slightly reducing their speed in the bottom layers.

In straits, narrow bays, and near coasts, tidal currents have a reverse (reversible) character, i.e., the tidal current is constantly directed in one direction, and the ebb current has a direction directly opposite to the tidal one.

In the open sea, far from the coast, and in the middle parts of fairly wide bays, there is no sharp change in the direction of the tidal current to the opposite, i.e., the so-called change of currents.

In these places, a continuous change in the direction of the current is most often observed, and a change in the current by 360 ° occurs with a semidiurnal tide pattern in 12 hours 25 minutes and with a diurnal tide pattern in 24 hours 50 minutes. Such currents are called rotating currents. The change in the direction of the rotating currents in the northern hemisphere, as a rule, occurs clockwise, and in the southern hemisphere - counterclockwise.

The change of the tidal current to the ebb current and vice versa occurs both at the time of high and low waters, and at the time of the average standing of the level. Quite often, a change of currents occurs in the time interval between high and low water. When the tidal current changes to ebb and reverse, the current velocity is zero.

The general scheme of tidal currents is often disturbed by local conditions. Accounting for the tidal current, as already mentioned above, is of great importance for the safety of navigation.

Data on the elements of tidal currents are selected from the Atlas of tidal currents, and for some parts of the seas, from tables placed on navigation charts. General indications about the currents are also given in the sailing directions of the seas.

Relatively constant currents are shown on the maps by arrows. The direction of each arrow corresponds to the direction of the current in this place, and the numbers above the arrow show the speed of the current in knots.

The direction and speed of tidal currents are variable, and in order to reflect them on the map with sufficient completeness, not one arrow is needed, but a system of arrows - a vector diagram.

Despite the visibility of vector diagrams, they unnecessarily load the map and make it difficult to read. To avoid this, the elements of tidal currents are usually shown on the map in the form of tables placed on free places on the map. A complete table is a table that contains the following data:

Watch for high water at the nearest tidal point; the inscription "Full water", corresponding to zero hours, is placed on

In the middle of the column, from it upwards, the numbers of hours to high water are put in ascending order, and downwards, also in ascending order, the numbers of hours after high water;

Geographical coordinates of points, usually denoted by the letters A; B; AT; G etc. ; the same letters are placed in the corresponding places on the map;

Elements of currents: direction in degrees and speed in syzygy and quadrature in knots (accurate to 0.1 knots).

The determination of the speed and direction of the current at a given moment in a given place according to the Atlas is found as follows.

First, according to the Atlas, the main port for a given place is determined, after that, using the Table of Tides (part I), they find the time of high water closest to the given one, calculate the time interval (in hours) before or after the moment of high water in the main port relative to the given moment. Then, for a calculated period of time before the onset or after the moment of full water, the direction of the current (in degrees) and speed (in knots) are found in the Atlas.

When sailing, the elements of tidal currents must be determined in advance; it is recommended to compile a table of currents for pre-calculated moments (in 1 hour) corresponding to the numbered places of the ship.

Below is an example of a table of tidal currents (Table 7).

Periodic fluctuations in the water level are caused by the manifestation of the periodic forces of attraction of the Moon and the Sun - the so-called tide-forming forces. Tidal fluctuations in level cover almost the entire coast of the World Ocean, and for brevity they are called tides. Thus, tidal phenomena are dynamic processes in the waters of the seas and oceans (including level fluctuations).
The limiting range of tidal level fluctuations at each point is between the highest and lowest theoretical levels calculated by calculation. Depending on the duration of the tidal cycle, there are:
- semidiurnal tides (P) - with a period of approximately half a day, i.e. having two full and low waters during the day;
- diurnal tides (C) - having high and low water during the day;
— irregular semidiurnal (ID) — with a noticeable diurnal difference in the values ​​of the corresponding level extrema;
- irregular diurnal (NS) - diurnal tides, which, at low declinations of the Moon, become semidiurnal with a significant decrease in their magnitude;
- mixed tides - irregular semidiurnal and (or) irregular diurnal tides.
Tides are characterized by the following inequalities:
- daily inequalities in height, which are the difference in heights of two successive high or low waters (for different points, daily inequalities have different values ​​- from a subtle difference in the heights of adjacent high or low waters to the complete disappearance of one high and one low water);
- semi-monthly inequalities in the heights and magnitudes of the tides (half-monthly inequality, depending on the phase of the moon, is most clearly manifested in semi-diurnal tides). During the full moon, the magnitude of the semidiurnal tides is maximum - the so-called spring tides occur. In the first and third quarters, the tides are the smallest - quadrature tides come;
- semi-monthly inequality, depending on the declinations of the Moon and the Sun (tropical inequality) is usually the main one in daily and irregular daily tides, with large declinations of the Moon, the tides are called tropical and are large, during the passage of the Moon through the equator, the tides are called equatorial and have small quantities;
- monthly inequality of tides (parallactic) manifests itself depending on the distance between the Earth and the Moon (perigee). The minimum values ​​of tides are observed at the greatest distance between the Earth and the Moon (apogee).
The main navigation aid containing pre-calculated levels for the World Ocean are tide tables. There are calendar-type tide tables, published annually on calendar dates, and permanent tables, calculated for many years. The precalculated levels in the tables are given for the seas of Russia relative to the lowest theoretical level (LTL), and for foreign waters relative to zero depths, which are accepted on foreign maps.

Depth is indicated at point B on the map: GLK = 4.7m; tPV = 03h27m; hMV =0.5m; tMV = 12h27m. Determine: the height of the water hset at tset = 10h12m and the actual depth at that moment.

Learned to read tide charts, count. But this is not enough for comfortable yachting. In yachting, you need to be able to take into account the direction and speed of the tidal current in planning your trip. This is what we will do now. Let's imagine that the Earth is absolutely round and all covered with water. If so, then the two formed "humps" on opposite sides of the Earth will gradually increase and decrease under the influence of the attraction of the Moon and the Sun, and the Earth will make its full revolution under this water blanket in one day. The speed of this movement at the equator will be: the length of the equator / 24 hours = 900 knots!

However, don't be alarmed, this is the speed of the wave, not the speed of the water. In the absence of continents, everything would be very calm - small changes in water levels and the almost complete absence of tidal currents. The situation changes radically, as we noted earlier, when interacting with the coastline. All the huge energy of the tides falls on the shores of the continents and, depending on the shape of the coastline and the topography of the seabed, raises water to a height of up to 17 meters in Fundy Bay (USA and Canada). It is obvious that such rises of water are accompanied by a very rapid movement of huge masses of water and, accordingly, by a swift and changing direction. And when the narrow passages between the islands get in the way of the tide, the current reaches a speed of 20 knots.

Just as with tide levels, humanity has accumulated enough information to predict the direction and speed of tidal currents.

For us, besides, of course, computer programs, there are two main sources of information about the current. These are already well-known tide charts (almanacs), containing Atlases of tidal currents, and tidal rhombuses.

Tide maps. Atlases of tidal currents.

Tide charts and tidal current atlases give a very clear picture, or rather 12 pictures: they are like frames from a cartoon showing a consistent change in the direction and strength of the current in your chosen zone. These "shots" seem to be taken one hour apart, providing coverage for the entire high tide period. In the figure, for example, a reversal of the tide map from the REEDS almanac for the Channel Islands is given.
Please note that the time axis in this example is set relative to HW in Dover (the port relative to which the time is set is called the Reference Port).

The time axis is always relative to HW in the Reference Port.
To determine the real time to which each picture belongs, you first need to get the HW value closest to the time you are interested in in the Reference Port from the tide charts (almanac), and then sign the time value to which it refers under each picture. Do not forget about the possible adjustment of summer time and, most importantly, that a port that is not only far from the place of interest to you, but even located in a different time zone can be selected as a standard port.

Make no mistake with the timing! The error can be several hours, and therefore, it can be critical!
Now let's look at one square of the tide map in more detail. Obviously, the arrows in the figure indicate the direction of the flow. It is intuitively clear that the longer and thicker the arrow, the stronger the tidal current, but the four-digit number separated by a comma next to the arrow needs clarification. You and I know perfectly well that at the same time, both in the syzygy and quadrature, the direction of the current should coincide, but the speed of the tidal current will be different, since more water should pass into the syzygy in the same time.

So, the mysterious inscription next to the arrow on the tide maps shows the values ​​\u200b\u200bof the tidal current velocity in syzygy and quadrature listed separated by commas. For better readability of tide charts, cartographers try to save signs, so the speeds are indicated in tenths of a knot. Thus, the inscription 58.97 tells us that at this time in this place the speed of the current is 5.8 knots squared, and 9.7 knots syzygy. Agree that, for example, such a record is 5.8; 9.7 - would be much more cumbersome.

Please note: these are real values, it is not for nothing that the proud inscription Race of Alderney flaunts on the tide maps in this place and there is a warning Heavy Overfalls! Avoid appearing in such places with strong tidal currents and headwinds. The collision of two elements generates large crashing waves. Be prepared for the whirlpools to turn the boat around at any moment, and if you are sailing, beware of involuntary gybes.

An inquisitive reader should have a question: how to determine the speed of the tidal current if we are planning a transition not to a syzygy or quadrature, but on another day? The answer is simple: you need to solve an interpolation problem. How this is done, we will consider below.

Another designation you will find on tide charts is slack. As follows from the translation, the tidal current is absent or negligible.

For example, let's determine when there will be a favorable current for leaving Alderney Island from Vgaue Bay if we go to Cherbourg on September 20, 2012. The crossing distance is about 30 miles. And if we assume our speed is 6 knots and do not take into account the speed of the tidal current, then we need 5 hours. If you plan the crossing so that the current is favorable all the way, then you can save a lot of time and fuel if you have to go under the engine. And if we make a mistake and end up there when the current is rushing at a speed of 7 knots towards Jersey Island, then we simply cannot cope with the current, and we will be carried south, where we will have to wait for the tidal current to change.

Based on the foregoing, it seems appropriate” to have the time to leave Alderney corresponding to the picture of the tide charts, on which HW-5 is written. At this time, the current is already becoming favorable in direction and will intensify, these are pictures HW-4, HW-3 and HW-2.

Now I need to solve the following problem: what will be on the clock in Alderney when I am in Dover (Reference Port) will be HW-5?
To do this, open the tide maps (almanac) for Dover for the date of interest to us - let it be September 20th. First of all, make sure that Alderney is in the same time zone as Dover. HW on this day falls on 0101 and 1326. We are interested in the time five hours earlier, and we still need to add one hour of daylight saving time, i.e. auspicious time to go:

0101-0500+0100=2101 of the previous date

1326-0500+0100=0926 today.

Decide when it is better for you to go - at night yesterday or in the morning today!

These are not all methods for determining the direction and speed of the tidal current. Many boaters prefer the simpler method. . But more on that in the next article.