Tidal Streams
and
Tides
An Explanation of the Tides
To the diver, tides are important because they affect
both the depth of water and generate horizontal water
movement. A rise and fall in water level can be expected in
all seaports. Almost everywhere, the tide will RISE from
LOW to HIGH water over a rough period of six hours, and
subsequently FALL again, to produce a succession of 4
tides over a period of just over 24 hours. The vertical
difference between LOW and HIGH water is known as the
RANGE of the tide, and is depicted below. The reason
that Tides are caused at all is due to the Gravitational pull
of the moon, and to a lesser extent, of the sun.
1
Tidal Streams & Tides
The Lunar Tide
According to Newton’s Law, the gravitational pull exerted by the Moon on that part of the Earth’s
surface nearest to the moon must be greater than the average exerted on the whole earth.
Consequently that pull exerted on the furthest part must be less.
These differences in attraction are termed LUNAR TIDE GENERATING FORCES
Only the water on the earth’s surface is free to move, and tends to be drawn away from those areas
where moonrise and moonset are occurring at B and D, causing a fall in tidal levels. It follows that
there will be a rise in tidal levels as water is drawn towards points A and C.
In the deep oceans, actual vertical tidal movement may be measured in inches, however it is the
presence of land-masses and changing sea bottom which can greatly amplify these tidal effects to
produce large ranges of tides which are common around the British Isles.
As the earth rotates once every 24 hours, it exposes a different side to the moon. For example, in 6
hours point A will have moved to point B and we would expect the tidal level to fall from HIGH WATER
to LOW WATER in that period.
It would also follow that tidal levels will rise again as the earth rotates to point C, and so on. We can
visualize from the diagram that in a period of just over 24 hours, there will be a succession of 2 high
and 2 low waters per day.
The Lunar Day
We set our watches to the SOLAR day of 24 hours, however the moon is the greatest tidal generating
force and has a LUNAR day of 24 hours 50 minutes approximately. For this reason the times of Low
and High water occur approximately 50 minutes later each day. The DURATION of one tide is very
roughly ¼ of a lunar day, or 6 hours 12 minutes; that is the time it takes for the water level to rise
from LOW to HIGH water, or vice versa with a FALLING TIDE.
The Solar Tide
The Sun also exerts a gravitational pull, but owing to the great distance between the EARTH and the
SUN, the SOLAR TIDE GENERATING FORCE is less than one half of those caused by the moon.
However, we would expect a rise in tidal levels at points directly beneath and opposite the sun, on the
earth’s surface, and a fall in tidal levels where SUNRISE and SUNSET are occurring.
2
Tidal Streams & Tides
The Phases of the Moon
As the EARTH and Moon orbit the sun and automatically alter their position in relation to the sun, so
their tidal generating forces interact, so producing varying RANGES of TIDES. The shape of the moon
as we see it in the sky, as it is struck by the sun’s rays, is a clue as to the actual PHASE of the moon, f
or the different phases have a direct bearing on the type of tide generated.
Ranges of Tides
When the Sun and Moon are in OPPOSITION (FULL Moon) as in Diagram 1, or in CONJUNCTION
(NEW Moon) as in Diagram 3; the two tide generating forces are acting together on the same line, and
their combined forces are at their greatest. Consequently, HIGH WATERS are particularly high and
LOW WATERS particularly low to produce the greatest RANGE OF TIDE. They are known as SPRING
TIDES.
When the sun and moon are at right angles to each other (in quadrature) their forces tend to
counteract each other and consequently the range of tide generated is less, as shown in diagrams 2
and 4 (the first & last quarters.)
3
Tidal Streams & Tides
Tidal levels do not always rise as high, or fall as low as they do during SPRING TIDES. These smallest
ranges of Tides are known as NEAP TIDES.
Neaps And Springs
The moon will orbit the EARTH approximately once a month (or 30 days), therefore it will follow that
the transition from SPRINGS at the time of the FULL MOON in diagram 1, to NEAPS at the time of
LAST QUARTER in diagram 2, will take approximately 7½ days. During this time period, there will be a
gradual reduction in the range of tide from the largest at Springs, to the smaller range at Neaps, and
so on.
Effect Of Lag
The Phases of the moon are calendar events, however there is a lag in response to the tide generating
force which means that in practice SPRING TIDES occur about 2 days after the NEW and FULL moon,
and for the same reason NEAPS also occur 2 days after the calendar event of the Moon’ s Quarters.
Actual response to tidal generating forces.
The magnitude and time lag of the response of the water to tide generating forces varies according to
terrestrial conditions, such as the depth, shape and size of the sea in which the port is located, so
that at no two places are the responses exactly the same.
Note : The Moon’s
orbit is elliptical and so its distance from the
Earth varies throughout the
month. When it is nearest to the Earth the Moon is
said to he In ‘Perigee’, and
when furthest away to be in ‘Apogee’. The greatest
attraction between Earth and Moon
occurs at Perigee and when this coincides with the time of
New of Full moon, the resulting spring tides are greater than average springs. As one complete orbit
takes approx. 27½ days the Moon will be nearly in Apogee when the next springs occur (about l5 days
after the Perigee tides). In consequence these tides will be less than average springs. It is of interest
that HIGH water Springs tend to occur at about the same time at any one place - i.e. DOVER 00:30 or
1230 hrs.
NOTE
As DIVERS, if we note the HEIGHT OF HIGH WATER in the LOCALITY, at least we will know the
maximum tidal increase we would encounter, adding this to charted soundings ON SITE, will determine
the MAXIMUM ACTUAL DEPTH.
4
Tidal Streams & Tides
TIDAL SUMMARY
Note the difference between HEIGHT, DEPTH, RISE and RANGE of the tide.
REMEMBER that SPRING TIDES occur a little after the NEW or FULL MOON, approximately every
15 days. NEAP TIDES occur approximately 71/2 days or halfway between each SPRING TIDE.
Tides are said to be TAKING OFF when decreasing, and MAKING when increasing to the spring range.
The duration of a tide is approximately 6 hours 12 mins., or ¼ of a lunar day of 24 hours, 50 mins.,
because there are generally 4 tidal levels. It follows that tides generally occur 50 mins. later each day.
However Tidal Duration may often be a 5-hour Flood, followed by a 7-hour Ebb.
How to Determine the ACTUAL DEPTH at any site, using Admiralty charts AND Tide Tables.
As Divers, we need to know the increase in water levels, caused by the tide, ABOVE CHARTED
SOUNDINGS.
Predictions given in tide tables of low and high water are HEIGHTS ABOVE CHART DATUM (Fig. 1).
We simply add predicted heights to charted depths to determine the ACTUAL DEPTH (Fig. 2).
It is relatively simple to determine the times and heights of tides for Standard and Secondary Ports
from the Tide Tables.
5
Tidal Streams & Tides
Standard Ports
Standard ports are placed where sufficient tidal information has been gathered, to enable future tidal
movements to be predicted, as in the example below.
Look at the
Spring range
Look at the
Neap range
Compare the times of successive neaps, then springs
Compare the times of H.W. on successive days
6
Tidal Streams & Tides
SECONDARY PORTS
Tidal data at places other than Standard Ports are included in Admiralty Tide tables, and detail time
and height differences between that place and the Standard Port of reference. These places are
known as Secondary Ports.
We know that H.W. Springs occur at approximately the same time at any one place. We can conclude
that other tidal levels also occur at about the same time. At Dover H.W. Springs occur approximately
at 0000 Hrs. and 1200 hrs., and L.W. Springs at 0700 and 1900 hrs. We can see that the other time
periods equate with the approximate neap tide period.
Time and height differences can be easily applied to the Standard Port. When times do not coincide
with the table, we can infer that we are in a transitional period (between neaps and springs) and make
simple adjustments.
In practise, these differences are often slight.
Tidal Information On Charts
Some information may be given on the chart in the form of average heights that can be expected at
various places during mean neaps and springs. From this information, average ranges at those places
can be determined. This information is not usually as accurate as Tidal Predictions from Tide Tables,
but they will give an approximation.
They do not show the TIMES of Low or High Water at those places, these will be shown under
Secondary Ports in the Tide Tables, or other sources.
Lat
Place
N
Chesil Cove
50° 34'
Portland
50° 34'
Lulworth Cove 50° 37'
Mulpe Bay
50° 37'
Swanage
50° 37'
Tidal Levels referred
Long Heights in metres
W
MHWS
MHWN
2° 28' 4.0
2.8
2° 26' 2.1
1.4
2° 15' 2.2
1.5
2° 13
2.2
1.5
1° 57' 2.0
1.6
to Datum of Soundings
above datum
MLWN
MHWS
Datum and remarks
1.7
0.6
0.8
0.1 0.93m below Ordnance Datum
1.0
0.2 1.02m below Ordnance Datum
1.0
0.2 1.05m below Ordnance Datum
1.2
0.5 1.40m below Ordnance Datum
7
(Newlyn)
(Newlyn)
(Newlyn)
(Newlyn)
Tidal Streams & Tides
To calculate depths at times other than Low or High Water
Note Well
Pick High & Low
water for the day
you want to dive
1.
2.
3.
4.
5.
6.
7.
8.
From the Tide tables select the date you wish to dive. (June 3)
Read off the low and high water heights (1.3m & 6.1m).
Transfer to the curve and join the points. (Black line)
Calculate H.W. not forgetting to add the hour for Summer Time. (10:12)
Select the dive time with respect to H.W. (07:30 for a fast drift)
Draw a line to the corresponding point on the curve (interpolate if necessary.)
Draw across to the tide height line
Read off the height above chart datum and add to the charted depth. (3.9m)
2) Link
points
5) Draw lines
6) Interpolate
if necessary
1) Mark heights
from tide table
7) Read
height
4) Chose
dive time
07:12 08:12 09:12 10:12
8
3) insert times
Tidal Streams & Tides
Having calculated the best time to dive, it may not coincide with Low or High Water, it follows it must
be between Low and High Water. In other words, within the RANGE of the TIDE.
The ACTUAL DEPTH will be - CHARTED DEPTH +
the height calculated from the tidal curve.
Tidal Rise and Fall
The Tide does not RISE and FALL at a uniform rate during the duration of the tide. Tidal movement at
first may be gradual, slowly building to a maximum then followed by a gradual reduction to SLACK
TIDE.
It follows that tidal height increase is not uniform, nor is the rate of the tidal STREAM.
At STANDARD PORTS, we would use the TIDAL CURVE. Having noted the TIME of HIGH WATER on
the day, we would compare this time with the REQUIRED THE TO DIVE. It will be so many hours
BEFORE, or AFTER HIGH WATER. We can then easily determine the Portion of the Range.
At Secondary Ports, provided we know the TIME OF HIGH WATER, and the RANGE AT THAT
LOCALITY, from the tables we could use the TIDAL CURVE of the referred STANDARD PORT; it will
usually prove to be sufficiently accurate.
SUMMARY
How to determine the actual depth at any site, using Admiralty tide tables and charts.
a) Note the required dive time (NOT FORGETTING TIME ZONE).
b) Using the tide tables, note the time and height of low and high water at the relevant port
(Standard or the Secondary Port) and note the time difference if the REQUIRED TIME does not
coincide with predicted times of low and high water on site.
c) Note the Hours before or after High Water it happens to be; and use the tidal curve to
determine the portion of the range (if you are using Admiralty Tables).
d) ADD the portion of the range to Low Water height, the Actual depth will then be this figure
added to the charted depth on site.
e) Remember to find the local range and duration of Tide, to enable you to compare the required dive
time to local law and high water times. You can use any method (Twelfths Rule, Approximate table,
tidal curve) to arrive at the portion of the range BETWEEN Low and High Water on the ACTUAL
DIVE SITE.
9
Tidal Streams & Tides
We can use ‘rule of thumb’ methods to simply predict the portion of the range (and in some cases the
TIDAL RATE).
1.
THE ‘TWELFTHS’ rule and the ‘THIRDS’ Rule.
As can be seen, if we assume
SLACK tide occurs at the time
of LOW and HIGH water, then
the Rate (in knots) will increase
in thirds of the maximum rate,
reaching the maximum at HALF
TIDE. TIDAL HEIGHTS will
increase by twelfths of the
total range. Again reaching the
maximum at half tide.
In practise, tidal movement
may not conform to this
movement at all.
2) THE RISE and FALL TABLE.
Here we need to know the tidal
range, and the required time to
enable us to determine the TIME
DIFFERENCE.
10
Tidal Streams & Tides
TIDAL STREAMS
If TIDES are the vertical movements of seawater, then Tidal streams are the periodical horizontal
movements of the sea in response to tidal generating forces. They are described by stating the
direction they are going towards (setting) and the rate of movement is described in knots. They flow
at their maximum rate during the springs and at their minimum rate during neaps.
A rising tide is commonly known as the FLOOD tide and a falling one the EBB tide. The rate of flow
may also vary during the duration of a tide. The stream is said to be SLACK at that instant when its
direction of travel is reversed. WE CALL THIS SLACK TIDE. If slack tide occurs at the time of low or
high water, the rate may steadily increase to its maximum at the half-tide period, then steadily
decrease up to the next turn of the tide. We can roughly calculate this movement by using the
THIRDS RULE.
However, slack tide may occur at any time during a tidal oscillation. In open channels, away from the
shore, the tidal stream may not necessarily turn at the time of low or high water, but commonly three
hours before and three hours after high water (as predicted at the standard port on the shore), or AT
ANY TIME.
Tidal characteristics commonly vary greatly from one locality to another. On Admiralty charts tidal
predictions of the stream are shown in the form of tidal diamonds. At these points, tidal data has been
made for a complete oscillation, and is shown in a table indicating tidal rate and direction. Because
times of tide vary each day, the predictions are made to a standard reference, the time of H.W. at a
specified Standard Port. When using diamonds, the times of high water at that STANDARD PORT
needs to be known.
Select the diamond nearest to your dive site. Looking in the table above for the minimum flow rate at
diamond ‘A’, Slack Tide is at –1hr H.W. and +5hr H.W. at the Standard Port However, at G it is very
different.
An example of a chart with tidal diamonds is shown below.
11
Tidal Streams & Tides
Tidal streams near the coast may follow the contours of the coast line, here tidal characteristics such
as duration, height, rate and direction will depend on local features, such as contours of the sea
bottom, depth and shape of coast line, etc.
Tidal Streams
How to Determine Tidal Rate at any site, using Admiralty Charts and Tide Tables.
i.
Choose the nearest, or most relevant, TIDAL DIAMOND.
ii.
Note the STANDARD PORT of REFERENCE, and from tables note the time of HIGH WATER
at THAT PORT, on the day. DO NOT FORGET the TIME ZONE (G.M.T. or SUMMERTIME).
iii.
From the tide tables determine the range on the day, then choose the appropriate NEAP or
SPRING rate from the Tidal Stream box on the chart. (or make simple adjustment).
iv.
Choose the best, or required time ON SITE, to AVOID DANGEROUS TIDAL MOVEMENT.
To calculate the Tidal Stream on admiralty charts using tidal diamonds, only two items need be known:a) The TIME of HIGH WATER at the Standard Port.
b) The Range of the Tide, on the day.
The diagram left indicates the general direction of the
main flood stream around the British Isles. The ACTUAL
direction in a particular locality may be considerably
different and we can refer to tidal diamonds as depicted
on charts.
However, Tidal direction on a chart may be indicated by
a winged arrow (1) to denote the FLOODSTREAM, and an
unwinged arrow (2) to denote the EBB STREAM. They
may also give approximate Neap and Spring rates.
1
12
2
Tidal Streams & Tides
Definitions
Terms in General use:Tides
The periodical vertical oscillations of the sea in response to the tidegenerating forces of the Moon and the Sun.
Tidal Streams
The periodical horizontal oscillations of the sea in response to the tidegenerating forces of the Moon and Sun.
High Water
The highest level reached by the sea during one tidal oscillation
Low Water
The lowest level reached by the sea during one tidal month.
Maximum Rate
The greatest rate reached in each of two more of less opposing directions by
the tidal streams in one oscillation.
Slack Water
The periods, preceding and succeeding maximum rate, when the tidal streams
are at their weakest.
Chart Datum
The low water plane to which the depths of features permanently covered by
the sea, and the heights of features periodically covered and uncovered by the
sea, are referred. The tide levels and predicted high and low water heights of
the tide are also referred to this plane. By international agreement, chart
datum should be a plane so low, that the tide will but seldom fall below it. The
heights of those features never, or rarely covered by the sea, are referred to
a high-water plane.
Ordnance Datum
The plane to which the heights of all features on maps are referred. It is the
datum of the land levelling system, and in most countries is based on mean sealevel at the point of origin of that system.
Height of the tide
The vertical distance between the level of the sea at any instant and chart
datum.
Range of the tide
The difference between the levels of successive high and low waters.
Rise of the Tide
The height of high water above chart datum
Mean Level
The average level of the sea as calculated from a long series of observations.
Mean sea level is the mean level of the sea at all stages of the tide. Mean Tide
Level is the level midway between Mean High Water and Mean Low Water. If
the frictional effects occurring in shallow and restricted waters distort the
tidal curve, Mean Sea Level and Mean Tide Level will differ.
Springs
The range of the semidiurnal tide varies mainly with the phases of the moon,
from new to full moon and vice versa. Springs are those semidiurnal tides of
greatest range, which occur in each of those periods.
Neaps
Those semidiurnal tides of least range, which occur in each period from new to
full, moon and vice versa.
Equinoctial
Spring tides which occur near the equinox. The tides have their greatest range
13
Tidal Streams & Tides
Springs.
at those equinoctial springs, when the moon is nearest to the earth.
Tropical Springs.
The range of the diurnal tide varies mainly with the declination of the Moon,
from maximum north or maximum south of the equator to zero declination.
Tropical Springs are those diurnal tides of greatest range, which occur in each
of these periods. Tropical derives from the fact that when the Moon has its
average maximum declination, it is over the tropic of Cancer or Capricorn.
M.H.W.S.
M.L.W.S.
Mean High Water Springs & Mean Low Water Springs:
The average heights of two successive high waters and of the corresponding
low waters at all spring tides.
M.H.W.N.
M.L.W.N.
Mean High Water Neaps & Mean Low Water Neaps:
The average heights of two successive high waters and of the corresponding
low waters at all neap tides.
Rectilinear Tidal
Streams.
Those tidal streams usually confined to rivers, estuaries and inshore waters,
which flow only in two more or less opposing directions during an oscillation.
Rotary Tidal
Streams.
Those tidal streams which flow at their maximum rates in two more or less
opposing directions, but which gradually turn from one to the other, and back
to the original direction, during oscillation.
Current
The horizontal movement of the water due to causes, mainly meteorological,
other than the tide-generating forces of the Moon and Sun. It may be a
progressive or a fluctuating movement.
Flow
The combination at any instant of tidal stream and current.
14
AN EXPLANATION OF THE MOON’S PHASES
Tidal Streams & Tides
When comparing Figures 1 and 2, remember that an observer looks from the Earth towards the
Moon. Thus, if Figure 1 is turned upside down to look at the Moon in Position 7, its
corresponding appearance in Figure 2 (not upside down) is obvious.
The Moon’s Phases are changes in the appearance of the Moon’s disc due to variations
in its position with reference to the Earth and Sun. Some knowledge of this is of good
practical use to the seaman because, at a single glance at the Moon he will know
without reference to books or tables, its phase, rough time of meridian passage and
the state of the tides in regard to Springs and Neaps.
Figure 2, when turned upside down, shows the appearance of the Moon at each phase (keeping
the same numbers as before) when seen from latitudes in which it passes North of the
observer at meridian passage.
Spring Tides do not occur in European waters until about 2 days after the New and Full
Moon. Similarly Neaps occur about 2 days after the Moon’s Quarters.
PHASES OF THE MOON
The Sun is so far away from us that, for all practical purposes, its fight is considered to
reach the whole of the Earth-Moon system in parallel rays.
Figure 1 looking down onto the North Pole of the Earth - shows 8 successive positions of
the Moon as it orbits the Earth in an anticlockwise direction. It also shows how at all times
the Sun’s rays illuminate one hemisphere of the Moon, whilst the opposite hemisphere is in
total darkness. The positions are numbered consecutively commencing at the New Moon,
Position 1.
Figure 2 illustrates the appearance of the Moon’s disc corresponding to each of the
positions, numbered 1 to 8, in Figure 1. This shows how the Moon looks to an observer in,
say, the British Isles or in any latitude from which the Moon bears South at its meridian
passage.
Position
No.
1
Moon’s Phase
Age
Remarks
New Moon
Days
0
Sun and Moon “in conjunction”. Moon not visible
because only the dark hemisphere faces the
Earth.
Visible as a crescent with its bow towards the
West. Waxing.
2
Between New
Moon and First
Quarter
3-4
3
First Quarter
7
4
Between First
Quarter and
Full Moon
Full Moon
11-12
Between Full
Moon and Last
Quarter
Last Quarter
18-19
Between Last
Quarter and
New Moon
25-26
5
6
7
8
15
22
Moon 90° East of Sun (in East Quadrature).
Visible as a half-disc with its bow towards the
West. Waxing.
Three quarters of the disc visible (called a
Gibbous Moon), the more rounded side towards
the West. Waxing.
Moon on Sun’s anti-meridian, i.e., “in opposition”.
The whole of the illuminated hemisphere is
visible.
Three quarters of the disc visible (called a
Gibbous Moon), the more rounded side towards
the East. Waning.
Moon 90° West of the Sun (in West Quadrature).
Visible as a half-disc with its bow towards the
East. Waning.
Visible as a crescent with its bow towards the
East. Waning.
Figure 1 Successive positions (1 to 8) of the Moon along its orbit round the Earth
Figure 2 Phases of the Moon as viewed from the Earth’s surface
15
Tidal Streams & Tides
16