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LESSON Tides and How
5.1 To Allow for Them
I
n this module we will be looking at the factors which cause the tides and how to calculate their effects. At first
this subject might seem a little daunting, but our easy explanation and methods will make things very clear and
simple.
Let’s make a start by looking at the amazing systems of tides and what causes them.
WHAT CAUSES THE TIDES
The major influences on the water surrounding the earth’s surface are the Sun and the Moon. The gravitational
pull of these bodies causes the water to have a tidal effect and this effect is maximised or minimised according to
the the relative positions of the Earth, Sun and Moon. The overall effect is also governed by the distance that Sun,
Moon and Earth are apart. As these bodies are in elliptical orbits you will begin to see that the systems can
become quite complicated. Add to this the fact that the earth consists of Continents, and very irregular land
masses, and you will see that the Tidal System can be, and is, very intrinsic.
However, from the user’s point of view tidal data is less complicated. It is derived from the history of water levels
at various places and therefore can be predicted to an extreme degree of accuracy.
As mentioned above the Sun and Moons gravitational effects cause the tides. The Moon being the nearer body
has the main influence. We will assume that the earth consists of all sea and no land. Looking at Fig 5-1 you will
see that the effect of sun and moon will cause the water to “hump” up on one side. In fact if you look at Fig 5-1
on the left hand side the moon is causing the ‘humping’ of water on the side nearest to it. Because of the laws of
equilibrium (we will not get into physics here) the water will also “hump” up on the side opposite to that of the
Moon.
MOON ORBITS EARTH EARTH SPINS ROUND
SUN
ONCE PER ONCE PER DAY
28 DAYS (APPROX)
FIRST QUARTER NEW MOON
LW
SPRINGS
NEAPS
MOON
HW NORTH HW
POLE NP
NEAPS
SPRINGS
LW
FULL MOON LAST QUARTER
Fig 5-1
Fig 5.1.1
The Moon takes approximately 28 days to orbit the earth and of course the earth is spinning on it’s own axis
once in 24 hours. Again looking at Fig 5-1, you will see that the Sun and Moon are in line with one another on
two occasions each month, New Moon and Full Moon.During this time the tidal influence is at its greatest and
will produce very high high tides,and very low low tides.
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NEAP RANGE SPRING RANGE
Charted heights of land objects
Heights of lights and cables are given above MHWS (Mean High
are measured above HAT (Highest Water Springs), which is the
Height of light Height of object
Astronomical Tide). average of all high waters.
HAT (Highest Astronomical Tide) HAT (Highest Astronomical Tide)
MHWS (Mean High Water Springs) MHWS (Mean High Water Springs)
Mean High Water Neaps
SPRING RANGE
NEAP RANGE
Height of Tide Tidal Rise
Depth
Tidal Rise Height of Tide
Mean Low Water Neaps MLWS (Mean Low Water Springs)
CHART DATUM LAT (Lowest Astronomical Tide) LAT (Lowest Astronomical Tide) CHART DATUM
The height of the tide is All Charted depths are The range of each tide All Charted depths are
how far above Chart Datum the measured below Chart is the difference between measured below Chart
tide stands at a given time. Datum (LAT). High and Low Water. Datum (LAT).
Chart Datum is at the level of
LAT.
Fig 5.1.2 Tidal Heights and Ranges
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Fig 5.1.2A Charted Depths and Drying Heights
MLWN
MLWS
DRYING HEIGHT
CHART DATUM LAT (LOWEST ASTRONOMICAL TIDE)
CHARTED DEPTH
For example:
3
7
( 16 )
Rock or Sandbank Rock or isolated point
Rock or Sandbank Showing drying height Showing drying height
awash at Chart Datum above Chart Datum above Chart Datum
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Fig 5.1.2A
HEIGHTS UNDER A BRIDGE
Mast clearance
Charted height of bridge
Mast height Total clearance
Above
waterline
HAT
Drop
MLWN
Air Draught (or clearance) under Bridges and Cables
As you can see from the diagram all clearances under Bridges and Cables are measured above
HAT (Highest Astronomical tide. The correct term for this is the Air Draught
On a recent trip to Norway on a cruise liner the ship had an air draught of 72 metres and the
height of a cable spanning the Fjord was 74 metres. From the deck it looked as though we
would hit the cable but the Captain and Norwegian Pilot had done their homework and the ship
cleared the cable with 2m to spare!
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Also twice per month the Sun and Moon are at right angles to one another, known as the first quarter and the last
quarter. This causes what we define as Neap tides where we have lower high waters and higher low waters.
This sounds complicated so have a look at Fig 5.1.2 which shows you the effect of Spring and Neap tides. You
will see here examples of what we call the Spring and Neap range of tides.
As a matter of interest, a Spring tide will not necessarily occur exactly at new or full Moon but about 2 days after
it. This is because the water does not respond to the gravitational pull of the Sun and Moon instantly, but in fact
lags for approximately two days.
Semi-Diurnal Tides
Around the UK we experience usually two high tides and two low tides each day. These are called Semi-Diurnal
Tides with successive High and Low waters being spaced just over six hours apart.
Now let’s have a more detailed look at Fig 5.1.2. You will see first the Spring and Neap Ranges of the tide.
Highest Astronomical Tide (HAT)
This is the the point above which all vertical clearances on a chart are shown. For example if the clearance height
of a bridge is 4 metres then it is 4m above HAT
All other heights on a chart are above MHWS. Underlined figures are drying heights above Chart Datum (see
below).
Mean High Water Springs (MHWS) & Mean High Water Neaps (MHWN)
These are the average or predicted heights of the Spring or Neap tides at High Water (HW) over a period of
18.6 years.
Mean Low Water Springs (MLWS) & Mean Low Water Neaps (MLWN)
These again are the average heights of Spring Low Water and Neap Low Water over the 18.6 year period.
Chart Datum and Lowest Astronomical Tide (LAT)
All Depths on a chart and Drying Heights are measured from the level of Chart Datum. This used to be from
MLWS but it was decided about 15 years ago to make this even safer. Therefore a new datum called Lowest
Astronomical Tide (LAT) was introduced. This is the lowest level to which the tide is ever predicted to fall (for
Astronomical reasons). It does not take into account any meteorological conditions which can effect the tide.
Chart Datum (LAT) therefore, can be defined as the level from which all heights of tide
are calculated and the level from which all depths on a chart are measured. Also any
areas that dry out (Sandbanks, Rocks etc.) are measured from this datum.
Range of the tide
This is the difference between the heights of High and Low Water. It can be either the Spring Range which is the
difference between MHWS & MLWS or the Neap Range which is the difference between MHWN & MLWN.
Now please take time to review the simulations on the next page:
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Now view the simulations shown below on your resource CD:
Tides SEARCH
Chart Datum SEARCH
Using Rule of Twelfths SEARCH
Calculating Height of Tide SEARCH
