Tide and time wait for no man ...

Tide and time wait for no man ...

An introduction, some definitions and refreshers

Chart datum is the reference level above which heights of tide are predicted and below which charted depths are measured. The datum used for most British ports is the lowest sea level predicted under average meteorological conditions for that port - unlike the land mapping datum for the ordinance survey which is fixed for the whole U.K. and is based on mean sea level (see footnote).

Range is important for tidal barrages and is the difference in height between successive High and Low waters.

Tides arise from the gravitational pull of the moon and sun on the major oceans of the world. The biggest tides occur when the sun, moon and earth are on the same axis and are called spring tides and occur about every two weeks. In the intervening week the sun - world - moon axis become at right angles -
resulting in a reduced gravitational pull on the oceans. These tides are known as neap tides and the range is about 64% of the spring range. In the Bristol Channel spring tides occur in the evening and early morning and neaps at midday and midnight.

Tidal predictions are for average conditions and in practice can be affected by meteorological conditions such as atmospheric pressure and gales.

So why are tides in some locations much higher/lower than the general levels? The main reason is the shore configuration e.g. the coast of South Wales and England together with the shelving sea bottom. The narrowing and shelving would not affect the level if the rate of rise was very slow but for the momentum of the thousands of tons of water entering at speeds up to 4 or 5 knots. Other effects occur in long closed estuaries and channels which have a natural wave resonance period which when coinciding with the tidal period gives a large amplification of the tide.

Together with changes in tidal heights come tidal currents or tidal streams. Off Portishead streams reach 4.8 knots at springs and 2.6 knots at neap tides. Near Lynmouth on the N.Devon coast, streams reach 4 and 1.9 knots.

The establishment of a UK Tide Gauge Network* (UKTGN) was recommended as a result of severe flooding along the east coast of England in 1953 and it is now owned and funded by the Environment Agency (EA).

*part of the National Tidal & Sea Level Facility which is hosted by the National Oceanography Centre (NOC) - a consortium consisting of NOC science groups, the British Oceanographic Data Centre (BODC) as well as the UK Tide Gauge Inspectorate.

The network comprises 45 gauges - related through the national levelling network to Ordnance Datum Newlyn.



Definitions:
Chart Datum (CD)
CD is the reference level from which heights of tide are predicted and charted depths are measured. In the UK it normally approximates to LAT, and the tide will not frequently fall below it. The actual depth of water in any particular position is the charted depth plus the height of tide.

Lowest Astronomical Tide (LAT)
LAT is the lowest level which can be predicted under average meteorological, and any combination of astronomical, conditions. This level will not be reached every year. Storm surges can cause even lower levels to be reached.

Highest Astronomical Tide (HAT)
HAT is the highest level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions, except storm surges. It is the level above which vertical clearances under bridges and power lines are measured; see 4.5.

Ordnance Datum (Newlyn)
Ordnance Datum (Newlyn) is the datum of the land levelling system on mainland England, Scotland and Wales, and to which all features on UK land maps are referred. The difference between Ordnance Datum (Newlyn) and CD is shown at the foot of each page of tide tables in this Almanac. Differences between CD and foreign land levelling datums are similarly quoted.

Charted depth
Charted depths are printed on charts in metres and decimetres (0.1m) and show the depth of water below CD. (Not to be confused with a sounding which is the actual depth of water (charted depth + height of tide) in a particular position.)

Drying height
A drying height is the height above CD of any feature which at times is covered by water. The figures, in metres and decimetres, are underlined on the chart. The depth of water over a drying feature is the height of tide minus the drying height. If the result is negative, then the feature is uncovered at that time.

Vertical clearances
Under bridges and power lines - these are measured above HAT. Some older charts may still show clearances above MHWS; see 4.5.

Elevation of lights
The charted height of a light (its elevation) is measured above MHWS.

Height of tide
The height of the tide is the vertical distance of the sea level above (or very occasionally below) CD. Predicted heights are given in metres and decimetres.

Rise/Fall of tide
The Rise of the tide is the amount the tide has risen since the earlier Low Water. The Fall of a tide is the amount the tide has fallen since the last High Water.

Duration
Duration is the time between LW and the next HW, normally slightly more than six hours. It can be used to find the approximate time of LW when only the time of HW is known.

Interval
The interval is a period of time quoted in hours and minutes before (–) or after (+) HW. Intervals are printed in hourly increments (–6hrs to +6hrs) along the bottom of each tidal curve diagram in Chapter 9.

Spring tides
Spring tides occur roughly every 16 days, near to Full and New Moon, when the tide–raising forces of Sun and Moon are at a maximum. See 4.9 for phases of the Moon.

Neap tides
Neaps occur roughly every 16 days, near the Moon's first and last quarters, when the tide–raising forces of Sun and Moon are at a minimum. See phases of the Moon.

Mean High Water and Low Water Springs/Neaps
MHWS and MHWN are the means of predicted HW heights of Sp or Np tides over a period of 18.6 years. Similarly, MLWS and MLWN are the means of LW heights for Sp and Np tides respectively. Mean tide level (MTL) is the mean of the above values.

Mean Sea Level (MSL or ML)
This is the average level of the sea's surface over a long period, preferably 18.6 years.

Range
The range of a tide is the difference between the heights of successive HWs and LWs. Spring range is the difference between MHWS and MLWS, and Neap range is the difference between MHWN and MLWN.

Standard Ports
have tidal characteristics observed over a long period and are suitable as a reference for secondary ports on the adjacent coasts.

Secondary Ports
Have similar tidal characteristics to those of their Standard Port. Time and height differences are applied to the Standard Port predictions. 'Secondary' does not imply lesser importance.

Tidal Coefficients
Indicate the range of a tide. Daily values are listed and explained in 9.0.9.

Tidal definitions


Footnote: In Great Britain, Ordnance Datum (OD) for the Ordnance Survey is ODN (Ordnance Datum Newlyn), defined as the MSL at Newlyn in Cornwall.