0. Marine Resource Management Hydrographic Module
190203LN(M)AM2435GSDI-P1 End product
Marine Resource Management Hydrographic Module
Tides
Dave Whitcombe

1. Learning Outcomes Why do we need to know about Tides Tidal Forces
How/Why are Tides Generated
Tide Measurement Methods
Tidal Levels and Datums
Tidal Prediction
Tidal Streams

2. Why do we bother about tides?
Bathymetry Data
Produce charts/
Produce Charts/Maps
Dredging of channels
Save Vessel Navigation
etc …….

3. The price of getting it wrong …..

4. Why Do We Need to Observe Tides?
When we measure depth it is from a moving platform
A Vessel Mounted Echo Sounder
The vessel will move relative to the seabed due to :-
Waves
Tidal Rise and Fall
We need to remove the effect of tide so as to:-
Establish a fixed height reference irrespective of:-
Time
Date
Geographical locations
Changes in Sea Level

5. Tide Generation Due to Moon/Sun Gravitational Pull
Moon Period 24 hours 50 mins
Sun Period 24 hours Days
Actual Tides Result from the Resultant Positions of Sun and Moon
Complete Tidal Cycle 29 days N
Sun/Moon S N
Earth S

6. Tidal Variation Springs Neaps Neaps Equinox/Solstice
Give Maximum Tidal Range
Highest HW/Lowest LW
Neaps
Give Minimum Tidal Range
Lowest HW/Highest LW
Equinox/Solstice
Sun/Moon on equal /opposite declination
Maximum Range/Minimum Range
21st March/September - 21st June/December

7. Springs and Neaps Cycle
Earth S
Moon
Sun
New Moon
Moon N
Earth S
Sun
Full Moon
Neaps
Sun N
Earth S
Moon
Moon
Last Quarter
1st Quarter

8. Real Tides(1) External Influences - Distortions Land/Coastline Weather
Pressure - 10mb = 0.1m change in MSL
Wind - Piling up against shore
Storm Surges: Long Period/High Amplitude
Seiches: Short Period/Low Amplitude
Shallow Water
Currents

9. Real Tides(1)
A storm surge with exceptional waves

10. Resonance Natural Period of Resonance
Combination of Tidal Cycle + Resonance
Atlantic - 12 hours = Semi-Diurnal
Pacific - 24 hours = Diurnal
N/S of 65o Tides are Diurnal but
Gulf of Mexico = Diurnal
North Cape Norway = Semi-Diurnal
Semi-Diurnal 2HW + 2LW per day
Diurnal 1HW + 1LW per day

11. Tidal Periods 25 hours HW HW Diurnal Tide LW HW HW LW HW
Semi- Diurnal Tide LW LW HW HW
Diurnal Inequality
LLW
HLW

12. Tide Gauges To measure Rise and Fall of Tide at a Fixed Location
Tide Pole
Float Gauge
Bubbler Gauge
Microwave/Acoustic Gauge
For Offshore Tide Measurements
U/W Pressure Transducer

13. Tide Gauges UK National Network
The UK national network of sea level gauges was established after violent storms in the North Sea in 1953 resulted in serious flooding in the Thames Estuary.

14. Aberdeen Tide Gauge
The tide gauge is located on the south east corner of Waterloo Quay, Aberdeen Docks

15. Tidal Levels and Datums
Tide Datum Related to Land Datum
In UK = Ordnance Datum Newlyn
Equivalent to MSL Has risen by ~0.15m
MSL- Mean Sea Level
Basic Reference level for all tidal measurements
Obtained by:-
Mean of all Tide Readings over a long Period
e.g. 30 days to 18.6 years
MTL - Mean Tide Level
NOT = MSL
Mean of all HW and LW - not all tide readings

16. Tidal Levels MHWS/MLWS Mean High/Low Water Springs
Maximum Tidal Range Occurring at Spring Tides
MHWN/MLWN
Mean High/Low Water Neaps
Minimum Tidal Range Occurring at Neap Tides
MHHW/MLLW
Mean High High Water/Mean Low Low Water
Where Tide is Diurnal

17. Tidal Datums LAT - Lowest Astronomical Tide
Lowest Tide Level Predicted to Occur
Due only to Astronomical Conditions - Not Weather
Sounding Datum
Height Level to Which Soundings are Reduced in the Field I.e. during the survey operations
Chart Datum
Height Level to Which Soundings are Reduced on the Chart I.e. as defined by Hydrographic Dept.
Defined as level below which the tide will very seldom fall

18. Datums Along an Open Coast
Above Land
Datum
MHWS
Fixed Land
Levelling Datum
MLWS
Below Land
Datum
Sounding
Datum
Distance Along Coast

19. Sounding Datums in an Estuary
River
River Mouth
Estuary
MHWS
MHNP
MSL
Land Datum
MLNP
MLWS
Sounding Datum
Below Land
Datum

20. Reduction of Soundings
Zero Roll on Echo Sounder( set to seal level
Transmission Mark (set at transducer depth
Height of Tide
Chart Datum Line
Raw Sounding
Reduced Sounding
Corrected Seabed Trace
Raw Seabed Trace

21. Tide Levels and Definitions
Charted Height
Bench Mark
Highest Astronomical Tide (HAT)
Datum for
Heights
Mean High Water Springs MHWS)
Mean High Water Neaps (MHWN)
Water Level
Ordnance Datum
(Newlyn)
Mean Sea Level (MSL)
Mean Neap Range
Mean Spring Range
Drying Height
Mean Low Water Neaps (MLWN)
Height of Tide
Actual Sounding
Mean Low Water Springs MLWS)
Charted Depth
Chart Datum
Chart Datum
Lowest Astronomical Tide (LAT)

22. Predicting Tides Harmonic Method Rigorous Mathematical Formula
Admiralty Method
At Primary Port - from published Admiralty Tide Tables
At Secondary Ports - modifications - Simplified Harmonic
Tidal Differences and Ratios - Co-Tides
Establish Datum for Soundings in Offshore Areas
Use Co-Tidal Chart to Predict Tidal Information at Sea
Co-Tidal Lines MHWI Equal Time of HW
Co-Range Lines MSR Equal Tidal Range
Relative to Nearest Standard Port

23. Example of Tide Prediction at Standard Port
HW Heights (m)
MEAN RANGES
Springs 3.7m
Neaps 1.8m 1 2 3 4 5 6
0.9
0.8
0.7
Chart Datum
0.6
0.5
Factor
0.4
0.3
0.2
0.1 1 2 3
LW Heights (m)

24. Predicting Co-Tides for Positions at Sea
To Obtain Tides at Location:-
Compute Tide Heights/Times for Standard Port
Add Time Difference to Standard Port Times
Multiply Standard Port Heights by Range Ratio

25. Tidal Streams/Currents
Currents are mainly due to Meteorological Conditions
Mainly Wind
Tidal Streams result from:-
Astronomical Conditions
Horizontal Progression/Movement of Water
Effects of Coastline - Channels/Seabed Topography
Effect of Wind
Tidal Stream Published Information :-
Admiralty Charts
Tidal Stream Atlases
Sailing Directions

26. UK Tidal Stream Diagram

27. Tidal Stream Prediction
From Admiralty Tide Tables
Tide Stream Diamonds
Table of Velocities and Directions
Related to Time of HW at Local Standard Port

28. Tidal Stream Prediction Example

29. Tides - More information