Marine Resource Management Hydrographic Module

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Marine Resource ManagementHydrographic Module

• Tides
• Dave Whitcombe

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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

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Why do we bother about tides?

• Bathymetry Data
• Produce charts/
• Produce Charts/Maps
• Dredging of channels
• Save Vessel Navigation
• etc .

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The price of getting it wrong ..
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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

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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 Earth S
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Tidal Variation

• Springs Neaps
• 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

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Springs and Neaps Cycle
Springs
New Moon
Full Moon
Neaps
1st Quarter
Last Quarter
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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

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Real Tides(1)
A storm surge with exceptional waves
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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

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

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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.
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Aberdeen Tide Gauge
The tide gauge is located on the south east
corner of Waterloo Quay, Aberdeen Docks
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Tidal Levels and Datums

• Tide Datum Related to Land Datum
• In UK Ordnance Datum Newlyn
• Equivalent to MSL 1915-1921 - 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

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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

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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

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Datums Along an Open Coast
Above Land Datum
MHWS
Fixed Land Levelling Datum
MLWS
Below Land Datum
Sounding Datum
Distance Along Coast
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Sounding Datums in an Estuary
River Mouth
Estuary
River
MHWS
MHNP
MSL
Land Datum
MLNP
MLWS
SoundingDatum
Below Land Datum
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Reduction of Soundings
Zero Roll on Echo Sounder( set to seal level
Transmission Mark (set at transducer depth
Height of Tide
Chart DatumLine
Raw Sounding
Reduced Sounding
Corrected Seabed Trace
Raw Seabed Trace
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Tide Levels and Definitions
Charted Height
Bench Mark
Highest Astronomical Tide (HAT)
Mean High Water Springs MHWS)
Datum for Heights
Mean High Water Neaps (MHWN)
Water Level
Ordnance Datum (Newlyn)
Mean Sea Level (MSL)
Mean Neap Range
Mean Spring Range
DryingHeight
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)
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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

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Example of Tide Prediction at Standard Port
HW Heights (m)
MEAN RANGES Springs 3.7m Neaps 1.8m
0
1
2
3
4
5
6
0.9
0.8
0.7
Chart Datum
0.6
Factor
0.5
0.4
0.3
0.2
0.1
0
1
2
3
LW Heights (m)
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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

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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

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UK Tidal Stream Diagram
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Tidal Stream Prediction

• From Admiralty Tide Tables
• Tide Stream Diamonds
• Table of Velocities and Directions
• Related to Time of HW at Local Standard Port

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Tidal Stream Prediction Example
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Tides - More information
www.nbi.ac.uk/home/insight/tidefaq.html www.nbi.a
c.uk/home/insight/tideinfo.html www.murorum.demon
.co.uk/sailing