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Marine Resource Management Hydrographic Module

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Marine Resource Management Hydrographic Module

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    1. Marine Resource Management Hydrographic Module Tides Dave Whitcombe

    2. 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

    3. Why do we bother about tides?

    4. The price of getting it wrong …..

    5. 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

    6. 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

    7. 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

    8. Springs and Neaps Cycle

    9. 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

    10. Real Tides(1)

    11. 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

    12. Tidal Periods

    13. 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

    14. Tide Gauges UK National Network

    15. Aberdeen Tide Gauge

    16. 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

    17. 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

    18. 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

    19. Datums Along an Open Coast

    20. Sounding Datums in an Estuary

    21. Reduction of Soundings

    22. Tide Levels and Definitions

    23. 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

    24. Example of Tide Prediction at Standard Port

    25. 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

    26. 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

    27. UK Tidal Stream Diagram

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

    29. Tidal Stream Prediction Example

    30. Tides - More information

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