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Marine Resource Management Hydrographic 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 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 Period24 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

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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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N Earth S Springs and Neaps Cycle Sun Springs Neaps Moon N Earth S Sun Moon N Earth S Sun Moon New Moon Full Moon Moon 1 st QuarterLast 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 65 o Tides are Diurnal but Gulf of Mexico = Diurnal North Cape Norway = Semi-Diurnal Semi-Diurnal2HW + 2LW per day Diurnal1HW + 1LW per day

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Tidal Periods LW HW 25 hours HW LW HLWLLW HW Diurnal Tide Semi- Diurnal Tide Diurnal Inequality

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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 MHWS MLWS Fixed Land Levelling Datum Above Land Datum Below Land Datum Distance Along Coast Sounding Datum

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Sounding Datums in an Estuary MHWS MLWS Land Datum Below Land Datum Sounding Datum River MouthEstuaryRiver MHNP MSL MLNP

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

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

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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 LinesMHWIEqual Time of HW Co-Range LinesMSREqual Tidal Range Relative to Nearest Standard Port

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Example of Tide Prediction at Standard Port HW Heights (m) LW Heights (m) 0123 0123456 MEAN RANGES Springs3.7m Neaps1.8m Chart Datum 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Factor

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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.ac.uk/home/insight/tideinfo.html www.murorum.demon.co.uk/sailing

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