Marine Resource ManagementMarine Resource ManagementHydrographic ModuleHydrographic Module

Tides

Dave Whitcombe

Learning OutcomesLearning 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

Why do we bother about tides?Why do we bother about tides?

Bathymetry Data

• Produce charts/

• Produce Charts/Maps

• Dredging of channels

• Save Vessel Navigation

• etc …….

The price of getting it wrongThe price of getting it wrong …..

Why Do We Need to Observe Why Do We Need to Observe Tides?Tides?

When we measure depth it is from a moving platformA Vessel Mounted Echo SounderThe vessel will move relative to the seabed due to :-WavesTidal Rise and Fall

We need to remove the effect of tide so as to:-Establish a fixed height reference irrespective of:-TimeDateGeographical locationsChanges in Sea Level

Tide GenerationTide Generation

Due to Moon/Sun Gravitational PullMoon Period 24 hours 50 minsSun Period 24 hours DaysActual Tides Result from the Resultant Positions of Sun and MoonComplete Tidal Cycle 29 days

N

Sun/Moon

SN

Earth

S

Tidal VariationTidal Variation

Springs NeapsGive Maximum Tidal RangeHighest HW/Lowest LW

NeapsGive Minimum Tidal RangeLowest HW/Highest LW

Equinox/SolsticeSun/Moon on equal /opposite declinationMaximum Range/Minimum Range21st March/September - 21st June/December

N

Earth

S

Springs and Neaps CycleSprings and Neaps Cycle

Sun

Springs

Neaps

Moon

N

Earth

S

SunMoon

N

Earth

S

Sun

Moon

New Moon

Full Moon

Moon 1st QuarterLast Quarter

Real Tides(1)Real Tides(1)

External Influences - DistortionsLand/CoastlineWeather

–Pressure - 10mb = 0.1m change in MSL–Wind - Piling up against shore–Storm Surges: Long Period/High Amplitude–Seiches: Short Period/Low Amplitude

Shallow WaterCurrents

Real Tides(1)Real Tides(1)

A storm surge with exceptional waves

ResonanceResonance

Natural Period of ResonanceCombination of Tidal Cycle + Resonance

Atlantic - 12 hours = Semi-DiurnalPacific - 24 hours = DiurnalN/S of 65o Tides are Diurnal butGulf of Mexico = DiurnalNorth Cape Norway = Semi-Diurnal

Semi-Diurnal 2HW + 2LW per day

Diurnal 1HW + 1LW per day

Tidal PeriodsTidal Periods

LW

HW HW

25 hours

HW HW

HW HW

LW

LWLW

HLWLLW

HW

Diurnal Tide

Semi- Diurnal

Tide

Diurnal Inequality

Tide GaugesTide Gauges

To measure Rise and Fall of Tide at a Fixed LocationTide PoleFloat GaugeBubbler GaugeMicrowave/Acoustic Gauge

For Offshore Tide Measurements U/W Pressure Transducer

Tide Gauges Tide Gauges UK UK NNational ational NNetworketwork

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.

Aberdeen Tide GaugeAberdeen Tide Gauge

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

Tidal Levels and DatumsTidal Levels and Datums

Tide Datum Related to Land DatumIn UK = Ordnance Datum NewlynEquivalent to MSL 1915-1921 - Has risen by ~0.15m

MSL- Mean Sea LevelBasic Reference level for all tidal measurements

Obtained by:-Mean of all Tide Readings over a long Periode.g. 30 days to 18.6 years

MTL - Mean Tide LevelNOT = MSLMean of all HW and LW - not all tide readings

Tidal LevelsTidal Levels

MHWS/MLWSMean High/Low Water SpringsMaximum Tidal Range Occurring at Spring

Tides

MHWN/MLWNMean High/Low Water NeapsMinimum Tidal Range Occurring at Neap

Tides

MHHW/MLLWMean High High Water/Mean Low Low

WaterWhere Tide is Diurnal

Tidal DatumsTidal Datums

LAT - Lowest Astronomical TideLowest Tide Level Predicted to OccurDue only to Astronomical Conditions - Not Weather

Sounding DatumHeight Level to Which Soundings are Reduced in the Field I.e. during the survey operations

Chart DatumHeight 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

Reduction of SoundingsReduction of Soundings

Raw Seabed Trace

Zero Roll on Echo Sounder( set to seal level

Chart DatumLine

Transmission Mark (set at transducer depth

Corrected Seabed Trace

Raw Sounding

Height of Tide

Reduced Sounding

Tide Levels and DefinitionsTide 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 DatumChart Datum

Ordnance Datum

(Newlyn)

Datum for

Heights

Bench Mark

Charted Height

Hei

ght o

f T

ide

Act

ual S

ound

ing

Mea

n N

eap

Ran

ge

Mea

n Sp

ring

R

ange

Charted Depth

DryingHeight

Predicting TidesPredicting TidesHarmonic 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

Example of Tide Prediction at Example of Tide Prediction at Standard PortStandard Port

HW Heights (m)

LW Heights (m)

0 1 2 3

0 1 2 3 4 5 6MEAN RANGES

Springs 3.7m

Neaps 1.8m

Ch

art Datu

m

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Factor

Predicting Co-Tides for Positions Predicting Co-Tides for Positions at Seaat Sea

Tidal Prediction for Position 57° 10’ N 00° 00’ E

At Required Location :From Chart 5058

MHWI(Locn) 1h 10m MSR(Locn) 1.95

At Selected Port :Aberdeen

MHWI(SP) 0h 56m MSR(SP) 3.7m

Time Difference :

MHWI(Locn) - MHWI(SP)

+0h 14m

Range Ratio :

MSR(Locn) / MSR (SP)

0.53

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

Tidal Streams/CurrentsTidal Streams/CurrentsCurrents are mainly due to Meteorological

ConditionsMainly Wind

Tidal Streams result from:-Astronomical ConditionsHorizontal Progression/Movement of

Water

Effects of Coastline - Channels/Seabed Topography

Effect of WindTidal Stream Published Information :-

Admiralty ChartsTidal Stream AtlasesSailing Directions

UK Tidal Stream DiagramUK Tidal Stream Diagram

Tidal Stream PredictionTidal Stream Prediction

From Admiralty Tide TablesTide Stream DiamondsTable of Velocities and DirectionsRelated to Time of HW at Local

Standard Port

Tidal Stream Prediction ExampleTidal Stream Prediction Example

Tides - More informationTides - More information

www.nbi.ac.uk/home/insight/tidefaq.html

www.nbi.ac.uk/home/insight/tideinfo.html

www.murorum.demon.co.uk/sailing