01 tidal theory

8/8/2019 01 Tidal Theory

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To give the student an understanding of:

` The main causes of Tides;

` The creation of Spring and Neap Tides;

` Causes of differences between PredictedHeights of Tides and Actual Heights of Tides;

` Calculations involving Underwater and

Overhead Obstructions, Shoreside Structures,

Leadlines & Echo Sounders.


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The Student will be able to:

` Explain how tides are created;

` Explain how Spring and Neap Tides are created;

` Explain the causes of the variation betweenPredicted and Actual Heights of Tides

` Perform Calculations involving Underwater and

Overhead Obstructions, Shoreside Structures,

Leadlines and Echo Sounders


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` The main factors causing tides is the combined effect of the

gravitational forces exerted on the earth by the moon and also by the

sun

` The approximate ratio of these forces can be represented as 7 : 3respectively

` Tide is also caused by the centrifugal forces produced by the earth

and the moon


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HW

LW

Point A

Under ideal conditions the earth is considered to have a uniform covering ofwater with no landmass

The difference in the gravitational and centrifugal forces exerted on the earths

surface by the moon causes the water to pile up towards the moon and also inthe hemisphere opposite to the moon

The earth is effectively rotating in an ellipsoid of water and point A will becarried round as the earth rotates

As the earth rotates once every 24 hours we would expect point A to see twoHWs and two LWs. This also applies to every part of the earth

LW

HW


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However, as the moon is in orbit around the earth, each daythe moon moves further along in its orbit and the earth

takes an extra 50 minutes to be back in conjunction withthe moon

Therefore a Lunar day is 24hours 50mins

HW

LW

P

LW

HW

P


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The time between two HWs is approximately 12hrs 25mins

The time between successive HW and LW is approximately

6hrs 12mins

This means that on some days it is possible to experience

only one HW or LW


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Spring Tides The tides with maximum range are known as Spring Tides

Spring tides occur at fortnightly intervals

They occur under the following circumstances:-

Opposition Conjunction

P SUN

New Moon

+7

+3

Full Moon

+7

The above figure represents the relative positions of the Sun and Moon at Spring tides

This is when the tidal generating forces of the Sun & Moon are acting together toproduce the highest High tides and the lowest Low tides

The Moon is said to be either in conjunction or opposition


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Neap Tides The tides with minimum range are known as Neap Tides

They also occur at fortnightly intervals

They occur under the following circumstances:-

Quadrature

Quadrature

PSUN

+3

1st. Quarter

+7

+7

LW

HW


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Neap Tides

1st. Quarter

P

SUN

+7 +3

+7

The figure represents the relative positions of the Sun and Moon at Neap tides

This is when the tidal generating forces of the Sun & Moon are acting at right angles toeach other so that effectively a lower High water and a higher Low water is produced

The Moon is said to be at Quadrature

The Moon exerts agreater influencein the ratio 7 : 3

The effect of theSun is to reducethe resultantheight of tide

HW

LW

Last Quarter


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` The predicted heights and times are shown in the Admiralty Tide Tables(ATT)

` The predictions are based on observations over at least one year and

are calculated for average meteorological conditions

` Other factors which affect heights of tide but which cannot be takeninto consideration in the tables are barometric pressure and wind


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Accuracy of Predictions - Survey

Predictions are usually based on a

survey period of at least 1 year,

preferably 3 years can be as short as3 weeks.

How recent the survey is.


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` Sailing Directions give the average barometric pressure for various

areas and if the actual pressure is less than this the sea level will tend

to be higher

` When the pressure is greater than the average the sea level will tend

to be lower


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` An onshore wind will pile up the water against the coast

`

An offshore wind will push the water away from the coast andtherefore tend to lower the coastal water level

` Winds blowing along the coast will set up long waves (storm surges)

which will raise or lower the level depending upon the position of the

crest or trough


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Exercises involving UnderwaterExercises involving Underwater

RestrictionsRestrictionsWaterWater InformationInformation Ship InformationShip Information

Shoal

WL

Chart Datum

Charted

Depth

draught

UKC

HoT


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RestrictionsRestrictions Example 1Example 1

oal

5.0m

4.0m

U

2.0m


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oal

5.0m

4.0m

U

2.0m

arted ept 5.0 m

Ht of Tide +2.0 mTotal ept 7.0 m

raug t -4.0 m

learance 3.0 m


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Charted Depth . m

Ht of Tide + . mTotal Depth 7. m

Draught - . m

Clearance 3. m

Shoal

WL

CD

. m

. m

UKC

. m

The Underkeel clearance is 3.0m


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RestrictionsRestrictions Exercise 1Exercise 1

oal

6.2

.6

U

.9

arted ept 6.2

Ht of Tide + .9

Total ept 1 .1raug t - .6

learance 2.

The under-keel clearance is 2.5m


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RestrictionsRestrictions ExampleExample

Shoal

WL

CD

3. m

8. m

1. m

HoT


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RestrictionsRestrictions ExampleExample

oal

.0m

.0m

.5m

oTraug t .0 m

learance + .5 m

Total ept .5 m

arted ept - .0 m

t of Tide .5m

The Height ofTide is 6.5m above Chart Datum


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RestrictionsRestrictions ExerciseExercise

oal

6.

.

.5

oT

raug t .

learance + .5

Total ept .7arted ept -6.

t of Tide .



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oal

4.0m

.5m

.5m

eig t of Tide


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Draught . mClearance 1. m

Total Depth . m

Drying Height (- -) +1. m

Ht of Tide 7. m.

Shoal

WL

CD

. m

1. m

1. m

Height of Tide



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1. Charted depth of shoal 3. m Height of tide 3. m Draught . m. Find the clearance over the shoal.Charted Depth 3. m

Ht of Tide +3. mTotal Depth . m

Draught - . mClearance 1. m

The under-keel clearance is 1.0m

Shoal

WL

CD

3. m

1. m

.7m

Height of Tide

Draught 3. m

Clearance +1. m

Total Depth . m

Drying Height (- -) + .7 m

Ht of Tide 7.3 m.



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Bill of Portland Lighthouse - Chart


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Bill of Portland Lighthouse - ALOL


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Springs and NeapsSprings and Neaps

NOAA National Ocean Service Education:NOAA National Ocean Service Education:Tides and Water LevelsTides and Water Levels


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MeasurementsMeasurements of Lighthousesof Lighthouses --

ALOLALOL


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Problems involving ShoresideProblems involving Shoreside

StructuresStructures


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Exercises involving ShoresideExercises involving Shoreside

StructuresStructures ExampleExample

CD

MHWS

8. m

3 . m

. m

SL

Heightof Light

Charted Height of light 3 . m. Height of MHWS 8. m. Height of tide . m.

Find the height of the light above the water.

Charted Ht of Lt 3 . m

MHWS + 8. m

Total Ht of Light .7 m

Height of Tide - . m

Ht of Lt above W/L 3 . m

The Height of the Light is 34.2m above the Sea Level


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StructuresStructures ExerciseExercise

CD

MHWS

.1m

. m

3. m

SL

Heightof Light

Charted Ht of Lt . m

MHWS + .1 m

Total Ht of Light .3 m

Height of Tide - 3. m

Ht of Lt above W/L .7 m



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StructuresStructures ExampleExampleA Light is 1. m above the water. The Elevation of the light is charted as 37. m.The height of MHWS is 8.7m. Find the height of tide.

CD

MHWS

8.7m

37. m

HoT

WL

1. m

Elevation of Lt 37. m

MHWS + 8.7 m

Total Ht of Light . m

Ht of Lt above W/L 1. m

Height of Tide .3 m



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StructuresStructures Exercise 5Exercise 5

CD

MHWS

1 . m

3 . m

. m

WL

Height

of Light

Height of light 3 . m. Height of MHWS 1 . m. Height of tide . m.

Find the height of the light above the water.

Charted Ht of Lt 3 . m

MHWS +1 . m

Total Ht of Light . m

Height of Tide - . m

Ht of Lt above W/L 3 . m



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` Charts published prior to 2005 measurement ofBridges and Electrical Wires from MHWS

` Charts published after2005 measurement ofBridges and Electrical Wires from HAT

` SQA measurement of Bridges and ElectricalWires from MHWS


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StructuresStructures Exercise5

Exercise5

CD

MHWS

5. m

2. m

HoT

WL

.7m

Elevation of Lt 2. m

MHWS + 5. m

Total Ht of Light 8.2 m

Ht of Lt above W/L .7 m

Height of Tide 1.5 m



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Problems involving OverheadProblems involving Overhead

ObstructionsObstructions


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ObstructionsObstructions Example 6Example 6

Charted Ht of Bridge 17.0 m

MHWS + 7. m

Total Ht of Bridge24

. mClearance - 2.0 m

Air Draught - 22.0 m

Height of Tide 0. m

The Maximum Height of

Tide is 0.6m


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ObstructionsObstructions Exercise 6Exercise 6

Charted Ht of Bridge 15.0 m

MHWS + 5.3 m

Total Ht of Bridge 20.3 m

Clearance - 1.0 mAir Draught - 18. 0 m

Height of Tide 1.3 m

The Maximum Height ofTide is 1.3m


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ObstructionsObstructions Example 7Example 7

Charted Ht of Bridge 14. m

MHWS + 7.4 m

Total Ht of Bridge 22.0 m

Height of Tide - .3 m

Air Draught -1 2.5 m

Clearance 3.2 m

The Clearance between the Mast and the Bridge is 3.2m


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ObstructionsObstructions Exercise 7Exercise 7

Charted Ht of Bridge 12. mMHWS + 5.3 m

Total Ht of Bridge 17. m

Height of Tide - 4.1 m

Air Draught - 8.3 m

Clearance 5.5 m

The Clearance between the Mast and the Bridge is 5.5m


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` Measures the Total Depth of Water

` If the HoT is subtracted, the Charted Depth is

found.

` HoT is therefore known as the Correction to theLeadline

` Useful for the creation of Charts


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Problems involving the LeadlineProblems involving the Leadline


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Example 8Example 8

Shoal

SL

Chart Datum

Charted

Depth

.7m

8.0mSounded Depth 8.0

mHt of Tide - .7 m

Charted Depth 1.3 m

The Charted Depth is 1.3m below Chart Datum


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Example 9Example 9

Shoal

SL

Chart at

Chart

th

5.

4.5

Soun th 4.5

Ht of Ti -5.

Chart th -1.2= rying H ight 1.2

The Drying Height is 1.2m above Chart Datum


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` Can be used to measure either:(a) Depth Under the Keel, or(b) Total Depth of Water.

` Care must be taken to ensure that theNavigator knows whether it is (a) or (b).

` If using Total Depth of Water, the correct up to

date draught must be entered into the EchoSounder


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Echo Sounder Measuring DepthEcho Sounder Measuring Depth

under the Keelunder the KeelWater Information Ship Information

Shoal

WL

Chart Datum

Charted

Depth

draught

Sounding

/UKC

HoT

Water Information Ship Information


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Echo Sounder Measuring TotalEcho Sounder Measuring Total

Depth of WaterDepth of Water

Shoal

L

Chart atum

Chart

th

Depthof

ater

oT

Water Information Ship Information


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` There may be several transponders, e.g. Fwd &Aft occasionally Port & Stbd.

` Care must be taken to know which Transponderis being used.

` If using the Fwd Transponder whilst the vesselhas a large stern trim the underkeel clearancecould be misleading!


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Problems involving theProblems involving the

EchoEcho--SounderSounder Example 10Example 10

Shoal

WL

Chart Datum

20m

5.2m

23.4m

HoT

Draught 5.2 mSounding +23.4 m

Total Depth 28. m

Charted Depth -20.0 m

Ht of Tide 8. m

The Height ofTide is 8.6m


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Complete Course Paper 1 starting in class,completing as homework.

Answers in Tides Course Papers Book at the frontof the Class

Answers also on Moodle

Good luck!

01 tidal theory

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