Hydrographic Surveys

Surveying deals with water bodies

mainly for the construction of

hydraulic structures, dredging and

navigation

- Prakash Kumar Sekar

Uses of Hydrographic survey

• Preparation of nautical charts

• Assesing silting in harbours

• For planning dredging

• Determination of countour

• For lake and river survey

• For design of projects like dams, bridges,

reserviours, marine structues, ports

harbours, offshore strucruresPrakash Kumar Sekar

Charts And Maps

• Hydrographic Chart

– an information medium and a tool for maritime

traffic for the safety and ease of navigation

– contains information on

• least water depths

• nature of ocean bottom

• sea and swell conditions

• surface navigational dangers

• other navigational significant dataPrakash Kumar Sekar

Soundings

• Sounding

– Measurement of the vertical depth from the

level surface of the water to the bed of the lake,

river or sea

– a series of soundings whether taken at random

points or on a grid can be used to prepare a

plan showing the topographic features of the

land covered by the water

Prakash Kumar Sekar

Tides

• Caused by the combined gravitational

effects of the sun and moon, with the moon

having the major effect

– influenced by

• terrestrial gravity

• earth’s rotation

• land masses

• weather systems

Prakash Kumar Sekar

Prakash Kumar Sekar

Prakash Kumar Sekar

Prakash Kumar Sekar

Tides - Semi-Diurnal Tide

• Consider the earth to be stationary and

covered with a layer of water. Assume that

the position of the moon is fixed such that it

has zero declination

Prakash Kumar Sekar

Tides - Semi-Diurnal Tide• On the side of the earth nearest the moon there is a net

force towards the moon

• whilst on the opposite side there is a net force away from

the moon.

• The small net forces at the North and South poles will be

towards the centre of the earth.

Prakash Kumar Sekar

Prakash Kumar Sekar

Tides - Semi-Diurnal Tide• At the two points on the equator, nearest and farthest from

the moon there will be permanent high water.

• At right angles to these points a band of low water will lie

along the meridian on either side of the earth

Prakash Kumar Sekar

Tides - Semi-Diurnal Tide• When the moon moves into perigee (closest point to the

earth during the moon’s elliptical orbit) the high waters

will be higher and the low waters will be lower.

• The opposite effect occurs when the moon moves out to

apogee (most distant point)

• These minimum and maximum tides are known as

perigean and apogean respectively

Prakash Kumar Sekar

Tides - Semi-Diurnal Tide• When the earth rotates the four tides, two high and two

low, move around the earth, in a 24 hour period.

• This is known as a semi-diurnal tide

• the poles have a permanent low tide

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Tides - Diurnal Tide• The moon's declination causes inequalities in the tides

that occur.

• successive high tides at a point on the earth with a

latitude equal to X (and Y) will NOT be equal.

– The high tide at X will not be as high as that at Y

Prakash Kumar Sekar

Tides - Diurnal Tide• At all points on the earth where the latitude on earth is

greater than the moon’s co-declination, there is only one

high tide and one low tide per day.

• This is known as the diurnal tide

• The form of tide varies between the extremes of diurnal

and semi-diurnal, with those exhibiting both

characteristics being known as 'mixed'

Moon’s co-declination

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Tides - Spring Tide• The sun has a similar tide producing effect on the earth to

that of the moon. However the forces involved are not as

great.

• At new and full moon the sun, moon and earth are nearly in

a straight line. The tide raising forces act together to

produce tides with a large range. These tides occur

fortnightly and are called spring tides

Prakash Kumar Sekar

Prakash Kumar Sekar

Prakash Kumar Sekar

Tides - Neap Tide• At the moon's first and last quarter the moon's tide raising

force is to some extent counteracted by the sun's tide

raising force producing tides with a small range.

• These tides are called neap tides (neap being high water

at its lowest value).

Prakash Kumar Sekar

Prakash Kumar Sekar

Soundings

• Tide Levels

– Mean Sea Level ( MSL )

• average height of the sea in all states of the

oscillation

• equivalent to the level which would exist in the

absence of all tidal forces

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Soundings

• Tide Levels (cont)

– Mean Tide Level ( MTL )

• average value of the heights of high and low water

– Mean High Water Springs ( MHWS ) and Mean

Low Water Springs ( MLWS )

• average values derived from a sufficiently long

series of high water springs and low water springs

Prakash Kumar Sekar

Soundings

• Tide Levels (cont)

– Mean High Water Neaps ( MHWN ) and Mean

Low Water Neaps ( MLWN )

• average values derived from a sufficiently long

series of high water neaps and low water neaps

Prakash Kumar Sekar

Tide Gauges - Tide Board

• merely a vertical staff with a broader face than a levelling

staff. This may be graduated every 5 or 10 centimetres,

according to the accuracy required. The tide board is

often difficult to read due to the surface chop or wave

action

Prakash Kumar Sekar

Tide Gauges - Float Gauge

• A float is attached to the bottom of a staff.

• The staff is then held in a box which is fixed in a vertical

position.

• Only the bottom of the box is open to the sea.

• The float rises and falls with

the tide, the staff running up

and down through guides on

the inside of the box.

• The staff can be read through a

special inspection opening in

the side of the box.

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Tide Gauges - Automatic Tide Gauge

• normally established permanently at an official tide

station.

• The tidal fluctuations are recorded on a chart attached to

a drum which revolves with time

• The gauge may need to be

visited only once every seven

days to change the paper chart

and reset the drive mechanism,

i.e. wind the clock

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Tide Gauges - Water Pressure Tide Gauge

• Operates on the changes of water pressure due to tide

rising and falling

• completely self contained instrument designed to

measure and record tidal movements when mounted on

an underwater offshore structure or on the sea bed

• Due account must be made for

barometric pressure reading

and the necessary corrections

applied

Prakash Kumar Sekar

Establishing Tide Levels

• If tide observations are made over a period of time,

statistical values for various types of tide can be arrived

at, such as mean sea level (MSL), MHWS, MLWN, etc.

• The degree of variation that can occur at a point for

observations taken over different time periods will of

course depend upon the range of the tide at that point.

Prakash Kumar Sekar

Soundings

• Sounding Equipment

– Sounding Rod

• 5m long, plate or shoe on end to prevent sinkage

into soft bottom

• commonly used in creeks, rivers, shallow dams or

lakes

– Lead Line

• lead weight attached to the line’s end to take it to

the bottomPrakash Kumar Sekar

Soundings

• Sounding Equipment (cont)

– Sonar Equipment

• Sound Navigation Ranging

• measure the range of an object by timing the two

way journey of pulse of sound energy and

converting the result to units of distance

Prakash Kumar Sekar

Soundings - Echo Sounder

– sounding device utilising a

fixed beam with a vertical

axis

– shape and width of the beam

varies

Prakash Kumar Sekar

Soundings - Echo Sounder

• Recorder

– paper record or trace

– digital display

– punch tape

– cassette recorder

– digital readout to a

computer

– several components

1

2

3

4

5

6

7

Prakash Kumar Sekar

Soundings - Echo Sounder

• Recorder

– paper record or trace

– digital display

– punch tape

– cassette recorder

– digital readout to a

computer

Prakash Kumar Sekar

Soundings - Echo Sounder– The shape of the beam is

assumed to be a cone

– the frequency of the sound signal

determines the beam width and

the reflective and penetrative

capabilities of the pulse

– the wider the beam width the

wider the lane widths and a

lesser number of runs are

required

Prakash Kumar Sekar

Soundings - Echo Sounder– the width of the beam varies with

the water depth

• possible to miss a prominent high

point if the line spacing does not

allow for any overlap

– If a wide beam is used on a

sloping seabed then an incorrect

depth which (will be that of the

first returning signal) will be

recorded for the depth

immediately under the boatPrakash Kumar Sekar

Soundings - Echo Sounder– Thus for precise work it is essential that a

narrow beam is used.

• disadvantage is an increase in the number of lines

necessary to cover the same area in order to achieve

a saturated examination.

Prakash Kumar Sekar

Soundings - Echo Sounder– A dual frequency echo sounder using a narrow

beam width frequency together with a wide

beam width frequency are used at the same

time to overcome this problem.

Prakash Kumar Sekar

Soundings - Reduction of Echo

Sounder Traces

– Calibration

• Squat

– determined by

» sailing over an area of known depth at different

speeds - the different depth readings record the

differences due to squat

» a staff is set up at the bow and the stern of the vessel

- the vessel is sailed past at different speeds and a

level used to read the staves - the squat is

determined from the differing levels

Prakash Kumar Sekar

Soundings - Reduction of Echo

Sounder Traces

• The soundings at each fix are not

reduced separately.

Prakash Kumar Sekar