The document discusses hydrographic surveys, which are essential for constructing hydraulic structures, dredging, and navigation, and includes applications such as preparing nautical charts and assessing silting in harbors. It explains the concepts of tides, soundings, and tide gauges, including methods for measuring water depth and fluctuations in tide levels due to gravitational effects from the sun and moon. Various types of tides, such as semi-diurnal, diurnal, spring, and neap tides, are explored, with emphasis on techniques used in measuring and recording these phenomena.

1. Hydrographic Surveys
Surveying deals with water bodies
mainly for the construction of
hydraulic structures, dredging and
navigation
- Prakash Kumar Sekar

2. 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

3. 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 data
Prakash Kumar Sekar

4. 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

5. 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

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9. 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

10. 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

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12. 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

13. 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

14. 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
Prakash Kumar Sekar

15. 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

16. 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
Prakash Kumar Sekar

17. 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

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20. 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

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22. 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
Prakash Kumar Sekar

23. 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

24. 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

25. 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

26. 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.
Prakash Kumar Sekar

27. 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
Prakash Kumar Sekar

28. 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

29. 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

30. 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 bottom
Prakash Kumar Sekar

31. 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

32. Soundings - Echo Sounder
– sounding device utilising a
fixed beam with a vertical
axis
– shape and width of the beam
varies
Prakash Kumar Sekar

33. Soundings - Echo Sounder
• Recorder
– paper record or trace
– digital display
– punch tape
– cassette recorder
– digital readout to a
computer
– several components
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2
3
4
5
6
7
Prakash Kumar Sekar

34. Soundings - Echo Sounder
• Recorder
– paper record or trace
– digital display
– punch tape
– cassette recorder
– digital readout to a
computer
Prakash Kumar Sekar

35. 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

36. 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 boat
Prakash Kumar Sekar

37. 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

38. 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

39. 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

40. Soundings - Reduction of Echo
Sounder Traces
• The soundings at each fix are not
reduced separately.
Prakash Kumar Sekar