E4014 Construction Surveying

1. E4014 Construction Surveying Hydrographic Surveys

2. 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

3. Charts And Maps • Hydrographic Chart (cont) • Also contains some non-coastal information • mariner is interested in the general shape of features that can be seen from the water • accurate height required only when judging at what distance a feature may be visible

4. Charts And Maps • Hydrographic Chart (cont) • only show topographical and cultural features when they will aid • a safe landfall upon an unknown coast and to safely enter a harbour • manoeuvring and docking within a harbour • the conduct of efficient harbour business

5. Charts And Maps • Hydrographic Chart (cont) • Three distinctive features • working space • chart is a navigational plotting and recording board • overlap • enables projection of a course from one chart to the next using connecting points of navigational importance • scale and shape

6. Charts And Maps • Hydrographic Chart (cont) • Pilot publications • contains information which cannot be included on the chart because of space legibility • ‘Notice to Mariner’ publications • recently acquired information • alerts to navigational hazards

7. Charts And Maps • Topographic Maps • Depicts the physical form ( in two dimensions ) of the surface of the earth and features resulting from human occupancy • information is • qualitative as to location and use • quantitative as to area and volume • format is rectangular with minimum overlap • use is for activities on land and geographical studies

8. Charts And Maps • Bathymetric Maps • Topographic maps of a nation’s submerged lands • depict the shape of the sea floor through the use of isobaths • do not show navigational information or shipping hazards

9. Charts And Maps • Bathymetric Maps (cont) • Primarily used as an aid to scientific studies associated with • marine mining • oil and gas exploration • coastal zone management • pollution control • boundary and other international jurisdictional matters • engineering and research purposes

10. 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

11. Soundings • simplest case • sounding survey of a lake with no tidal or wave pattern • water would be assumed to be level and accepted as datum for the soundings i.e. RL = 0.00m • soundings would be taken in a systematic way and the fix positions are plotted • depth of the soundings are plotted so that the decimal point represents the the position of the sounding on the chart e.g. • 6.5 would indicate a depth of 6.5m at that point on the chart • 6.5 indicates 6.5m above datum • Sounding (cont)

12. Soundings - Sounding Datums • In coastal areas soundings are normally related to a tidal datum such as Mean Sea Level • Hydrographic charts must always have a lowest water as their datum • mariners reading a depth of 10.0 m expect a boat drawing 9.0m to be safe during all tide ranges

13. Soundings • Sounding Datums (cont) • Each sounding made on tidal water must be corrected for the height of the tide at the time the sounding was taken • a tide board is attached to a suitable structure and set vertically in the water in the vicinity of the survey area • preferable to be visible from the boat • tide board readings and the time are observed and recorded at constant intervals e.g.15min

14. Soundings • Sounding Datums (cont) • the zero mark on the tide board is determined by levelling from a BM • differences between AHD and Mean Sea Level are available and the soundings can be corrected accordingly

15. Soundings - Sounding Datums (cont) • conventional levelling found that • the 3.000 graduation on the gauge is exactly 19 metres below the BM. • Therefore the zero mark on the tide board has an RL of -2.375m. • Conversely the datum (AHD) zero is at 2.375m on the tide board.

16. Soundings - Sounding Datums (cont) • During a sounding survey the tide board was observed every 15 minutes. The results of soundings taken during one 15 minute period are shown below. Reduce the soundings to AHD

17. Soundings - Sounding Datums (cont) (2) Interpolate with respect to time

18. Soundings - Sounding Datums (cont) (2)

19. 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

20. 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

21. 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.

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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).

29. 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 • approximates the geoid

30. 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

31. 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

32. Soundings • Tide Levels (cont) • Highest Astronomical Tide ( HAT ) and Lowest Astronomical Tide ( LAT ) • highest and lowest levels that can be predicted to occur under average meteorlogical conditions • may not be reached each year • do not take storm surges into account and hence considerably higher and lower levels may still occur

33. Soundings • Tide Levels (cont) • Indian Spring Low Water ( ISLW ) • level suggested by Sir Charles Darwin for Indian waters • mathematically derived from tide data

34. 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

35. 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.

36. 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

37. 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

38. 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.

39. Establishing Tide Levels • As a guide, the following figures are appropriate: • Figures based on one full day's observation may vary considerably from observations made on another day as meteorological conditions play a big part. • Figures based on one full lunar month's observations (291/2 days) may vary by up to 25cm from values taken from another month's observations on the East coast of Queensland. This figure is the semi-annual and annual variation of MSL. • Figures based on one full year's observations may vary from values taken from another year's observations.

40. Establishing Tide Levels • Thus it can be seen that to obtain the figure for tidal variation at a point observations should be taken over at least one lunar month. • More accurate results are obtained when observations are taken over a full year. • The most accurate results cannot be obtained until continuous observations have been made over at least 19 years.

41. Establishing Tide Levels • Ships are concerned with the least amount of water that may be below them. • the datum for depths at sea, and navigation charts is normally a low water datum. • Two datums which have been used in the past for navigation charts are • LWST and ISLW, • both of which are to be progressively replaced by LAT.

42. 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

43. 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

44. Soundings - Echo Sounder • sounding device utilising a fixed beam with a vertical axis • shape and width of the beam varies

45. Soundings - Echo Sounder • several components • Recorder • paper record or trace • digital display • punch tape • cassette recorder • digital readout to a computer 6 1 7 2 5 3 4

46. Soundings - Echo Sounder • Recorder • paper record or trace • digital display • punch tape • cassette recorder • digital readout to a computer

47. Soundings - Echo Sounder • Recorder • when a horizontal fix occurs a fix button is pressed and the moment is recorded on the trace • fix is numbered so that it can be correlated with position fixes made • time is recorded at the first and last fix and for every tenth fix - enables the tide gauge readings to be correlated with the depth readings

48. Soundings - Echo Sounder • Transducer • transmitting transducer vibrations generates pressure waves which are projected into the water medium and detected by a receiving transducer • may be • hull mounted • mounted in a shoe, or • towed in a ‘fish’ enabling the sounder to operate at any depth • Receiving Amplifier • amplifies the weak return signal

49. 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

50. 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