Hydrographic survey for std not print
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CHAPTER 3 : HYDROGRAPHIC SURVEY Prepared By: Asst. Prof. Pramesh Hada MSC in Urban Planning B.E In Civil Asst. Professor, Civil Engineering Dept. Nepal Engineering College
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Transcript of Hydrographic survey for std not print
CHAPTER 3 : HYDROGRAPHIC
SURVEYPrepared By:
Asst. Prof. Pramesh Hada
MSC in Urban Planning
B.E In Civil
Asst. Professor, Civil Engineering Dept.
Nepal Engineering College
CONTENTS
� 3.1 Introduction, Definition of Basic Terms(pu08)
� 3.2 Vertical & Horizontal Controls
� 3.3 Equipment & Methods of
Sounding(pu06,07,09,10,11,14)
� 3.4 Measurement of Cross-Section
� 3.5 Measurement of Velocity & Flow (pu07,09)
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� 3.5 Measurement of Velocity & Flow (pu07,09)
� 3.6 Instruction on Field Works
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3.1 INTRODUCTION, DEFINITION OF BASIC TERMS
HYDROGRAPHY
� Branch of science that deals with themeasurement of bodies of water.
� Is the branch of applied sciences whichdeals with the measurement anddescription of the physical features ofoceans, seas, coastal areas, lakes andrivers, as well as with the prediction of
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rivers, as well as with the prediction oftheir change over time, for the primarypurpose of safety of navigation and insupport of all other marine activities,including economic development,security and defence, scientific research,and environmental protection.
� Is a important tool for hydrologicstudies.
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HYDROGRAPHY
� Hydrographic surveying deals with the configurationof the bottom and adjacent land areas of oceans,lakes, rivers, harbors and other water forms onEarth.
� In strict sense, it is defined merely as the surveyingof a water area; however, in modern usage it mayinclude a wide variety of other objectives.
- such as measurements of tides, current, gravity, earthmagnetism, and determinations of the physical and a
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magnetism, and determinations of the physical andchemical properties of water.
� The principal objective of most hydrographic surveys, is to obtain basic data for the compilation of nautical charts with emphasis on the features that may affect safe navigation.
� Other objectives include acquiring the information necessary for related marine navigational products and
for coastal zone management, engineering, and science
Er.Pramesh
Engineering
SIGNIFICANCE OF HYDROGRAPHIC SURVEY
PU 2006,2007,2008
The Purpose of hydrographic survey are as follows:
� Used for controlling and planning of engineering projects
like bridge,dams,tunnels,reservoirs,harbours etc.
� To determine mean sea level and observation of tides.
� For the measurement of discharge of rivers.
� For the measurement of areas subjected to scour or silting
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� For the measurement of areas subjected to scour or silting
and to ascertain the quantities of degraded material.
� For the determination of shore lines.
� For making subsequent investigation to secure information
needed for the construction, development & improvement
of port facilities.
� Determination of coastal erosion, silting.
� Determination of site of construction of harbours,
dockyards.
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Er.Pramesh
Engineering
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Prof,civil
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Er.Pramesh
Engineering
BASIC TERMS
� Sounding : A Measurement of depth of water.
� Harbour : A place where ships are parked for cargo
& others.
� Reef : A submerged ridge of rock near the surface of
the water
�M.S.L (Mean Sea level) : Average of Sea level,
Height according to mean sea level
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Height according to mean sea level
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3.2 VERTICAL & HORIZONTAL CONTROLS� Vertical control – Depth Calculated (R.l calculated)
� Horizontal control – Northing and Easting calculated (X & Y)
The main objective in hydrographic surveying is to determine the
depth of water at a certain point.
�Hence Sounding is the measurement of depth below the water
surface.
�For Sounding, A vertical control is needed and to locate the
sounding, a horizontal control is needed.
Horizontal control may consist of either a triangulation or a
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�Horizontal control may consist of either a triangulation or a
traverse. Triangulation may have to be employed for horizontal
connection of widely seperated points.
�Traverse may be done by chain,tape, plane tabling or by theodolite,
depending upon the stations.
�Hence the underwater & near water points may be located with
reference to the traverse or triangulation.
�Vertical connection may be affected by the determination of the
depth of the point below the water surface. Such determinations are
known as “Soundings”.
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3.3 EQUIPMENTS NEEDED FOR SOUNDING (IMP)� Sounding Boat
� Sounding Poles
� Lead lines
� Sounding Machine
� Eco – Sounding (Fathometer)
� Sounding Boat
� Used for taking sounding.
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� Used for taking sounding.
� should be sufficiently spacious and stable.
� For quiet water. a flat bottom boat is more suitable, but for rough water round-bottomed boat is more suitable.
� For regular soundings, a row boat may be provided with a well through which soundings are taken.
� Should be extended far enough over the side to prevent the line from striking the boat.
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SOUNDING POLES/RODSEr. P
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� Is a pole of a sound straight-grained well
seasoned tough timber usually 5 to 8 cm in
diameter and 5 to 8 metre long.
� Suitable for shallow and quiet waters.
� An arrow or lead shoe of sufficient weight
is fitted at the end.
Helps in holding them upright in water.
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� Helps in holding them upright in water.
� lead or weight should be of sufficient area
so that it may not sink in mud or sand.
� Pole of 6 m can be used to depths upto 4
metres.
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LEAD LINES
� lead line or a sounding line is usually a length of a such cord, or a brass chain with a sounding lead attached to the end.
� Due to prolonged use, a line of hemp or cotton is liable to get stretched.
� It should be soaked in water for about one hour before it is used for taking soundings.
� length of the line should be tested frequently with a tape.
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tape.
� For regular sounding. a chain of brass, steel or iron is preferred.
� usually used for depths over about 6 metres.
� Sounding lead is a weight (made of lead) attached to the line.
� weight is conical in shape and varies from 4 to 12 kg depending upon the depth of water and the strength of the current.
� often has cup-shaped cavity at the bottom.
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SOUNDING MACHINE
� Where much of sounding is to be done, asounding machine as very useful.
� May either be hand driven or automatic. Fig.1 shows a typical hand driven Weddele’ssounding machine.
� lead weight is carried at the end of a flexiblewire cord attached to the barrel and can belowered at any desired rate, the speed of thedrum being controlled by means of a brake.
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drum being controlled by means of a brake.
� Readings are indicated in two dials — theouter dial showing the depth in feet and theinner showing tenths of a foot.
� Handle is used to raise the level which can besuspended at any height.
� Is mounted in a sounding boat and can beused up to a maximum depth of 100 ft.
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ECO – SOUNDING (FATHOMETER) (V.V.IMP)
� Echo sounder is used for measuring depths of large rivers, in excess of 10 m, and those of seas.
� Eco sounding is a technique of using sound pulse to find the depth of water.
� Is an electrical instrument, in which a sound impulse from the surface of water is Sent towards the bottom of the river or sea (Fig. 13.8).
� Sound waves are reflected back From the bed in the form of an echo which is arrested
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� Sound waves are reflected back From the bed in the form of an echo which is arrested by the receiver.
� There is an automatic recording of the time of onward and backward travel of the sound wave.
� Taking the velocity of sound in water to be approximately 1470 m/s, the depth of water can be computed.
� Generally the echo sounder is placed just at the water level to eliminate transmission loss and obtain an accurate result.
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ECO – SOUNDING (FATHOMETER)� Uses for Ocean sounding where depth is too much.
� It is adjusted to read depth in accordance with the velocity of sound in the type of water in which it is being used.
� For Eco sounding Instrument called “Fathometer” is used.
� A fathometer may indicate the depth visually or indicate graphically on a roll which
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or indicate graphically on a roll which continuously goes on revolving and provide a virtual of the lake or sea.
� The main parts of echo-sounding apparatus are : 1. Transmitting and receiving oscillator. 2. Recorder Unit 3. Transmitter/Power unit
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PRINCIPLE OF ECO SOUNDINGEr. P
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•Fig Below illustrates, the principle of
echo sounding.
•Consists in recording the interval of
time between the emission of a sound
impulse direct to the bottom of the sea
and the reception of the wave or echo
reflected from the bottom.
•If the speed of sound in that water is V
and time interval between the
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and time interval between the
transmitter and receiver is t, then the
depth is given by , h = 1/2(Vt)
•Velocity varies with salinity & temp. of
sea water, so accoustic velocity in sea
water is -
S – in terms of per 1000.
ADVANTAGES OF ECO SOUNDING
It has the following advantages over the older method of lead
line and rod :
� Used when a strong current is running and when the
weather is unsuitable for the soundings to be taken with the
lead line & other methods.
� Is more accurate as a truly vertical sounding is obtained.
Under normal water conditions in ports and harbours an
accuracy of 7.5 cm may be obtained.
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accuracy of 7.5 cm may be obtained.
� Is more sensitive than the lead line.
� Record of the depth is plotted immediately and provides a
continuous record of the bottom as the vessel moves forward.
� Speed of sounding and plotting is increased.
� Error due to estimation of water level in a choppy
sea(irregular) is reduced owing to the instability of the boat.
�
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�Shorelines are where oceans and seas meet land.
�The change is abrupt and sudden.
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METHODS OF LOCATING SOUNDING
� Soundings are located with reference to the shore traverse by observations made (I) entirely from the boat, (ii) entirely from the shore or (iii) from both,
The following are the methods of location
1. By cross rope 2. By range and time intervals
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2. By range and time intervals
3. By range and one angle from the shore 4. By range and one angle from the boat 5. By two angles from the shore 6. By two angles from the boat 7. By one angle from shore and one from boat 8. By Intersecting ranges 9. By tacheometric Observation
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1. Location by Cross Rope
� most accurate method of locating the soundings and may be used for
rivers, narrow lakes and for harbors.
� Used to determine the quantity of materials removed by dredging,
the soundings being taken before and after the dredging work is
done. (dredging - Remove with a power shovel, usually from a bottom of a body of
water)
� Single wire or rope is stretched across the channel and is marked by
metal tags at appropriate known distance along the wire from a
reference point or zero station on shore.
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Methods of Locating Sounding
reference point or zero station on shore.
� The soundings are then taken by a weighted pole.
� The position of the pole during a Sounding is given by the
graduated rope or line.
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2) Location by range and time interval :
� Method used when the width of channel is small and when great degree of accuracy is not required.
� In this method, boat is kept in range with the two signals on the shore and is rowed along it at constant speed.
� soundings are taken at different time intervals, knowing the constant speed and the total time elapsed at the instant of sounding.
� Distance of the total point can be known along the range.
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Methods of Locating Sounding
3. By range and one angle from the shore
� Boat is ranged in line with the two shore signals and rowed along the ranges.
� Point where sounding is taken is fixed on the range by observation of the angle from the shore.
� As the boat proceeds along the shore, other soundings are also fixed by the observation of angles from the shore.
contd……..
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•Let “B” is the instrument station, A1-A2 is the
range along which the boat is rowed and α1, α2, α3
etc. are the angles measured at B to points 1, 2, 3
etc.
•This method is very accurate and very convenient
for plotting.
•If the angle at the sounding point is less than 30°,
a new instrument station must be chosen.
By range and one angle from the shore Er. P
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a new instrument station must be chosen.
•After first and last soundings & every tenth
soundings are fixed by angular observations and
intermediate points are fixed by time interval.
4. Location by range and one angle from the boat
� Is similar to the previous one except that the angular fix is made by angular observation from the boat.
� Boat is kept in range with the two shore signals and is rowed along it.
� At the instant the sounding is taken, the angle subtended at the point between the range and some prominent point B on the shore is measured.
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Methods of Locating Sounding
some prominent point B on the shore is measured.
This method has the following advantages :
� Since all the observations are taken from the boat, the surveyor has better control over the operations.
� Mistakes are reduced since the recorder books the readings directly.
� On important fixes, check may be obtained by measuring a second angle towards some other signal on the shore.
� To obtain good intersections throughout, different shore objects may be used for reference to measure the angles.
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5) Location by two angles from the shore
� In this method, a point is fixed independent of therange by angular observations from two points onthe shore.
� Used to locate some isolated points.
� Two instruments & two surveyor are required & instrument is selected with accuracy.
� If A & B are two instrument station and ‘d’ is measured accurately.
� Instrument stations A and B are precisely connected to the ground and their positions on plan are known.
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Methods of Locating Sounding
to the ground and their positions on plan are known.
� Surveyor on the boat holds a flag for a few seconds, and on the fall of the flag the sounding and the angles are observed Simultaneously.
� co-ordinates of the position P of the sounding may be
computed from the relations :
Method has the following advantages
� Preliminary work of setting out and erecting range signals is eliminated..
� Suitable for strong current of water due to which it is difficult to row the boat a1on the range line.
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6. LOCATION BY TWO ANGLES FROM THE BOAT
� In this method, the position of the boat can be located by the solution of the three-point problem by observing the two angles subtended at the boat by three suitable shore objects of known position.
� Three-shore points should be well-defined and clearly visible.
� Prominent natural objects such as church spire, lighthouse, flagstaff, buoys etc. are selected for this purpose.
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spire, lighthouse, flagstaff, buoys etc. are selected for this purpose.
� Let A, B and C are the shore objects and P is the position of the boat from which the angles α and β are measured.
� Both the angles should be observed simultaneously with the help of two sextants, at the instant the sounding is taken.
� The method is used to take the soundings at isolated points.
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7. Location by one angle from shore and
one from boat
� This method is used to locate the isolated
points where soundings are taken.
� Let two points A and B are chosen on the
shore, one of the points (say A) is the
instrument station where a theodolite is set
up, and the other (say B) is a shore signal or
any other prominent object.
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Methods of Locating Sounding
any other prominent object.
� Sounding is taken at P, the angle α at A is
measured with a theodolite while the angle β
at the boat is measured with the help of a
sextant.
� Knowing distance d between the two points A
and B by ground survey, the position of P can
be located by calculating the two co-ordinates
x and y.
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8. Location by Intersecting Ranges
Method is used when it is required to
determine by periodical sounding at the
same points, the rate at which silting or
scouring is taking place.
� This is very essential on the harbors
and reservoirs.
� The position of sounding is located by
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Methods of Locating Sounding
� The position of sounding is located by
the intersection of two ranges, thus
completely avoiding the angular
observations.
� Suitable signals are erected at the shore.
� The boat is rowed along a range
perpendicular to the shore and
soundings are taken at the points in
which inclined ranges intersect the
range.
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9. Location by Tacheometric Observations
� Method is very much useful in smooth waters.
� Position of the boat is located by tachometric observations from the shore on a staff kept vertically on the boat.
� Observing the staff intercepts at the instant the sounding is taken, the horizontal distance between the
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Methods of Locating Sounding
horizontal distance between the instrument stations and the boat is calculated by :
� Direction of the boat (P) is established by observing the angle (α) at the instrument station B with reference to any prominent object A.
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3.4 MEASUREMENT OF CROSS-SECTION
Case I – When the River is small
� Two poles are fixed on both banks of the river & a rope isstretched between them.
� The water width of the river is divided into severalcompartments of equal width (say b) and the points aremarked by hanging tags or pendants.
� Depth of water corresponding to each pendant is measuredby a sounding rod or sounding cable.
� Let the depths of water be d0,d1,d2…..Considering the endcompartments as traing1es and other compartments as
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compartments as traing1es and other compartments astrapeziums, the area is calculated as follows.
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CASE II – WHEN THE AREA IS LARGE
� Two centre line AB is taken perpendicular to the river.
� Two stout pegs T1,T2 are fixed on AB.
� Then following procedure is adopted:
1. At station A, a theodolite is set up and the line is ranged properly. The width of the river is measured by stadia method. 2. The width is divided into equal parts, and the widths of these compartments are fixed. Thus the distances AP1, P1P2, P2P3,... are fixed. 3. Another theodolite is set up at C, so that
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3. Another theodolite is set up at C, so that AC is perpendicular to AB. The distance AC is measured (say it is equal to D).
4. Then the angles ( α1, α2… are calculated, in the following manner.
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5. The calculated angles α1, α2, ... arc set out from the theodolite at C.
Thus, the line of sight of the theodolite at A and that of the theodolite at C
coincide at the points P1, P2, P3, ... which are the points along the cross
section where soundings are to be taken.
6. The depths of water at these points arc measured by a sounding cable or
echo—sounder mounted on a boat.
7. Then the cross—sectional area of the water section is calculated in the
usual manner.
MEASUREMENT OF VELOCITY
� Measurement of velocity is an important aspect of many direct
stream flow measurement techniques.
� Different methods used for the measurement of velocity:
1. Surface float method
2. Sub-surface float method
3. Velocity rod method
4. Pitot tube method
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5. Current meter method
1. Surface float method
� Surface floats are made of cork. and can easily float on water.
� Are generally in the form of 10 cm cubes.
� The floats are painted red or white and have a small flag
attached on top [Fig. (a)].
(cork –cap or Outer bark of the cork oak; used for stoppers for bottles etc)
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1. SURFACE FLOAT METHOD
Velocity is measured in the following way:(a) At the discharge site, a rope is stretchedbetween two poles fixed on both banks ofthe river and the water section is divide dinto several compartments by hanging tagsfrom a rope [Fig. 13.9(b)].
b)velocity of each compartment ismeasured by the float, which is releasedslightly ahead of C/S-1. When the floatjust crosses the section, the stop watch is
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just crosses the section, the stop watch isstarted. When it just crosses C/S-2, thestopwatch is closed. The time taken bythe float to cover the ‘run’ (knowndistance) is noted, and from this thevelocity is calculated [Fig. I3.9(c)j.
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2. SUB-SURFACE FLOAT METHODEr. P
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� Sub-surface float is a hollow cylinder which is attached
by a cord to a surface float.
� Position of the sub-surface float is adjusted according to
the depth of the river. Generally, it is kept 0.2D meters
above the bottom.
� In this case, the measured velocity is equal to the mean
velocity.
� The procedure of measurement is the same as that in the
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� The procedure of measurement is the same as that in the
surface float method (Fig. 13.10). Here, D is the depth of
water.
3. VELOCITY ROD METHOD
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� Velocity rod made of hollow metal or wood. Its
diameter is about 2.5 to 5 cm.
�Weight provided at the bottom of the rod to keep it
vertical. length of the rod is adjustable.
� Rod is generally submerged to the extent of O.6D.
� The procedure of measurement is exactly similar to
that in the surface float method (Fig. 13.11).
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that in the surface float method (Fig. 13.11).
4. THE PITOT TUBE
� Pitot tube is a glass tube bent at anangle of 90° at the lower end.
� Upper end is open and kept above thewater level. The lower end is in theform of a nozzle, and is directedupstream.
� Due to the velocity of water, the water column rises in the right limb (Fig. 13.12). Let the rise in water
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(Fig. 13.12). Let the rise in water level be h. Then applying Bernoulli’s theorem between points A and B.
�Measuring the rise of water column the velocity can be calculated.
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5. CURRENT METER METHOD� Commonly used for measuring the velocity of water.
� It consists of the following components. (a) A tail which keeps the meter in the direction of alignment, (b) A system of conical buckets fitted to a spindle which rotates due to current, (c) A counterweight which keeps the meter vertical, and (d) A headphone for registering the sound produced by the spindle at every revolution.
� Current meter is lowered to a depth of 0.6 D (D � Current meter is lowered to a depth of 0.6 D (D being the depth of water) by a suspension cable.
� Conical buckets is kept on the upstream side and the tail on the downstream side.
� Due to the velocity of water, the system of conical buckets rotates about the spindle and a ‘tick’ sound is emitted once for every revolution. Thus, a continuous “tick-tick” sound is heard through the headphone. The number of revolutions per minute 15 counted with the help of a stopwatch.
� velocity can be ascertained from the rating table (supplied by the manufacturer with the meter). This velocity gives the mean velocity of flow.
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Current meter Method
� Different structures like notch,weirs,sluice gates are used for
measuring flow or discharge.
� Notch – device used for measuring the rate of flow of liquid
through a small channel or a tank.
� Weir – concrete or masonary structure placed in an open channel
over which the flow occurs.
A/c to the shape of the opening, it is classified as:
Rectangular weir or notch
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3.4 Measurement of Flow
� Rectangular weir or notch
� Triangular weir or notch
� Trapezoidal weir or notch
� Broad crested weir
� Discharge over a Rectangular weir or notch
� Discharge through a Triangular weir or notch
� Discharge through a Trapezoidal weir or notch
� Discharge through a Broad crested weir
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Discharge over a Rectangular weir or notch
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Discharge over a Rectangular weir or notch
Discharge through a Triangular or V-notch
dhh
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Discharge through a Trapezoidal notch or weirEr. P
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Discharge through a Broad - Crested weirEr. P
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TUTORIAL 1 – HYDROGRAPHIC SURVEY(IMP PU QUESTIONS)
� What is Sounding? Explain two most accurate methods of locating sounding
Position. (7)
� What is the significance of underwater mapping in hydrographic survey?
Explain Rectangular weir & V-notch methods to find the volume of water in
the river?(8)
� Write the purposes of hydrographic survey & Explain the methods of locating
sounding positions.(9)
� Explain the terms i)Sounding ii)Echo-sounding iii)sextant (6)
Explain various methods used in the measurement of flow.(6)
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� Explain various methods used in the measurement of flow.(6)
� What is Echo sounding? Enlist the advantages of Echo Sounding.(7)
� What is Sounding ? Explain any three methods of sounding.(7)
� Define hydrographic survey. Write the purpose of hydrographic survey . (8)
� How do you find the discharge of a river in Hydrographic survey. Describe at
least two methods.(8)
� What is Eco-sounding? How it measures the depth of water bodies?
Discuss. (7)
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