HYDROGRAPHIC SURVEYING 1.0 Introduction: o The survey conducted on bodies (stream, river, lake or an ) is known as hydrographic surveying. The objectives of the hydrographic surveying may be listed as below: o To determine navigation routes/ preparation of nautical charts using surface data of shore area. o To make underwater investigations for designing port and harbour facilities. o To find scouring and silting data on subaqueous floor. o To collect discharge data of rivers. o To plan engineering projects like bridges, dams and reservoirs. o To determine the shoreline of water bodies. o To collect data of to find mean level. 1.1 Shoreline Survey o Shoreline survey is to record location of shorelines, prominent features on the shoreline and data of high and low tides.

o Shoreline is located by running a traverse and taking offsets from the traverse lines to the shoreline points. o For a narrow river, an open traverse on one bank of river is sufficient to locate shorelines on both banks. In case of wide rivers traverse are run on both the banks to determine the shorelines. o The two traverses on both the banks may be interconnected by observations as a check on the work being done. o It may also be required to use triangulation network along the banks of a wide river. o In case of tidal water surface, the low and high water lines are observed from the benchmarks on the shore. o Shore deposits and marks on the rocks can be used for determining high and low water lines. o Contouring can be done to locate the points of high water line. o Generally, interpolation from soundings is done to save the time and efforts. 1.1.1 Controls o Hydrographic surveying involves measuring the depth of water at points on the water body surface and location of the points at which depth is measured. o Finding depth of water at a point on the water body surface is known as sounding. For sounding data to be useful vertical control (benchmark) on the shore is necessary. o As the water level in a water body may frequently vary, the reduced levels of points on the water surface have to be related to the depth of water as well. o To locate the points where soundings are taken, horizontal control is required and triangulation points/ shoreline traverse points are used for this purpose. o Wherever precision is required, the location is determined with reference to the triangulation points. 1.1.2 Tides o The is based upon Newton’s equilibrium theory. All celestial bodies exert a force of attraction (gravitational force) which is directly proportional to masses and inversely proportional to the square of the distance between them. o Thus the ocean on the Earth are under influence of the Sun and the Moon, however influence of the Moon is more due to its proximity with the Earth. o The generally accepted tidal theory has two important assumptions: (i) the Earth is covered all around by an ocean of uniform depth (ii) the sea is capable of taking, instantaneously, any new position as per the forces exerted on it. 1.1.2.1 Lunar Tides o Lunar tides are the variations in the ocean surface level due to the moon. o The moon has two types of motion, i. e., rotation about its own axis and revolves around the Earth.

o The fig above shows the situation of lunar tides. Let; Me be the centre of mass of the earth Mm be the centre of mass of the moon Mc be the common centre of gravity o At first, consider the gravitational attraction between them without the motion. This force is not uniform as the distance of mass particles from the moon varies for different locations of the earth. o Secondly, the rotation of the earth will cause centrifugal forces on its mass particles. This force will be almost uniform all around. o The non-uniform force of moon will have more effect the earth face nearer to the moon and less effect on the surface away from the moon. o The water rises to the maximum level on the side facing the moon and this is known as superior . On the opposite face, the water rises to a minimum level and known as inferior lunar tide. o On the other two surfaces, the water level will be low and this is the phase of low tide. o Rotation of the earth about its own axis, will keep on changing the positions of the numbers shown as 1, 2, 3, 4 facing the moon with time and further the motion of the moon also changes due to its motion around the earth. Thus the tidal positions keep on changing and all the points will experience high and low tides at some point of time. 1.1.2.2 Solar Tides o Tidal phenomenon due to the sun is of the same pattern as the same due to the noon. If Dm is the distance between the earth and the moon.

o As known that (Ms/ Mm)= 27111716 and (Dm/ Ds) = 0.00257; the tide producing force due to the Sun is 0.46 times that due to the moon.

1.1.2.3 Spring and Neap Tides o The combined effects of the lunar and solar tides result in spring and neap tides.

o Spring tides occur during full moon when the sun and the moon have the same celestial longitude. o Assuming that the sun and the moon lie along the same horizontal with the equator, the effect of tidal forces of the sun and the moon are additive giving a maximum tide known as spring tide. o After about 7.5 days, the longitudes of the sun and the moon are at 90° and crest of the moon tide coincides with the trough of the sun tide counteracting the influence of each other and known as neap tide. o In the spring tide, the high water level goes above the average tide and the low water level is below the average. In contrast, the neap tide is characterized by high water level below the average tide and low water level is above the average. o The cycle of spring and neap tides repeats after about 29.5 days. o Limitations of the equilibrium theory: (i) The orbits of celestial bodies are not circular (they are elliptical). (ii) The relative positions of the sun and the moon varies with time. (iii) The relative attraction between the sun and the moon may influence the tides. (iv) The deviation from the equator, i.e. with non zero declination of the sun and the moon. (v) The distribution of masses may not be uniform. (vi) The effect of land masses replacing water on the surface. o The prediction of tides is thus very difficult and should be mainly based on the observational data of actual occurrences of tides.

1.1.2.4 Measurement of tides o The elevations of high and low or tidal positions are measured using various types of gauges. o There are two types of gauges: Self registering and Non-registering types o Commonly used gauges are shown below. (a) Staff gauge

A staff gauge is a simple scale graduated to 5 or 10 cm. The scale is fitted vertically. The zero of the scale is fixed arbitrarily and its elevation is determined by leveling. The scale is read at intervals and the readings recorded. The scale should be sufficiently long to record the low and high water levels. (b) Float gauge

It is a simple device consisting of a container in which a float is hung with a wire. Water enters the gauge through the openings at the bottom and lifts the gauge up to the level of the water. A graduated vertical rod with an index mark is used to read the position of the float. As the float rises with the rising water level, the reading against the index mark is taken to determine the water level. (c) Weight gauge

A weight gauge consists of a wire or chain to which the weight is attached. The arrangement of weight and chain is passed through a pulley to maintain vertical position of the hanging weight. A graduated scale with an index mark is placed near the wire for taking the reading. During measurement, the weight is lowered to touch the water level and a reading is taken against the index mark. The reduced level of the zero of the graduated scale is determined using a level and staff. The staff is held touching the bottom of the weight when it touches the water level, at the same time placing the index over the zero mark of the scale. The reduced level of the zero mark is thus established. (d) Self-registering gauge

These gauges are designed to automatically record water levels either on paper or store them electronically along with the time of the day. This essentially consists of a float that is attached to a float wheel by wires and kept under constant tension. The float moves with the water level and the motion is transferred to the wheel. Through an appropriate gear system, such motion is recorded on paper on a drum. The drum maintains a constant speed and this establishes the time interval between readings.

1.2 Soundings o Sounding is the determination of the depth of water at different points on the surface of a water body. o This requires two measurements- vertical measurement giving depth or R. L. of the bed and horizontal measurements to locate the points where depths were taken. o In flowing and turbulent water as in sea or river, sounding has to be done carefully, as the data collection of soundings may be disturbed by silting and scouring that changes the bed level. 1.2.1 Sounding Equipment

(i) Sounding Boat: It is a simple boat with a sounding platform. A rowing canoe or flat bottomed boat may be used in placid water. Sometimes, a special sounding boat with a central well is used. In flowing waters and difficult conditions, a motor launch is preferred.

(ii) Sounding rod or pole: It is a thick wooden pole 5-8 m long and about 80 mm in diameter. Sounding rods are suitable for shallow water sounding only. A weight of lead is attached at the end for stability and for holding it vertical. (iii) Lead lines: These are made of cord, rope or a brass chain with a sounding lead weight attached to it. The line is graduated in a stretched position after wetting it. It should be dried and stored. It is wetted for about one hour before it is used for sounding. The lead weight may be hollow at the bottom for sampling the material. It is bell shaped with a ring at the top to attach the lead line. The weight should be sufficient for the line to be stable in flowing water.

(iv) Sounding machine: It is used where extensive sounding work is expected. The sounding machine is a drum with a lead line and may be operated manually or by electric power. The attached lead weight is lowered slowly in water. The depth of the sounding can be read from the dials. The sounding machine can be used for depths up to 30 m.

(v) Fathometer: It works on the principle of echo sounding. It can be used for greater depths and hence mostly used in ocean sounding. A fathometer determines the water depths by measuring the time taken by waves to travel through water and back. The instrument can directly give the depth or may record it on paper giving the profile of the bed ground. The velocity of sound waves in water depends on many factors, the instrument can be adjusted to the velocity in a particular stretch of water based on its properties. A fathometer has a transmitter for generating and transmitting sound waves and a receiver unit for receiving the echo sound waves. It also has recording and power units. The signal is transmitted to travel through the water and the reflected signal is received in the receiving unit. The time of travel of signal is recorded. As the distance travelled is two times the depth of the water and V is the velocity of sound that takes the time t, then D = V t/ 2, where D is the depth of water. A correction may be applied if the boat is in motion, i. e., the position of transmission and receiving of the signal is not the same. The circuitry is so designed that when the sounding is taken, the depth is automatically calculated, displayed and recorded as a graph.

Advantages of Fathometer: (i) It is more accurate than other methods. (ii) It can be used in strong currents where other methods may not suit. (iii) A true vertical sounding is obtained. (iv) It is very fast method. (v) It can be used in adverse weather conditions. (vi) It provides a continuous record of soundings, the profile of the ground can be seen later after the field work is over.

(vi) Modern Systems: Oceanographic studies or large scale harbour projects use echo -sounding systems with side-scan/ single or multiple beam scanning systems. Multiple -beam scanning systems use a number of transducers and scan the floor in parallel lines in two perpendicular directions. These airborne devices are known as (sound detection and ranging) and LIDAR (light detection and ranging) 1.2.2 Methods in sounding

Fieldworks in sounding is done with following steps; (i) As the sounding party moves along in a boat, the sounding man, standing at the end of the boat, dips the rod at a forward position. The reading is taken when the rod becomes vertical. A large number of such readings are taken.

(ii) If a lead line is used, it is thrown forward and the boat keeps on moving. The reading is taken when the line becomes vertical. This action requires experience so for soundings experienced staff is required.

(iii) In placid water, the above methods are used. In turbulent water, is required and the water level variations are noted with time. The sounding party will also note the time of reading of the sounding. It may now be possible to reduce the readings to a common datum. 1.3 Locating Soundings In order to determine the topography of the floor of a river or an ocean, it is essential to know the locations of soundings in addition to depth of water. There are many ways to locate sounding positions. The following relevant terms are required in this regard.

Range Line: The soundings are generally taken along straight lines laid at intervals, at right angle to the shore or banks of a river. Fig. below shows the range lines. In case the shoreline is not straight, the range lines are laid out as radiating from a prominent point on the shore.

Signals: In order to range the line, shore signals are placed on both the banks or at two different points far apart. The signals are rods or tripods fixed to the ground with a coloured flag on top. The signals must be clearly visible from a considerable distance. They are accurately located with respect to the traverse or triangulation points. The location of soundings can be related to shore points on the plan through the signals. 1.3.1 Location by Cross Ropes One of the commonly used methods is to locate soundings by a Cross Rope as shown below. This method consists of stretching a rope or wire across the width of a river, lake or harbour area. The stretched line has tag marks indicating distances with respect to a zero position on the shore. As soundings are taken, there positions with respect to these tags are noted and recorded along with the depth and time. This a simple technique and commonly applied.

1.3.2. Range and Time Intervals In this method, the sounding boat is rowed along the river at a constant speed and the soundings are taken at regular time intervals are recorded. The range line is first fixed by shore signals and the sounding line is kept as the line joining the shore signals. The sounding taken and time is noted. From the speed of the boat and the noted time or thus known time interval, the location of the sounding can be ascertained. This method does not give accurate results hence may be avoided for precision works. 1.3.3 Range Line and One Angle from Shore As shown in the fig. the above method is executed in the following steps: (i) Fix two shore signals in line with the range line. Set instrument to zero and observe the shore signal with the angle set to zero. (ii) The sounding operation and angle observation will require great coordination. Boat is moved along the range line. The line of sight from the instrument to the bow of the boat or the sounding man should be fixed. (iii) Appropriate signal system should be used for mutual information exchange between the sounding and instrument persons. The angle measurement to sounding point must be coordinated well so that there is no time mismatch and the sounding operation and angle measurement must be done at the same point of time. The time should be noted by the boat persons as well as by instrument persons. (iv) Angle should be measured to an accuracy of 1´ to 5´. (v) Sometimes, the angles are observed only for a few of the points and other points are located by the speed and time method by moving boat at constant speed.

1.3.4 Range Line and One Angle from the Boat It is similar to the previous method and involves the following steps: (i) A convenient range line is fixed on the plan. The shore station is preferably fixed on a prominent object, if no prominent feature is available station is set up using tripods and flags. (ii) The boat carries the angle measuring instrument. The angle to the shore station is measured at the time of sounding. The process is followed for all sounding points as shown in the fig. The advantage of this method is that there is a Better control over the measurements. The method can also be used in conjunction with time and speed method with the motion of boat being constant.

1.3.5 Two Angles from Shore The method requires two instruments and following procedure may be adopted. (i) Two stations on the shore may be fixed with the precaution that measured angle be more than 30°. (ii) There is no need to accurately fix a range line and range signals. However boat may be run approximately along the range line. (iii) Angles and sounding should be measured simultaneously. Appropriate signaling exchange among the boat person and instrument persons should be followed. (iv) If 1 and 2 are the instrument stations and  and  are the angles measured. The coordinates of the sounding station S can obtained as:

The advantage of this method is that the exact range line is not required. This method is suitable when there is difficulty in rowing the boat accurately along a range line. 1.3.6 Two Angles from the Boat Similar to the previous method, in this method two angles are measured from the boat. As shown in the fig. angles  and  are measured from the boat. The two angles are measured at the time of sounding. The three stations P, Q and R on the shore should either be the prominent features or the three points established by traverse or triangulation. The position of the three points are known in the plan.

With two known angles and  and positions of three points P, Q and R the sounding position S can be located. This is known as Three Point Problem and shall be discussed separately.

Range lines and range signals are not required. The method is best suited for measuring soundings at isolated points. There is a better control over the operation as the whole party works from one boat.

1.3.7 One Angle from Shore and One Angle from Boat As measuring two angles from the boat or shore simultaneously is difficult. Other method, which uses one angle from shore and one from boat may be used. As shown in the figure two shore signals are required in this case. P and Q are the shore signals and S is the location of sounding. Both the angles and sounding measurements should be done simultaneously. Let  be the measured QPS and  be the PSQ, then PQS = 180 – ( + ) = γ. Thus the problem reduces to the same as the measurement of two angles from the shore. Therefore,

1.3.8 Intersecting Range Lines If soundings are to be taken at number of times on the same point (to determine silting and scouring), two sets of range lines can be set out by suitable signals from the shore. Sounding points are fixed on the intersection of the range lines. The advantage of this method is that no angles are to be measured. The signal points are fixed in the plan by the intersecting lines, which are located by the traverse or triangulation points on the shore. 1.3.9 Reduction and Plotting Sounding directly measures the depth of water and the data obtained may be used to know the ground profile of the bed as done in leveling. If the water level changes frequently due to tides, the tidal gauge readings are taken at the time of the soundings. Tidal readings can be used to reduce the sounding readings to a common datum. The most commonly used datum is the mean . The datum used can be the mean level of low water of spring tides (LWOST) or the mean low water springs (MLWS). A correction is applied to reduce the soundings to mean datum reading of the sea.

Example: The following soundings were taken when the gauge reading was 3.65 m. 1.75 m, 3.8 m, 4.5 m, 7.8 m, and 8.6 m If the mean datum reading at LWOST is 2.85 m, reduce the sounding to LWOST. Gauge Reading (3.65 m) Sol: Sounding (1.75 m) Mean datum reading(LWOST) (2.85 m) Reduced sounding (0.95m) Mean datum reading at LWOST = 2.85 m Ground level Gauge reading at sounding = 3.65 m Correction to sounding = 2.85 – 3.65 = - 0.8 m Reduced soundings are: s1= 1.75 – 0.8 = 0.95 m, s2=3.8-0.8=3.0 m, s3=4.5-0.8=3.70 m s4=7.8-0.8=7.0 m, s5=8.6-0.8=7.8 m. Plotting: Soundings are plotted on the plan. If the range lines are fixed on the plan, the soundings are plotted on the range lines depending upon the distances measured on the cross rope or by any other means. If angles are used, distances are calculated or angles are laid out to locate the soundings.

Problems for Practice: Q1. During a sounding field work, P and Q were two stations on the shore. S was a sounding station. The angles measured were SPQ = 42°32´ and SQP = 64°36´. Find coordinates of S with respect to P if the distance PQ = 1580 m. x R P Q Q2. A sounding S was located by y one angle from a boat and one angle from the shore. P and Q were shore signals 934 m apart. S The angles measured were SPQ = 76°32´ and PSQ = 32°56´. Find coordinates of S with respect to P. Q3. The following observations refer to tidal gauge readings and soundings. Find the corrected soundings referred to the datum. At 10:00 am, the gauge reading is 3.25 m. Mean datum reading is 4.65 m. Soundings at different points taken are 1.85 m, 2.15 m, 2.55 m, 2.80 m, 3.10 m. Reduce the soundings to the datum. 1.4 Three-Point Problem The three point problem can be stated as “locating the unknown position S on a plan in which three points (say P, Q and R) are known and the angles subtended by the lines PS, QS and RS are also known.” The three point problem can be solved by graphically as well as analytically.

1.4.1 Tracing Paper Method As shown in the fig. the three given points , P, Q and R are available in the plan. From the sounding station S, angles subtended by PQ and QR at S have been measured. Now, on a tracing paper, three lines S-1, S-2 and S-3 are drawn with the help of known angles as shown in the fig. Keep the tracing paper on the plan containing P, Q and R and adjust the position of the tracing paper such that the line segments S-1, S-2 and S-3 pass through the points P, Q and R simultaneously. The position of S can be marked on the plan. 1.4.2 Using Station Pointer A station pointer is a special protractor with three arms. The central arm is fixed to hold the ring protractor and the other two arms are movable. The central ring is graduated in degrees and minutes and the angles between the arms can be set to desired value. The arms of the station pointer have bevelled edges which indicate the lines between which the angles are set. As in the tracing paper method, the angles are set in the station pointer between the arms. These angles are the angles subtended at the sounding point S by the lines PQ and QR. Once these angles are set, the station pointer is moved and rotated on the plan such that the bevelled edges of the arms simultaneously pass through the points P, Q and R. The station S is located as the centre of the circular ring holding the arms. 1.4.3 Graphical Method There are many graphical methods, one of these is described as follows.

Start with the three points P, Q and R as shown In the fig. as available in the plan. Join the line PR. Mark the angles TRP =  and TPR = to get the point T. Draw a circle passing through P, R and T. Join TQ and extend it to cut the circle at S. S is the position of sounding station in the plan.

This can easily be proved using the properties of a circle. Chord PT subtends angles at S and R. These angles should be same. Therefore, PSQ = TRP= 

Similarly, chord TR subtends angles at P and S. These angles should be equal. Therefore, RSQ = TPR=  Therefore, the angles at S are  and  and hence the position of S is located as per angles measured from the boat.

1.4.4 Analytical Method S is the sounding station from which angles  and  have been measured. As shown in the fig. P, Q and R are the shore stations which are located on the plan through traversing or triangulation. It is required to find any two of the distances PS, QS and RS or the angles and y as shown. The known angles are PQR = γ, PSQ =  and QSR = . Let the sum of angles x and y be .

1.5 Mean Sea Level Mean sea level is the average height of the sea for all stages of the tides. In India, Mean Sea Level (MSL) adopted by survey of India for reference, is located at Mumbai High. For most hydrographic use, the mean sea level may be found by observation over 12 lunar months, which will yield sufficiently accurate results. However, a very precise determination of mean sea level is obtained by observations over a period of 19 years. This the period over which the lunar nodes will have completed one complete cycle. Referring survey data to the mean sea level is necessary in order to have a common benchmark. A relation between the MSL and terrestrial level is also necessary for executing many engineering projects. The mean sea level is determined at a place by observations over a period of time and by relating all the vertical measurements to a common datum. PSMSL (Permanent Service for Mean Sea Level) is a global data bank for mean sea levels from across the world. PSMSL keeps records of tide gauge readings and bottom level pressure recorders obtained from various countries. PSMSL started in 1933 and it is located at the National Oceanographic Centre, Liverpool, UK. The centre obtains data from nearly 2000 tide gauge stations from around the world. Revised Local Reference (RLR) data has been introduced and it is based on a datum which is 7 m below MSL at a place. The data pertaining to India is supplied by Geodetic and Research Branch of the Survey of India and is periodically updated. The details may be seen at its website. Problem for Practice on three-point problem

1. From a sounding boat at sea, the following angles were measured;  = 32°46´ and  = 41°24´. The three shore stations P, Q, R are located by traversing. PQ=596 m, QR=678 m, and PQR = 132°52´. Find location of S by calculating the distance PS, QS and RS.