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HYDROGRAPHIC SURVEYING 1.0 Introduction: O the Survey Conducted on Water Bodies (Stream, River, Lake Or an Ocean) Is Known As Hydrographic Surveying

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HYDROGRAPHIC SURVEYING 1.0 Introduction: O the Survey Conducted on Water Bodies (Stream, River, Lake Or an Ocean) Is Known As Hydrographic Surveying HYDROGRAPHIC SURVEYING 1.0 Introduction: o The survey conducted on water bodies (stream, river, lake or an ocean) 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 tides to find mean sea 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 theory of tides 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 tide. 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 waters 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.
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