Hydrographic Surveys Lecture Notes PDF

HYDROGRAPHIC SURVEYS Hydrographic surveying is the process employed in gathering information concerning anybody of water and its adjacent land areas. Purpose of Hydrographic Surveying 1. Provide the data for the preparation of hydrog...

HYDROGRAPHIC SURVEYS Hydrographic surveying is the process employed in gathering information concerning anybody of water and its adjacent land areas. Purpose of Hydrographic Surveying 1. Provide the data for the preparation of hydrographic maps and nautical charts for needed for navigation, harbor improvement, yardage and dredging projects. 2. Flood control, water supply, hydro-electric power development, pipeline and underground cable crossings, and irrigation. 3. Determining the volume of impounded water, direction, and velocity of currents, drainage areas, and capacity of a containing basin. 4. Gathering data needed for the establishment of tidal datum. 5. Obtaining information needed for the design of bridges, dams, seawalls, culverts, sewage disposal plants, lighthouse, docks, and other structures. 6. Locating navigational hazards such as submerged obstructions, sunken vessels, sand bars, rocks, shoals, and coral reefs. TERMS in HYDROGRAPIC SURVEY Hydrography is the study, description, and mapping of oceans, seas, lakes, and rivers especially with reference to their navigational and commercial or other uses. Hydrographic chart is a type of map or chart used for navigation purpose. It is similar to a topographic map except that the tide fluctuations, navigation markers, water depths, and character of bottom material are indicated. Sounding is the process of measuring the depth of a body of water at a particular point. Sounding line is a length of hemp or cotton cord, or a sash cord or chain with a cylindrically shaped lead weight attached to one end which is used in the measurement of depths. Range lines are reference lines or courses that are established along which soundings are to be taken. These lines are tide to points of known location or to a net of triangulation stations along the shorelines and which can be plotted on a map. Buoy is a float of wood or other suitable material used for marking. It is anchored in place by a heavy weight to which it is attached by a rope or chain. Stream gaging is the process of measuring the flow of water passing a given cross-section of the stream in a unit of time. Discharge, the discharge of a stream, or run-off, is the volume of water passing a given cross- section of the stream in a unit of time. Gaging station is a selected site on a stream which is equipped and operated so as to furnish the basic data from which systematic records of discharge can be derived. Current meter is an instrument used for measuring, indirectly, the velocity of flow at various depths in a stream. It is equipped with cups or vanes which rotate about a vertical or horizontal axis upon impact with flowing water. The rate of rotation bears a direct relationship to the velocity of the water. Subaqueous contour is an imaginary line on the surface of the earth found underwater, all points of which are of the same elevation. They are also known as depth curves or bathymetric contours. Fathom is a unit used for the measurement of water depth and is equivalent to six (6) feet. Tidal datums are specific tide levels which are used as surfaces of reference for depth measurements in the sea and as a base for the determination of elevation on land. Stream is a current or steady flow of water running along the earth’s surface. Dredging is the process of scooping or sucking up mud, sand, rocks and other material underwater for the purpose of enlarging, deepening or clearing channels, harbours, rivers, and other bodies of water. The Hydrographic Survey A hydrographic survey is characterized by the observations and measurements which are undertaken to determine and subsequently portray the topography above and beneath the water surface, as well as to locate important marine features. Basic operations involved in a hydrographic survey 1. Reconnaissance. A preliminary step is the careful reconnaissance of the area to be surveyed. This is done in order to select the most suitable manner of executing the survey and planning all phases of work. It is always seen to it that the project is completed in accordance with plans ansd specifications provided for such work. Reconnaissance may be performed by ground, on water, or by air. In many instance, aerial photographs or previously prepared maps and charts are used for reference and in planning the survey. 2. Establishment of Horizontal Control. The second step is the establishment of the framework by which land and water features are held in their true relationship to each other. It is desirable to take sights on all objects which will be useful as control points for the topographic survey of the nearby land area. Similarly, the position of hydrographic signals should be determined as these will be referred to later when locating soundings. These signals should be erected at locations along the shore and which could be used to strategically control the defined boundaries of the survey site. 3. Establishment of Vertical Control. Prior to the execution of sounding operations, vertical control must first be defined in order that the elevation or stage of the water surface can be correctly referenced. Vertical control data are also needed for the limited topography which will be portrayed on a nautical chart. At times it will be necessary to establish a tide station to observe the tidal fluctuations which will be needed to define the plane of reference for the soundings. The established datum is then tied to nearby bench marks by differential levelling. 4. Topography. In a hydrographic map or nautical chart, only a relatively narrow fringe of the nearby ground surface is shown since the expected users are not particularly interested in this area except for a few prominent landmarks which could be used for referencing positions. A topographic survey is performed to include this limited topography. 5. Hydrography. The measurement of the water depths and establishing their respective locations are the most important operations in a hydrographic survey. Different procedures and techniques have been devised to suit various field conditions, scope of work, purpose and the desired accuracy of the hydrographic survey. 6. Preparation of Hydrographic Map. The final product of a hydrographic survey is the preparation of a hydrographic mar or nautical chart. In the case of subsurface surveys for engineering requirements, the end result may be the calculation of water volume contained in a body of water, amount of dredged material, discharge of a stream, or the fluctuations of level in a body of water. Sextant The sextant is a portable and han-held instrument used for measuring horizontal and vertical angles. It functions by the physical principle that a light ray striking a plane mirror causes the angles of incidence and reflection to be equal. By sighting through the small telescope, and adjusting the movable arm until the correct reflections of the objects being sighted appear in the mirrors, the angle between these objects is measured. Because it is not necessary for the observer to remain stationary during observation, the sextant is well suited to hydrographic work and has the added advantage of measuring angles in any plane. It is used principally by navigators and surveyors for measuring angles from a boat, but it is also employed on exploratory, reconnaissance, and preliminary surveys of land. With the use of a sextant the subsequent solution of the three-point problem is made possible for determining the position of the sounding vessel at various selected times. It should be noted, however, that the sextant is not an instrument of precision for measuring small angles and where short distances are involved. The precision of horizontal measurement with the sextant depends upon the size of the angle and upon the length of sight.  Types of Floats. Importance of the Tides Practical and economic aspects of everyday life are affected in many different ways by the tides. For example, tides play an important part in the launching of ships, and in floating vessels free from drydock. They are of vital importance to navigation in enabling ships to clear reefs, sandbars, and shoals, and to enter shallow channels. Most diving and salvaging operations are contingent upon the tides, as are dredging, harbour construction projects, and marine engineering. Our fishermen are dependent upon the tides for their day’s catch. Boating, swimming, and aquatic sports activities are affected by tide-induced conditions of surf and sea. Cause of Tides The principal cause of the tides is the difference in gravitational attraction exerted by the moon upon different parts of the earth. The secondary cause is the similar difference in the attraction of the sun. Gravitational attractions are also modified by certain hydrological and meteorological factors. The periodic nature of the tides is a purely astronomical phenomenon resulting from the daily rotation of the earth about its axis, its annual revolution about the sun, and the moon’s monthly revolution about the earth. Although the elevation and depression of the tides may occur in any large body of water, in the open sea, tidal changes are masked by the action of waves and swell, hence their main effects are made noticeable along shorelines, and in harbours, bays, and channels. The advance of the tides with respect to beaches is evidenced accordingly by a visible rise in water level. Gravitational attraction varies as the square of the distance between the earth and the moon (or sun), so that the portion of the earth’s surface nearest the moon is attracted more powerfully than the central portion.  Tidal Definitions 1. Tide is the half-daily cycle of rhythmic rise and fall of the surface of oceans, seas, lakes, bays, rivers, and other bodies of water connected with them due to the gravitational attraction of the sun and the moon acting upon the rotating earth. 2. Tidal current is the horizontal movement of water accompanying tides and which is produced by the combined action of astronomical, hydrological, and meteorological factors. 3. Flood tide is the period within which the water surface is rising and moving in towards the shore. 4. Ebb tide is the period within which the water surface is falling and moving seaward. 5. Slack water is the instant at which the tidal current is changing direction and flows neither in nor out. 6. Set refers to the direction of current flow. 7. Drift refers to the speed of current flows. 8. High water refers to the maximum height to which the water surface rises above the standard datum plane during a given period. It is also known as high tide. 9. Low water or low tide refers to the greatest depression of the water surface below the standard datum plane during a given period. 10. Tide range is the difference between the high and low tide at a certain location on a given date. It may vary both for different locations and for high and low tides measured on different days at the same location. 11. Tidal day consist of an idealized average period of 24 hours and 50 minutes between the occurrence of two successive high tides of the same type at the same location. 12. Stand is the period around which the greatest peak or depression of the tides is reached and during which any change in height of the tide is scarcely noticeable. 13. A tidal wave is a shallow water wave caused by the gravitational interactions between the Sun, Moon, and Earth ("tidal wave" was used in earlier times to describe what we now call a tsunami.) 14. A tsunami is an ocean wave triggered by large earthquakes that occur near or under the ocean, volcanic eruptions, submarine landslides, or by onshore landslides in which large volumes of debris fall into the water. Types of Tides Due to the combination of various tide-raising forces, there exist three main types of tides: 1. Semidiurnal tides are tides characterized by two high and two low waters each tidal day. This tide is found mostly along Atlantic coast of the US, and Europe. 2. Diurnal tides are tides, in which in a long run, display but one high and one low water each day. This tide is found in the Philippines and coast of China 3. Mixed tides are type of tide which has two high waters and two low waters each day. Tidal Datums Tidal Datums are specific tide levels which are used as surface of reference for depth measurements in the sea and as a base for the determination of elevation on land. The height of these surfaces will depend on local conditions and therefore it cannot be transferred from some other locality by levels or computed from geodetic mean sea level. Many different datums have been used, particularly for levelling operations and one should be careful in ascertaining the datum used in the establishment of bench marks for a particular levelling work. The Principal Tidal datums 1. Mean sea level (msl) is the datum of first-order level net of many countries and is increasingly used as the base for general 6evelling operations. It is defined as the average height of the sea for all stages of the tide. 2. Mean low water (mlw) is the mean of all low waters as observed over long periods. It is used as a datum to which depths are referred. 3. Mean lower low water (mllw) is the average of all height of the lower of the two low waters that occur in each lunar day. 4. Mean low water springs (mlws) is the defined as the mean of the low waters of the spring tides occurring a day or two after new full moon. It is obtained by subtracting one-half of the range of the spring tides from the mean tide level. Sounding Party 1. Chief of party – directs all surveys operations. 2. instrumentman – set up the instrument to be used in hydrographic survey. 3. Recorder – the one who records all the data gathered during the surveyoperations. 4. Leadsman – handles the leadline or the sounding rod. 5. Coxswain – the one who is responsible for steering the boat. 6. Lookout – the one who looks up and watch over the water surroundingsfor ay hazardous materials. 7. Signalman –the one who alerts the shore party that sounding is about tobegin.