Hydrographic Surveys

Hydrographic SurveysHydrographic SurveysHydrographis Surveying - is the process employed in gathering information concerning any body of water and its adjacent land areas.

Puspose of Hydrographic Survey:1. Providing data for the preparation of hydrographic maps and nautical charts needed in navigation, harbor, improvement, yardage and dredging projects.2. Flood control, water supply, hydro-electric power development, pipe line and underground cable crossings and irrigation.3. Obtaining information needed for the design of bridges, dams, seawalls, culverts, sewage disposal plant, light houses, docks, and other structures.Purpose of Hydrographic Survey:4. Determining the volume of impounded water, direction and velocity of currents, drainage areas and the capacity of a containing basin.5.Gathering data needed for the establishment of tidal deturns.6. Locating navigational hazards such as submerged obstructions, sunken vessels, sand bars,rocks, shoals, and coral reefs.

Hydrographic Survey Terms

1. Hydrography - the study, description, and mapping of oceans, seas, lakes, and rivers especially with reference with their navigational and commercial or other uses.2. Sounding - the process of measuring the depth of a body of water at a particular point.3. Stream Gaging - the process of making measurements in streams and rivers for the purpose of predicting the rate of discharge at various water levels or stages.4. Fathom - a unit used for the measurement of water depth and is equivalent to six (6) feet.5. Hydrographic Chart - a map or chart used in navigation. It is basically similar to a topographic map except that water depths, navigational markers, and the character of underwater surface are indicated.

Hydrographic Survey Terms6. tidal Datum's - 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.7. Discharge - the volume of water flowing past, a section of a stream, canal, flume, and other water ways. It is usually expressed in cubic feet per second or cubic meters per second.8. Stream - a current or steady flow of water running along the Earth's surface.9. Dredging - 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.10. Subaqueous Contour - an imaginary line on the Earth's surface found underwater, all points of which are of the same elevation. They are sometimes known as depth curves or bathymetric contours.

Basic Operations Involved in a Hydrographic Survey1. Reconnaissance2. Establishment of Horizontal Control3. Establishment of Vertical Control4. Topographic Survey5. Hydrography6. Preparation of Hydrographic MapEquipment for Hydrography1. Sounding Craft2. Sounding Pole3. Leadline4. Sounding Machine5. Fathometer6. Signals7. Tide Gage8. Sextant

Importance of TidesPractical 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 dry rock. 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.The Cause of the TidesThe 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.Tidal Definitions1. Tide - 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 - the horizontal movement of water accompanying tides and which is produced by the combined action of astronomical, hydrological, and meteorological factors.3. Flood Tide - the period within which the water surface is rising and moving in toward the shore.4. Ebb Tide - the period within which the water surface is falling and moving seaward.Tidal Definitions5. Slack Water - 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 flow.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.Tidal Definitions9. 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 - the range of the tides 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 - consists of an idealized average period of 24 hours and 50 minutes between the occurrence of two successive high tides of the same type of the same location.Tidal Definitions12. Stand - the stand of the tide is the period around which the greatest peak or depression of the tides is reached and during of which any change in height of the tide is scarcely noticeable.13. Tidal Waves- consists of a large head or front of water generated at the surface of the ocean by force winds of hurricanes or typhoons, or by isostatic adjustment of the ocean floor accompanying a submarine earthquake.

Tidal DatumsTidal 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 Principal Tidal Datums are:1. Mean sea level - the datum for first-order level net of many countries and is increasingly used as the base for general levelling operations. It is defined as the average height of the sea of all stages of the tide.2. Mean low water - the mean of all waters as observed over long periods.The Principal Tidal Datums are:3. Mean lower low water - the average of all heights of the lower of the two low waters that occur in each lunar day.4. Mean low water springs - the mean of the low waters of the spring tides occurring a day or two after new or full moon. It is obtained by subtracting one-half of the range of the spring tides from the mean tide level.

Types of Tides1. Semidiurnal Tides2. Diurnal Tides3. Mixed TidesTidal CurrentsHorizontal movement of water to and from the shore accompanies the rise and fall of the tides. The flow of water, known as tidal current, may be large or small, depending upon the existing tidal head of water.Tidal BoresIn certain narrow channels subject to extremely high tides at the entrance, a "piling up" of water occurs at a formidable rate. At a time halfway between low and high water, the tide is usually rising at its maximum speed. The resultant wall of water rushes up the river with a characteristic roaring and hissing sound which makes its arrival a readily detectable in advance. This type of "cascade in reverse" is known as a Tidal Bore.Tide GagesTide Gages are instruments for measuring the height of the tide and are classified into the following groups:1. Non-recording Gages2. Automatic GagesSounding Party1. Chief of Party - the person who directs all survey operations of the party.2. Instrument man - the person who is responsible for setting up the different instruments to be used in the hydrographic survey.3. Recorder - the person whose task is to record all the survey data gathered.4. Leadman - handles the leadline or the sounding rod.Sounding Party5. Coxswain - the person who is responsible for steering the boat on selected compass bearings or ranges.6. Lookout - in waters where there are expected dangers to navigation, a lookout is employed.7. Signalman - the person who alerts the shore party that sounding is about to begin.8. Fathometer Attendant - in deep water surveys where a fathometer is employed, the attendant focuses full attention on this instrument.Survey BuoysA buoy is a floating object anchored in place in a body of water by a heavy weight to which the buoy is attached by a rope or chain. It may be made of wood, plastic metal or other suitable material which floats in water.Locating SoundingsVarious methods used for locating sounds are the following:1. Time Intervals Along a Range Line2. Range Line and an Angle From Shore3. Intersecting Range Lines4. One Angle and Stadia Distance from Shore5. Two Angles From Shore6. Two Angles From Boat7. Distances Along a Cross rope or Wire8. Direction and Vertical AngleElectronic Positioning Systems1. Short-range Navigation (Shoran)2. Long-range Navigation (Loran)3. Electronic Position Indicator (EPI)4. Radio Acoustic Ranging (RAR)Echo SoundingEcho sounding is a method of measuring the depth of water by determining the time required for sound waves to travel from a point near the surface of the water to the bottom and back.An echo-sounding equipment (fathometer) is designed to produce the sound, transmit it downward, receive and amplify the echo, measure the intervening time interval, and automatically convert this interval into units of depth measurement, such as feet, fathoms or meters.Types of Echo Sounders1. Portable Fathometers2. Non-portable FathometersAdvantages and Limitations of Echo sounders1. Variation in the water temperature2. Degree of salinity in the water3. Absorption of transmitted sound waves by the water4. Turbulence5. Characteristic and slope of the bottom surface6. Aeration or air entrainment along the bottom of the vessel

New Developments in Electronic Sounding Equipment1. Light Detection and Ranging (LIDAR)2. Airborne Lasers3. Multispectral Scanners (MSS)4. Orbiting Satellites5. Photo bathymetryMultispectral TechniquesThe interpretation of multispectral imagery is primarily based on the tones of the object viewed. Depending upon the spectral reflectance of the object, tone varies with the wavelength.Earth Resources Technology Satellite (ERTS)The first Landsat Satellite (originally known as the Earth Resources Technology Satellite) was launched by the United States of America in July 23, 1972 into a sun-synchronous polar orbit around the earth at an altitude of 919 km. It was a butterfly-shaped observatory weighing 891 kg which circled the earth every 1 hour and 43 minutes. Drags And SweepsTo decate and locate such obstructions or menaces to navigation and to provide definite assurance that a certain minimum navigational depth exists throughout the given area, wire drags, wire sweeps, or sweep bars are employed. these devices came into general use sometime in 1900 when sweeps 60 to 300 m long were first used. This method has been widely adapted that sweeps 3000 to 4500 m long were later employed to cover several square kilometers of areas in one working day. Drags And Sweeps1. The Wire Drag2. Wire Sweep3. Sweep BarDredging OperationsDredging is the process of widening, enlarging, cleaning or deepening of channels in harbors, rivers and canals. In removing the dredged material to scows or hoppers dredges are employed. Dipper and clamshell dredges are used when the excavated material are loaded into scows which are towed to a deep water dumping site.Scow MeasurementsThe quantity of material dredged from any body of water can be determined either by soundings or scow measurements. Another method commonly employed in determining volume of dredged material is to measure the draft of a scow before and after loading.Reservoir Sedimentation SurveyA survey is made of the reservoir site to determine its capacity at various stages of the water.Marine Construction and Maintenance SurveysThe involvement of a Civil Engineer during the early stages in the planning and development of ports, wharves, harbors, and other marine structures is highly essential. His task usually includes the preparation of plans design, supervision of all new marine construction, the execution of related hydrographic surveys, and subsequent maintenance of existing marine structures.Marine Construction and Maintenance SurveysIllustrative Problem:A rectangular deck scow 30.50m long, 6.10m wide, and 3.66m high has a draft of 1.22m when light and a draft of 3.05m when loaded. The bottom length of the scow is 23.15m. The water line is 29.26m long when the scow is loaded with rocks and 25.60m long when light. If seawater weighs 1026 kg/cu m and the loaded dredged material weighs 3208 kg/cu m, determine the following:a. volume of water displaced when unloaded or lightb. displacement loadedc. weight of the water displaced by the loadd. volume of the loaded rockThree-point ResectionIn a hydrographic survey there is usually a large number of sounding points to be plotted on a chart or map. There are numerous approaches to the solution of three-point problem such as graphical, mechanical or analytical methods. The Three-point problem involves the determination of the position of the instrument (usually a sextant) by measuring the horizontal angles between three points of known position.Indeterminate and Weak ResectionsA three-point resection becomes insoluble if the point P happens to be on the circumcircle joining points A, B, and C.Resection on Four PointsA three-point resection can always be made to check mathematically, however, the mathematical checks do not provide a check on the field work or on the control points used.Derivation of Equations for the UnknownInstruments for Measuring Stream VelocityStream velocity are commonly determined either by the use of floats or by the use of current meters.FloatsThe measurement of stream velocity by floats is well suited for reconnaissance work, in determining flood velocities or in locating gauging stations.Classifications of Floats1. Surface Floats - the commonly used type of float. It is intended to measure stream surface velocities and is particularly suitable for rough determinations of velocity and for gaging streams in high flood.

2. Subsurface Floats - usually in the form of double float. It is unsuitable for use on small and shallow streams since the submerged part of the float could get easily entangled with objects in the stream bed.

3. Rod Floats - it is weighed at the bottom to allow it to float vertically upright with only a short length exposed above the surface of the water.Current MetersA current meter is an instrument used for the indirect measurement of velocity in streams. In velocity measurements along a stream, current meters are lowered into the water either from a boat, bridge, especially constructed cable system, or by wading. Vertical-Axis MetersA current meter falling under this classification has a vertical axis of rotation normal to the direction of stream flow.Classifications of Vertical-axis meters:1. Price Meter2. Pygmy meter3. Vane MeterHorizontal-Axis MeterThe axis of rotation of the meter is parallel to the direction of stream flow and the rotor is fitted with spiral or helicoidally shaped vanes.

Classifications of Horizontal-Axis Meter

1. Hoff Meter2. Ott Meter3. Haskel MeterMethods of Determining Mean Velocity1. Vertical Velocity-Curve Method2. Two-Point Method3. Single-Point Method4. Integration MethodMeasurement of Stream DischargeMeasurement of stream discharge are usually made in connection with the design of water supply systems, flood protection works, hydroelectric power development, irrigation systems and fish farming structures. The principal information sought in a discharge measurement is the volume of water flowing past a measuring section of a stream in a given period of time.Methods in Detemining Discharge in Streams1. Velocity-Area Method2. Slope-Area Method3. Weir MethodMethods in Detemining Discharge in StreamsIllustrative Problems:Capacity of Lakes and ReservoirIn the design of water supply systems, irrigation projects, structures for aquaculture development, and hydroelectric power generating stations, it is necessary to determine the volume of water which could be contained and generated by a supporting reservoir or lake.Methods in Determining Capacity of Lakes and Reservoir1. Cross-Section Method (pictures ng figures page 346 -347)2. Contour MethodMethods in Determining Capacity of Lakes and ReservoirIllustrative Problems: