Wednesday, December 18, 2019

HYDROGRAPHIC SURVEYING


HYDROGRAPHIC SURVEYING
A hydrographic survey is one whose principal purpose is to secure information concerning the physical features of water areas. Such information is essential for the preparation of modern nautical charts on which are shown available depths, improved channels, breakwaters, piers, aids to navigation harbor facilities, shoals, menaces to navigation, magnetic declinations, sailing courses, and other details of concern to mariners. Also, a hydrographic survey may deal with various subaqueous investigations which are conducted to secure information needed for the construction, development, and improvement of port facilities to obtain data necessary for the design of piers and subaqueous structures, to determine the loss in capacity of lakes or reservoirs because of silting, and to ascertain the quantity of dredged material.
            Hydrographic surveying has obvious application in the field of dock and harbor engineering and coast protection works. There are many more applications in civil engineering projects involving irrigation, water supply, water power, flood control, river works, land reclamation, and sewage disposal. The measurement of discharges of rivers and streams is also one of the activities of hydrographic surveys.
            The most common uses of the hydrographic surveys can be listed as:
1.     Measurement of tides
2.     Determination of bed depths
3.     Determination of scour, silting, and irregularities of the bed
4.     Determination of shore line
5.     Establishment of the mean-sea level
6.     Preparation of navigation charts
7.     Measurement of discharge of rivers and streams
8.     Determination of direction of currents to locate sewer fall
9.     Providing help in the planning of projects like bridges, dams, harbours, reservoirs.

METHOD OF HYDROGRAPHIC SURVEYS
To locate the subaqueous details on hydrographic charts, a number of suitably special control points are established along the shore line. The depth of water at various points forming a network is determined by making soundings from stationary boats. The locations of the soundings can be determined by making observations either from the boat to the control points or from the control points to a target fixed in the boat.
            Before locating the details in hydrographic survey, the following preliminary steps are required to be carried out:
1.     Reconnaissance
2.     Establishing horizontal control
3.     Establishing vertical control.

Reconnaissance
Although the principal operation of the hydrographic survey is obtaining the hydrograph or making the soundings. This phase of the project cannot be performed until certain preliminary steps are taken. The first of these is a careful reconnaissance of the area to be surveyed in order to select the most- expeditious manner of performing the survey, and to plan all operation so that the survey is satisfactorily completed in accordance with the requirements and specifications governing such work. The use of aerial photographs can be of considerable help in this preliminary study.

Establishment of horizontal control:
The horizontal control is a framework by which land and marine features are held in their true relative positions. It usually consists of a series of connected lines whose lengths and azimuths are determined accurately. Triangulation and, to a lesser extent, traverse are most commonly executed to provide horizontal control. For rough works a tacheometric traverse or plane table traverse may be run. However, for more precise control, a theodolite and a tape traverse is usually run. For extended surveys, where great precision is required, horizontal control is provided by triangulation.

                       As the general topography, vegetation, type and size of water body railways and highways of the coast line are the factors which decide the character of control, no definite rules can be given for providing horizontal control for hydrographic surveys. However, the following guide lines may help in selection of the type of control.

1.     If the water body is more than one kilometer wide, it is advisable to run two traverse along each shore, connecting to each other by frequent tie lines.
2.     If the water body is narrow, i.e., rivers, inlets, etc, and shore conditions are favorable it is advisable to run a traverse line only along one of the banks.
3.     If the shore lines are obscured by vegetation it is advisable to adopt triangulation system.
4.     For large lakes and ocean shore lines, the horizontal control consists of a network of connected triangulation system on shore.
These triangulation systems are usually, supplemented by traverses run along the shore and connected to the primary triangulation system.
Previously established control is very important asset in any hydrographic survey. Every effort should be made to obtain and utilize data from earlier surveys.

            Establishment of vertical control
                        Before sounding operations are begun it is essential to establish the vertical control in order that the stage or elevation of the water surface can be known when the soundings are obtained. Vertical control data are also needed for the limited topography shown on all nautical charts.
            A number of bench marks are established near the shore line at close spacing to serve as vertical control, the bench marks are used for setting and checking the levels of gauges to which the sounding refer to.

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