HYDROGRAPHIC TERMS

HYDROGRAPHIC TERMS

v MEAN SEA-LEVEL

The elevation of the points generally refers to the mean sea-level (M.S.L) which is considered as datum for all types of surveys. The mean sea level is determined by making observations extended over a period of 19 years.

           The procedure for establishing the mean sea-level as datum is as below:

1.     A gauge is set at location where it is protected from rough waves and where the water level is not affected by local conditions.

2.     The gauge should be located at a low level in sufficient depth of water so that the gauge reading can be taken even at low tide.

3.     The zero of the gauge is referred to a bench mark located on shore.

4.     The reading of the elevation of water surface are taken continuously for 19 years and the mean of all the readings is the mean sea-level (M.S.L).

5.     The gauge reading corresponding to the mean sea-level is transferred to a permanent point on the shore which serves as a bench mark for determining the elevations of other points with reference to the datum, i.e., M.S.L.

The observations extending over one lunar month give results quite close to the average of 19 years. These results are sufficiently accurate for use in most of the hydrographic surveys.

v SHORE-LINE SURVEYS

Shore-line surveys are conducted for one or more purpose like delineation of shore lines, determination of high and low-water lines, and the location of prominent points on the shore to which the horizontal positions of soundings may be referred. The work involved may be for horizontal or vertical control.

            Normal methods of chain survey and plane table survey are adopted to survey irregularities in the shore line. Points which are clearly visible from the water surface, are selected as reference points such as churches, temple, light houses, wind mills, transmission pylons, etc., and their locations plotted easily.

            Determination of high-water line may be easily judged roughly from the marks on permanent rocks. But to locate the line accurately, method of contouring may be adopted in which a number of points are established at the time of high tide. The line connecting these points so obtained, is the required high-water line. The determination of low-water line is usually done by interpolation from sounding since the low-water line does not remain exposed for a longer time.

v TIDES

Tides are periodical variations in the level of the surface of a large body of water like a sea or an ocean. These variations are mainly due to the attraction of the sun and the moon. Tides have a bearing on a number of aspects of hydrographic surveys.

v SOUNDING

One of the main objectives of hydrographic surveys is determination of general topography of the bottom of water bodies. The process of determining depths below the water surface is called sounding. Sounding is analogous to leveling on land, and therefore, the reduced level of any point on the bottom of a water body is obtained by subtracting the sounding from the mean sea-level.

            Sounding, are required for the following purpose:

1.     Preparation of accurate charts for navigation.

2.     Determination of the quantities of the material to be dredged.

3.     Location of the area from where material to be dredged, and where to be dumped.

4.     Obtaining information for the design of breakwaters, wharves, sea-wells, etc.

  EQUIPMENT FOR SOUNDING

The essential equipment and instrument employed for taking the sounding may be grouped as below:

1.     Shore signals and buoys

2.     Sounding equipment

3.     Angle measuring instruments.