HOW IS QUANTITY OF WATER IN DAMS AND RESERVOIRS MEASURED?

The quantity of water in dams or reservoirs is estimated by a survey method called contour survey, by the engineering surveyors. This ‘quantity survey’ is actually done prior to the reservoir construction. It can also be estimated while water is present. In survey, the levels of the bed of reservoir are taken by leveling instruments and contour are drawn. This is called contour map. A contour is an imaginary line representing equal elevation points. The area enclosed by a contour is flat. This is simply true in case of reservoir. Two successive contour lines at one foot interval provide the area of water at top and bottom for each segment or slice. The volume of each segment is the average area multiplied by the difference of contour height, which is one foot. All volume of segments are added to get the total volume of water. This is how the volume is computed prior to reservoir construction or when dries. When water is present, the bed levels have to be taken under water. To get the bad level, an equipment called Echo sounder is used. To know the point at which bed level is taken an electronic equipment called total station is employed. Earlier a couple of theodolites were used. A boat with Echo sounder is navigated in water on predetermined parallel lines at specific intervals of 10 metre. Knowing the depth and location of each point of bed, levels are calculated and contour map is drawn. The volume is computed from contour map. Some surveyors calculate on simple cross sectional method also in which the water is divided into several sections like potato chips and all average sectional area is multiplied by distance between sections to provide the volume. There is no change in the method of data collection using boat.