In a reservoir water is stored in an irregular shaped geological formation. The volume of such a formation is arrived with the help of contours and linear approximation formulae.
Contour is an imaginary line at the same elevation. Numbers of such contours are drawn before the formation of reservoir with the help of surveying techniques. The height difference between two such adjacent contours is called as contour interval. Area of two adjacent contours are arrived with linear approximation is or by an instrument called Plani Meter. By knowing the areas and their height difference (contour-interval) the volume is arrived at. The reservoir formation can be assumed as lot many of such segments with a contour on either side.
For example, if we consider the datum (bed formation) as +100m, and the reservoir level (say full reservoir level, FRL) is +110m and if the contour interval considered is 1m, we have 10 contour areas at +100m +101m, +102m… +110m. Volumetric measurement used to quantify the water storage in reservoir is TMC (thousand million cubic metres). These calculations are represented graphically as, water capacity versus reservoir elevation, commonly called as reservoir capacity curves. With such graphs, volume of water at any fractional elevation may also be obtained. Another point needing exposure here is with the increasing life of a reservoir. Flood brought silt may accumulate in the reservoir bed and reduces the reservoir capacity. This can be ascertained with the sounding techniques.
The quantities of water in dams or reservoirs are 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 contours 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 the 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 (or peel). 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 dry.
When water is present, the bed levels have to be taken under water. To get the bad level, equipment called Echo sounder is used. To know the point at which bed level is taken 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.