• Mass inflow is a curve which represents the cumulative flow in a reservoir at any particular instance.
  • It is a plot between cumulative inflow in the reservoir with time.
  • It can be prepared with the help of a hydrograph of the river for the dam site for a large number of years.
  • We know hydrograph is a plot or curve between discharge versus time.
  • Figure 9.4 (a) shows a hydrograph of a river for the dam site for a number of years and Fig. 9.4 (b) a corresponding mass curve prepared from the hydrograph of Fig. 9.4 (a). Let starting year of the hydrograph is 1998.
  • The total quantity of water that has flown through the river from 1998 to a time t1 (say 2001) has been represented by the hatched area.
  • In mass curve, the corresponding ordinate at time t1 (ordinate AB) will represent the total volume of water indicated by the hatched area of hydrograph curve.
  • Similarly the ordinates of the mass curve corresponding to other years can be determined from the hydrograph curve and plotted.
  • When all such points are joined free hand we get the mass curve corresponding to the hydrograph.
  • It is evident that a mass curve will continuously rise as it is the plot of accumulated inflow versus time.
  • Periods of no inflow would be indicated by horizontal lines on the mass curve.
  • The slope of the mass curve at any time gives the rate of inflow at that time.
  • Mass inflow curve is sometimes known as Ripple diagram also.

Flood hydrograph and mass inflow curve