This is the simplest and easiest method of drawing frequency curve. According to law of probabilities, the possible percentage of future floods that will equal or exceed a given flood Q may be obtained by the following relation.

(5.3.1)

Where,

p = Calculated % possibilities of future flood i.e. in 100 year how many times this magnitude of flood will be exceeded.

m = the number of years during the period of record that a flood was equaled or exceeded.

n = total no of years of record.

After calculating p,

(5.3.2)

Where;

p = Calculated % possibilities of future flood i.e. in 100 year how many times this magnitude of flood will be exceeded.

m = no of years the discharge Q was exceeded.

I = Future flood frequency i.e. after how many years Q will be exceeded.

An estimate of the probable frequency of flood flows of the Sabarmati River at Dharoi Dam has been made in Table 5.1 and frequency curve is shown in Fig 5.1.

According to Normal Frequency Evaluation Graphical Method the lowest magnitude of flood 118 is came in every year and calculated % possibilities of it is 100% and the highest magnitude of flood 13186 is came once in 21 year and calculated percentage possibilities of it is 3.45%.