Following procedure is followed in the determination of the maximum runoff of a large locality:
(i) The total area of a the locality is divided into smaller units, from where the water flows in various inlets.
(ii) The time-area diagrams for each area so divided is obtained and plotted.
(iii) Now the cumulative time-area diagram is plotted step by step.
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(iv) From the cumulative time-area diagram plotted above the time of concentration and the corresponding contributing area are determined,
(v) Now the maximum rainfall intensity is obtained from the intensity duration curve described in article 4.10, by taking its duration equal to the time of concentration,
(vi) The run-off coefficient ‘C is obtained from the table 4.1.
(vii) Now the maximum storm flow is determined by the formula 4.1.
Example:
Using rational formula determine the run-off from an area of 3.5 hectares for a rainfall of 12 mm lasting for 20 minutes, assuming the time of concentration to be equal to the precipitation time. If the frequency of this rainfall is 1, then also determine the runoff at a frequency of 0.3 and time of concentration 15 minutes.
Use the following relationship:
Solution:
