Water Supply Engineering
For determining the velocity of flow of underground water, the most commonly used non-empirical formula is

Hazen’s formula
Lacy’s formula
Darcy’s formula
Slichter’s formula

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Water Supply Engineering
Time of concentration

time taken for all the ran off to reach the drain
duration of rainfall.
is the time taken, for precipitation
time taken for the storm water to travel from the most remote point to the drain.

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