The time of concentration for a given storm water drain is “the total time required by the flow to reach to the maximum limit”.
It generally consists of two parts; viz.
(i) The Inlet Time or Overland Flow Time or Time of Equilibrium (T):
The time taken by the water to flow overland from the critical point upto the point where it enters the drain mouth.
It may be estimated by using the eqn. (4.10), given below:
Where Ti = Inlet time in hours.
L = Length of overland flow in kilometres from the critical point to the mouth of drain
H = Total fall of level from the critical point to the mouth of the drain in metres.
ADVERTISEMENTS:
L and H can be found from the survey plan of the area and Ti can be easily calculated
(ii) The Channel Flow Time or Gutter Flow Time (Tf):
The time taken by the water to flow in the drain channel from the mouth to the considered point. This may be obtained by dividing the length of the drain by the flow velocity in the drain. This may be obtained by dividing the length of the drain by the flow velocity in the drain.
