The first step in designing a storm drain system is to determine the peak runoff flow. The , discussed in Chapter 4, is the method that applies to the vast majority of watersheds for storm drains.

The time of concentration in a storm drainage design is comprised of the time required for water to flow from the most distant point of the drainage area to the inlet (t

c

, also called inlet time) and the travel time (t

t

)as the water flows through the storm drain line under consideration (travel time through a conduit). See in Chapter 4 for more information. The hydraulic designer must be careful to document the actual inlet time and travel time for each segment, because the total time is summed through the system and used for sizing the conduit. The temptation to use a default t

c

of 10 minutes for every drainage area usually results in the flowrate in the conduit being underestimated.