Time of Concentration - Time or Distance?

[JasonG]

The Time of Concentration is often described as the time for water to travel from the "most hydraulically distant point". My Civil Engineering Reference Manual adds "timewise" to this definition.

What is the general consensus out there? Do you use the path that gives the longest Time of Concentration (in minutes) or the longest path (in feet)?

For example, a short path with a flat slope and long sheet flow will give a longer Tc then a much longer path with a steep slope and short sheet flow.

 

[psmart]

Since the Tc is a measure of how long it takes for all parts of the watershed to start contributing to the runoff point, you should use the flow path with the longest travel time.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

I have a guide I use when training fellow workers in stormwater management. Here is an excerpt (the shallow slopes may appear odd to some - we are in a coastal plain):

TC a. Investigate scenarios for the hydraulically most distant point in the watershed.
i. This may not be the longest path by distance.
ii. It is a function of length, slope, and ground cover.
iii. The SHEET FLOW portion of the travel path WILL most likely determine the hydraulically most distant. (To get the same travel time as 100 ft. of sheet flow through short grass @ 1%, you would need:
(1) 655 ft. of unpaved shallow concentrated @ 0.3% or
(2) 1720 ft. of 5 ft. bottom grass trapezoidal channel @ 0.3%
iv. One can enter trial Tc information in Hydrocad to help determine the most distant path.