2.0

Table Of Contents
Time of Concentration and Time of Travel
557
Calculating the Watershed Time of
Concentration
The time of concentration is the time it takes for runoff to travel from the
hydraulically most distant point of the watershed subarea to a specified outflow
point.
Tc can be a combination of three types of flow: Sheet, Shallow Concentrated,
and Open Channel. Each type uses different equations and requires different
input. For more information, see Specifying the Sheet Flow, Specifying the
Shallow Flow, and Specifying the Channel Flow in this chapter.
NOTE
The TR-55 time of concentration method has the following limitations:
n
The sheet flow length should not exceed 300 feet.
n
The minimum Tc is 0.1 hours.
n
The maximum Tc is 2 hours for the Tabular Hydrograph Method and
10 hours for the Graphical Peak Discharge Method.
For more information about the Tc equations and other limitations of TR-55,
see the SCS TR-55 manual, Chapter 3, Time of Concentration and Travel
Time.
To calculate the watershed time of concentration
1 From the Hydrology menu, choose Runoff
ä
Time of Concentration (Tc) to
display the Time of Concentration Calculator dialog box.
NOTE
The Time of Concentration Calculator is also accessible from the TR-55
Tabular Hydrograph dialog box, the TR-55 Graphical Peak Discharge
Method dialog box, the Rational Method dialog box, and the SCS TR-20
Unit Hydrograph Method dialog box, because those dialog boxes require
you to enter the Tc value.
2 In the Description box, type a description for the time of concentration
calculation.
3 In the Sheet box, enter the sheet flow. You can either enter this value directly,
or click Select to display the Sheet Flow Time Calculator dialog box. For more
information, see Specifying the Sheet Flow in this chapter.