Lag Time versus Time of Concentration

  Applies To 
  Product(s):  PondPack, SewerGEMS, CivilStorm
  Version(s): 08.11.XX.XX
  Area:  Modeling
  Original Author: Dan Iannicelli, Bentley Technical Support Group

The documentation mentions an "SCS Lag" with a built in factor of 0.6. Should I enter a lag time or a time of concentration for my catchments?

Explanation

Lag time = 0.6 * Tc

The "SCS Lag" help topic is specific to the SCS Lag Tc method, not a general comment about the Tc (Time of Concentration). In general, the hydrology calculations in PondPack, SewerGEMS and CivilStorm always work with Tc, so if you're entering a user defined Tc, you would enter the Tc, not the lag time. The "SCS Lag" and other methods refer to the optional Tc calculators, which help you establish the total Tc if you don't already have it. Basically you click the ellipsis button next to the "Tc Data Collection" field in the properties of the catchment, then click the new button in the Tc Data Collection Dialog. You'll see numerous methods such as Eagleson, Carter, TR-55 sheet flow, SCS Lag, etc.

These are standard methods established over the years for calculating the total Tc based on other known parameters. You'll see that when you select "SCS Lag", you must enter the hydraulic length, slope and CN. This tool will then use the equation mentioned in the help (which automatically applies a 0.6 factor) to compute the Tc, and then populates the Time of Concentration field for your catchment with that value, and uses it in the calculations. Again, if you already know the Tc, you would select "user defined" as the method, or select "user defined" as the "Tc input type" in the properties of the catchment.

See Also

How is Initial Abstraction considered with the SCS Unit Hydrograph method?

SCS unit hydrograph method TR-20 or TR-55

Catchment's total runoff and peak flow are different than HEC-HMS

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