The Kerby-Kirpich method offers an alternative means of calculating
the time of concentration for a watershed. With this method, the
overall time of concentration
is divided into two components: overland flow and channel flow. The
Kerby equation is used to calculate overland flow, and the Kirpich
equation is used to calculate channel flow.
Kerby equation (overland flow)
Tov = K (LN)0.467 / S-0.235
where: Tov = Overland Flow Time of Concentration
(min)
K =
Unit Conversion Coefficient, in which K = 0.828 for traditional
units and K = 1.44 for SI units
L =
Overland Flow Length (in feet or meters as dictated by K)
(ft/ft)
N =
Retardance Coefficient - The Select button will show Common Kerby
Coefficient
S = Slope
of Terrain conveying the overland flow (ft/ft)
Kirpich equation (channel flow)
Tch = KL0.0.770 S-0.385
where: Tch = Channel Flow Time of Concentration
(min)
K =
Unit Conversion Coefficient, in which K = 0.0078 for traditional
units and K = 0.0195 for SI units
L =
Channel Flow Length (in feet or meters as dictated by K)
(ft/ft)
S = Slope
of Terrain conveying the main channel flow (ft/ft)
Select Kerby-Kirpich Method from Watershed > Time of
Concentration menu.
In the dialog for the Overland Flow, input the Kerby - Overland
Flow Length (can be user entered), or Select Flow Line. The flow
data can be obtained from picking a 3D polyline and the program
calculates the length and slope of the flow automatically. User can
also Select Surface Data and choose a surface model to get the flow
slope. Enter or Select Retardance Coefficient. For the Channel
Flow, input the Kirpich - Channel Flow Length (can be user
entered), or Select Flow Line. User can also Select Surface Data
and choose a surface model to get the flow slope. Calculate button
to see the result.
Pulldown Menu Location: Watershed -> Time of
Concentration > Kerby - Kirpich Method
Keyboard Command: flowtc8
Prerequisite: None
