Time of Concentration - Regression Equation, LA County

The regression method uses an iterative process to calculate the time of concentration. The study of regression equation relied on Tc computations from a large number of subareas. The subareas were taken from diverse hydrology studies that used the kinematic wave theory equations to calculate Tc. The following equations are from Hydrology Manual of Los Angeles County Department of Public Works.

where:    Tc    = Time of concentration, in minutes
               L     = Longest flow path length from watershed boundary to outlet, in feet
               Cd = Developed runoff coefficient, ratio of runoff rate to rainfall intensity, in in/in
               It     = Intensity at time it, in in/hr
               S      = Slope of longest flow path, in ft/ft
               IMP = Percent Impervious, percent expressed as 0.0 to 1.0
             Cu = Undeveloped runoff coefficient, ratio of runoff rate to rainfall intensity, in in/in

Select Regression Equation Method from Watershed > Time of Concentration menu. In the dialog, specify the Soil Type, Proportion Impervious Data, Flow length and Slope, and the Rational rainfall data. The flow path data can be obtained from picking a 3D polyline and the program calculates the length and slope of the flow automatically. Click on Calculate button to see the result.

The Soil Type library has all the tables from the Apprendix C in the Hydrology Manual. New Soil Type and Runoff Coefficient Data can be added to the library. The Import CSV function can easily import existing spreadsheet to the library. Graph dialog shows the Cu/Intensity curve.

The Proportion Impervious Data library is from the Apprendix D in the Hydrology Manual. New data is allowed to add to the library.

The Tc report is in Carlson report format.

Pulldown Menu Location: Watershed -> Time of Concentration > Regression Equation (LA County)
Keyboard Command: flowtc7
Prerequisite: None

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