WMS:Dekalb Rational Hydrograph

The Dekalb Rational Hydrograph was developed by Dekalb County Georgia, and like the Universal Rational Hydrograph, ordinates are computed by scaling the peak discharge by an appropriate value. The time to peak occurs at 5*tc, while the time base is 10*tc. All coefficients occur at increments of tc, and are different depending on whether tc is less than 20 minutes or not. The following table lists the coefficients and a typical hydrograph is shown below.

Dimensionless Time and Hydrograph Ordinates
t/tc	Q/Qp for tc < 20 min	Q/Qp for tc >= 20 min
0	0.00	0.00
1	0.16	0.04
2	0.19	0.08
3	0.27	0.16
4	0.34	0.32
5	1.00	1.00
6	0.45	0.30
7	0.27	0.11
8	0.19	0.05
9	0.12	0.03
10	0.00	0.00

where:

• t_c = time of concentration.
• Q = Flow at time t, in cfs.
• Q_p = Peak flow.

Related Topics

• Computing Hydrographs with Rational Method
• Rational Method Equation