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 * t_c, while the time base is 10 * t_c. All coefficients occur at increments of t_c, and are different depending on whether t_c 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
$t$/$t_{c}$	$Q$/$Q_{p}$ for $t_{c}$ < 20 min	$Q$/$Q_{p}$ for $t_{c}$ >= 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