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 $5\ast t_{c}$, while the time base is $10\ast t_{c}$. 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