Runoff Hydrograph - Rational Method

The Rational Method is intended for modeling one runoff node, represented by a weighted runoff coefficient and total watershed area. The Rational Hydrograph can be created using one of four methods. They are Modified Rational, Dekalb Rational, Universal Rational and User Defined Rational. All four methods require a predefined IDF curve and the time of concentration to determine the peak flow using the Rational formula.

The Modified Rational method assumes the time of concentration is also the time of peak for the hydrograph. For a duration greater than the time of concentration, this method creates a trapezoidal shaped hydrograph. You may also apply a Receding Limb Factor in order to changed the slope of the receding limb. The default factor value is 1.

Modified Rational Hydrograph

The Deltalb Rational method uses the Modified Rational method as its foundation. Peak discharge is computed using the Rational formula, then the time to peak and the time of base are adjusted to five and ten times the time of concentration. Ten discharge values at increments of time of concentration are computed by scaling the peak discharge appropriately according to the following template.

T/Tc	Q/Qp if Tc < 20.0 min	Q/Qp if Tc >= 20.0 min
0	0	0
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	0.0	0.0

The Universal Rational method uses multiples of predefined coefficients (see the following table) and the peak discharge to compute ordinates at different times. Peak discharge is assumed to be reached at a time equal to three times the time of concentration. The time of base of the hydrograph is computed as eleven times the time of concentration.

Time (min)	Q
0	0.0
Tc	0.21Qp
2Tc	0.30Qp
3Tc	Qp
4Tc	0.54Qp
5Tc	0.39Qp
6Tc	0.25Qp
7Tc	0.18Qp
8Tc	0.15Qp
9Tc	0.14Qp
10Tc	0.13Qp
11Tc	0.0

The User Defined Rational method is similar to the Dekalb Rational method and the Universal Ration method, but with different templates provide by users. This method computes a hydrograph by using a dimensionless hydrograph template that has values for T/Tc versus Q/Qp.

Select Rational Method from Watershed > Hydrograph Routing > Watershed Hydrograph menu. This command opens a dialog for hydrograph calculation. First choose what method to use, then enter the sub-basin and rainfall data. The IDF curves will be computed after you choose a rainfall from the rainfall library. When use the User Defined Ration method, you will need to define the Q/Qp template.

After entering all information, click Calculation button to generate the runoff hydrograph. Runoff Hydrograph button opens a dialog with the tabular and graphic data, from there you can draw the hydrograph on screen and save the hydrograph data to a file.

Runoff Hydrograph - Rational Method

Pulldown Menu Location: Watershed > Hydrograph Routing > Watershed Hydrograph > Rational Method
Keyboard Command: runoffhyd3
Prerequisite: None