What is the runoff coefficient?

The runoff coefficient C is a dimensionless empirical coefficient related to the abstractive and diffusive properties of the basin. Basin abstractions including infiltration, depression storage, evapotranspiration and interception are lumped into the coefficient.

The runoff coefficient ranges between 0 and 1.0, where a value of 0 indicates that none of the rain falling on the basin generates runoff, and a value of 1.0 indicates that all of the rain falling on the basin generates runoff. A basin that has low land-surface slopes, high infiltration rates, high ground-water storage, extensive vegetation and surface storage will have a low runoff coefficient. A steep basin with an impervious surface, little vegetation and no surface storage will have a high runoff coefficient.

How do I determine the runoff coefficient?

Tables of runoff coefficients can be found in many hydrology textbooks. A document containing runoff coefficient tables along with a description of how to best determine the coefficients can be downloaded here. Alternatively, an online table is available.

What is the time of concentration?

Time of concentration has several definitions including:

• The minimum time required after runoff begins for the entire basin to contribute flow to the outlet.
• The time required for a particle of water to travel from the most hydraulically distant point in the basin to the outlet.
• The time required for a flood wave to travel from the most hydraulically distant point to the outlet.

The time of concentration is employed by the rational method when selecting an appropriate rainfall intensity duration.

How do I determine the time of concentration?

StormNET allows for the time of concentration to be calculated using one of the following methods:

• Carter
• Eagleson
• FAA
• Kirpich
• NRCS (SCS) TR-55
• User-defined

A document describing how to calculate the time of concentration using the NRCS (SCS) TR-55 method can be found here.

How do I determine the rainfall intensity?

StormNET’s IDF Curve Manager can quickly determine rainfall intensities required by the rational method. Alternatively, rainfall intensity can be selected from an intensity-duration-frequency (IDF) curve generated from point rainfall data collected in the local area.

Can I generate a runoff hydrograph using the rational method?

Yes. The shape of the runoff hydrograph as predicted by the rational method is dependent upon the relationship between the storm duration and the time of concentration. The peak flow occurs at the time of concentration. After the rainfall stops, the flow recedes to zero over a timeframe approximately equal to the time of concentration.

What are the important assumptions of the rational method?

The rational method makes the following assumptions:

• Precipitation is uniform over the entire basin.
• Precipitation does not vary with time or space.
• Storm duration is equal to the time of concentration.
• A design storm of a specified frequency produces a design flood of the same frequency.
• The basin area increases roughly in proportion to increases in length.
• The time of concentration is relatively short and independent of storm intensity.
• The runoff coefficient does not vary with storm intensity or antecedent soil moisture.
• Runoff is dominated by overland flow.
• Basin storage effects are negligible.

It is important to note that all of these criteria are seldom met under natural conditions. In particular, the assumption of constant, uniform rainfall intensity is the least accurate.

What is the modified rational method?

The modified rational method allows for the storm duration to be longer than the time of concentration. As with the rational method, the peak flow occurs at the time of concentration and this peak flow is maintained for the remaining duration of the storm. After the rainfall stops, the flow recedes to zero over a timeframe approximately equal to the time of concentration.

What is the DeKalb rational method?

The DeKalb rational method was developed by DeKalb County, Georgia. The peak discharge is calculated using the rational method formula discussed previously. In order to generate a hydrograph for a particular project, the dimensionless ordinates are scaled using the peak discharge value. One of two curves must be selected based upon the time of concentration.

t/tc	Q/Qp (tc < 20 min)	Q/Qp (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

Ordinates of the DeKalb dimensionless hydrograph

What is the origin of the rational method?

The origin of the rational method is unclear. In the United States, Kuichling (1889) was the first to mention the method in the scientific literature, yet some engineers attribute the principles of the method to Mulvaney (1851). In England, the method is often referred to as the Lloyd-Davies method, which was published in 1906.

Where can I get additional information about the rational method?

The following documents contain a more detailed explanation of the rational method. Additionally, the USGS report compares peak discharges generated using the rational method to field observations.

Connecticut DOT Drainage Manual (Ch. 6)
USGS Scientific Investigations Report 2005–5254

Can I use the rational method in StormNET?

Yes, with StormNET the rational method can be quickly implemented along with variants of the approach including the modified rational method and the DeKalb rational method.

What time of concentration methods does StormNET support?

StormNET allows for the time of concentration to be calculated using one of the following methods:

• Carter
• Eagleson
• FAA
• Kirpich
• NRCS (SCS) TR-55
• User-defined