Environmental Engineering Reference
In-Depth Information
given in Chapter 5, section 2.4. A simplified procedure, which could be used in cases
where data is not available and the required accuracy is not high, is described in Chapter
4.
4.3
Determining surface runoff
4.3.1
The rational method
This method, also known as Lloyd-Davis method, is the oldest and most widely used
method in engineering practice for determination of the design runoff quantity, during the
process of drainage structures design for flood prevention. Its purpose is to determine a
design surface runoff flow rate, which would be the base for the sizing of the conveying
structure to transport the runoff from a given area to a point where it could be discharged
safely into a natural water body or disposed on land. Thus, this method focuses on a
selected rainfall event, which is the most probable one to cause flooding within a given
period of time. In other words, the method determines the runoff from high intensity
storms with a relatively low probability of occurrence. It should be applied with caution
when used for the determination of pollution loads and the design of pollution abatement
structures, as in most cases, diffuse pollution is attributed to medium storms with a high
frequency of occurrence. Therefore, the selection of an appropriate “design” storm in
terms of pollution control and abatement is very important. The method is based on the
following assumptions:
• The peak rate of runoff at any point is a direct function of the average rainfall intensity
during the time of concentration to that point.
• The frequency of the peak discharge is the same as the frequency of the average rainfall
intensity.
• The time of concentration is the time required for the runoff to become established from
the most remote part of the drainage area to the point under consideration. It includes
the overland flow time (inlet time) and the time of flow along the channel, governed
by channel hydraulics.
Reported practice generally limits the use of this method to urban areas of less than 13
km
2
(White 1978). For larger areas, the application of hydrograph methods is
recommended. The rational method is represented in the following formula:
Q = C i A
where
Q
= the peak runoff rate;
C
= runoff coefficient, which depends on
characteristics of the drainage area; i = the average rainfall intensity (i
av
); and
A
= the
drainage area. The drainage area information should include the following:
• Land use - the present and predicted future practice-as it affects the degree of protection
to be provided and the percentage of imperviousness.
• The character of soil and cover as they may affect the runoff coefficient.
• The general magnitude of ground slopes, which, with pervious items and shape of the
drainage area, will affect the time of concentration.
