FLOOD09746

• County: Statewide
• Basin: Statewide
• Title: Colorado Statewide Drainage Floodplain Management Criteria Manual
• Date: 8/1/2002
• Prepared By: CWCB & WRC Engineering Inc
• Floodplain - Doc Type: Educational/Technical/Reference Information

.

.

.

CHAPTER 5
HYDROLOGY

SECTION 2.0
RUNOFF

COLORADO STATEWIDE
DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL

2.2.1 METHODOLOGY

The Rational Formula method is based on the following equation:

Q=CIA

(Eq. CH5-200)

Q is defined as the maximum rate of runoff in cubic feet per second (actually,
Q has units of acre inches per hour, which is approximately equal to the units
of cubic feet per second). C is a runoff coefficient and represents the runoff-
producing conditions of the subject land area. I is the average intensity of
rainfall in inches per hour for a duration equal to the time of concentration. A
is the contributing basin area in acres. The time of concentration is defined
as the time required for water to flow from the hydraulically most distant part
of the drainage area to the point under consideration.

2.2.2 ASSUMPTIONS

The basic assumptions made when applying the Rational Formula method
are as follows:

1. The computed maximum rate of runoff to the design point is a
function of the average rainfall rate during the time of concentration to
that point.

2. The maximum rate of rainfall occurs during the time of concentration,
and the design rainfall depth during the time of concentration is
converted to the average rainfall intensity for the time of
concentration.

3. The maximum runoff rate occurs when the entire area is contributing
flow. However, this assumption has been modified from time to time
when local rainfall/runoff data was used to improve calculated results.

2.2.3 LIMITATIONS ON METHODOLOGY

The Rational Formula method can adequately
approximate the peak rate of runoff from a rainstonn in
a given small basin. The critics of the method usually
are unsatisfied with the fact that the answers are only
approximations. A shortcoming of the Rational Fonnula
method is that only one point on the runoff hydrograph
is computed (the peak runoff rate), therefore, the
estimated total runoff volume using the triangular
hydrograph is not very accurate.

2.2.4 TIME OF CONCENTRATION - RATIONAL METHOD

As previously mentioned, the time of concentration is
defined as the time required for runoff to flow from the
hydraulically most distant part of the drainage basin to
the desired point in the basin.

AUGUST 2002

RUNOFF

The time of
concentration
is defined as
the time
required for
runoff to flow
from the
hydraulically
most distant
part of the
drainage
basin to the
desired point
in the basin.

CH5-204