Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Rational Method
3. Compile the site-specific depth-duration-frequency (D–D–F) and intensity-durationfrequency
(I–D–F) statistics for the project site using NOAA Atlas 14 (see Section 2.2
and Section 9.1).
4. Calculate the time of concentration. This is to be done as an iterative process.
a. Determine the K b parameter from Table 3.1 or Figure 3.1. If the drainage subbasin
contains subareas of different K b values compute a K b for each surface
roughness class using the total area of the subbasin when
applying Table 3.1 or Figure 3.1. Then arithmetically area-weight the values
of K b .
b. Make an initial estimate of the duration and compute the intensity from the D–
D–F data, or derive from the I–D–F curve for the desired frequency.
c. Compute an estimated T c using Equation (3.2). If the computed T c is reasonably
close to the estimated duration, then proceed to Step 5, otherwise repeat
this step with a new estimate of the duration. The minimum T c should not be
less than 5-minutes.
5. Determine the peak discharge Q by using the value of i in Equation (3.1).
6. As an alternative to the above procedure, the DDMSW program may be used to calculate
peak discharge.
3.6.2 Multiple Basin Approach
The Rational Method can be used to compute peak discharges at intermediate locations within a
drainage area less than 160 acres in size. A typical application of this approach is a local storm
drain system where multiple subbasins are necessary to compute a peak discharge at each proposed
inlet location. Consider the schematic example watershed shown in Figure 3.3. A peak
discharge is needed for all three individual subbasins, subbasins A and B combined at Concentration
Point 1 and subbasins A, B and C combined at Concentration Point 2. This can be
accomplished using two different approaches: the combined watershed approach and the triangular
hydrograph approach. The triangular hydrograph method is incorporated in the DDMSW
computer program, but the combined hydrograph method is not. The combined hydrograph
method is intended for use by engineers/hydrologists without access to a computer and DDMSW.
Either method may be used but the engineer/hydrologist should receive prior approval from the
jurisdiction before applying the combined watershed method. Steps for applying both
approaches follow.
3-8 August 15, 2013