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

Drainage Design Manual for Maricopa County
Hydrology: Application
9.2 RATIONAL METHOD
9.2.1 Procedures for the Peak Discharge Calculation
1. Determine the area within the development boundaries.
2. Select the Runoff Coefficient, C from Table 3.2. If the drainage area contains subareas
of different runoff characteristics, and thus different C coefficients, arithmetically
area-weight the values of C.
3. Tabulate the depth-duration-frequency (D-D-F) statistics for the project site using Figure
A.1 through Figure A.60 (see Section 9.1.3.1) for an example. If many subbasins
for the same area are to be analyzed, compute an I-D-F table and prepare an I-D-F
graph to more efficiently select rainfall intensities. Refer to APPENDIX B for an example
I-D-F table and graph. Alternatively, if the project site lies within the Phoenix
Metro area, the I-D-F graph in APPENDIX B can be used to compute intensity, but a
site-specific I-D-F is preferred.
4. Calculate the time of concentration. This is to be done as an iterative process.
a. Determine the K b parameter from Figure 3.1 or Table 3.1. If the drainage area
contains subareas of different K b values, arithmetically area-weight the values of
K b .
b. Make an initial estimate of the duration and compute the intensity from the sitespecific
I-D-F 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 peak discharge Q by using the above value of i in Equation (3.1).
6. As an alternative to the above procedure, the DDMSW program may be used to calculate
peak discharges.
August 15, 2013 9-9