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

Drainage Design Manual for Maricopa County
Hydrology: Application
9.1.3 Rainfall Examples
9.1.3.1 Rainfall for the Rational Method
Duration
A watershed to be modeled using the Rational Method has its centroid located at 33 o 42'
40'' N and 112 o 14' 50'' W. Use Figure A.1 through Figure A.60 of Appendix A.1 to
develop D-D-F and I-D-F tables and an I-D-F curve for all storm frequencies and durations.
The resulting D-D-F data is shown in Table 9.1.
Table 9.1
EXAMPLE DEPTH-DURATION-FREQUENCY STATISTICS FROM FIGURES
(Source: NOAA Atlas 14 Arizona, Figures in Appendix A.1)
Point Rainfall DepthData in inches
Storm Frequency, years
2 5 10 25 50 100
5-min 0.30 0.40 0.43 0.53 0.61 0.70
10-min 0.41 0.54 0.68 0.82 0.93 1.06
15-min 0.52 0.70 0.83 1.02 1.17 1.31
30-min 0.70 0.93 1.12 1.38 1.58 1.78
1-hour 0.86 1.17 1.40 1.70 1.95 2.20
2-hours 0.98 1.32 1.58 1.92 2.20 2.46
3-hour 1.02 1.35 1.61 1.96 2.23 2.50
6-hour 1.20 1.52 1.79 2.13 2.42 2.70
12-hour 1.34 1.70 1.96 2.35 2.61 2.90
24-hour 1.55 1.99 2.34 2.84 3.22 3.62
To obtain I-D-F data, divide each rainfall depth value from Table 9.1 by the corresponding
duration using Equation (9.1):
i j
k
=
P j
k
------
D k
(9.1)
where:
i j
k
P j
k
D k
= Rainfall intensity in in/hr for duration j in hours and frequency k in
years.
= Point rainfall depth in inches for duration j and frequency k .
= Rainfall duration in hours for frequency k .
9-4 August 15, 2013