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

Drainage Design Manual for Maricopa County
Hydrology: Application
9.1 RAINFALL
9.1.1 Procedure for the Development of the Design Rainfall
9.1.1.1 Procedure for the Rational Method
1. Determine the size of the drainage area.
2. Locate the drainage area and determine the point rainfall depth for every duration,
and all frequencies of interest from Figure A.1 through Figure A.60 of Appendix A.1.
Summarize in a Depth-Duration-Frequency (D-D-F) table.
3. Create an Intensity-Duration-Frequency (I-D-F) table by dividing the individual rainfall
depth values from Step 2 by the duration associated with the rainfall depth. The units
should be in terms of inches per hour.
4. Plot the results for each frequency on log-log paper and examine the results to be
sure they plot as smooth curves. Any anomalies should be checked against Appendix
A.1 to be sure the correct depth value was read.
Note: Steps 2 through 4 are performed automatically in DDMSW.
9.1.1.2 Procedure for the Unit Hydrograph Method.
1. Determine the size of the drainage area.
2. Determine the point rainfall depth or the areally averaged point rainfall depth, from
Figure A.1 through Figure A.60 of Appendix A.1, depending on the desired storm
duration and frequency.
3. For a single storm analysis, determine the depth-area reduction factor using or Table
2.1 for a 6-hour local storm and Table 2.2 or Figure 2.2 or a 24-hour general storm.
For a multiple storm analysis, determine the drainage areas at key points of interest in
the watershed. For each drainage area, determine the depth-area reduction factor
using or Table 2.1 for a 6-hour local storm and Table 2.2 or Figure 2.2 for a 24-hour
general storm.
4. Multiply the point rainfall depth by the appropriate depth-area reduction factor(s).
5. For a 6-hour local storm, use Figure 2.5 to select the appropriate pattern number(s)
(rounded to the nearest 0.1 pattern number).
9-2 August 15, 2013