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

Drainage Design Manual for Maricopa County
Hydrology: Rainfall
2.5 PROCEDURE FOR THE DEVELOPMENT OF THE DESIGN RAIN-
FALL
The following is the procedure for the development of the design rainfall. Notes and general
guidance on the application of this procedure and the methodologies presented in this chapter
are provided along with a detailed example in 9.1 RAINFALL.
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 Table 2.1
or Figure 2.1 for a 6-hour local storm and Table 2.2 or Figure 2.2 for 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 Table 2.1 or Figure 2.1 for a 6-hour local storm and Table 2.2 or Figure 2.2 for a
24-hour general storm.
As drainage area increases, the average depth of rainfall over that area decreases.
For situations that require runoff magnitudes at only one point in the watershed, the
effective rainfall over the watershed can be simulated by a single storm. The single
storm approach can be applied regardless of the number of subbasins used to define
the runoff characteristics of the watershed.
For situations that require runoff magnitudes at multiple points within a drainage area,
the effective rainfall depth at each of those points is simulated using a set of index
storms. The drainage areas of the index storms and thus the rainfall depth adjustment
factors are selected to be representative of the contributing drainage areas at
the points of interest. This implies that the watershed will be delineated with multiple
subbasins.
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).
6. For a 6-hour local storm, use the dimensionless rainfall distributions of or Table 2.4 to
calculate the dimensionless distribution(s) by linear interpolation between the two
bounding pattern numbers.
August 15, 2013 2-19