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

Drainage Design Manual for Maricopa County
Hydrology: Rainfall
characterized by relatively small areas of high intensity rainfall resulting in depth-area reduction
curves that decrease rapidly with increasing area. The U.S. Army Corps of Engineers studied
historic storms in Arizona and published the results of those studies (U.S. Army Corps of Engineers,
1974). For local storms (6-hour duration), the depth-area reduction curve that is to be
used in Maricopa County is the curve developed by the U.S. Army Corps of Engineers for the 19
August 1954 Queen Creek Storm. That curve is shown in Table 2.1 and Figure 2.1. For the 24-
hour general storm, the depth-area reduction curve that is to be used in Maricopa County is
shown in Table 2.2 and Figure 2.2. This curve is taken from Figure 15 of the National Weather
Service HYDRO-40 (Zehr and Myers, 1984).
Use these depth-area reduction values to adjust the point rainfall depths from the isopluvial maps
(Figure A.1 through Figure A.60 of Appendix A.1). For the design of stormwater storage facilities,
refer to the Drainage Policies and Standards for Maricopa County or the local jurisdiction for
depth-area reduction values to adjust the point rainfall depth from the isopluvial map for the 100-
year, 2-hour storm (Figure A.56 of Appendix A.1).
For design storms other than what is specified in this manual, the depth-area reduction and temporal
distribution will need to be developed on a case-by-case basis depending on the purpose of
the study, location of the watershed, and other meteorological and hydrological factors.
TABLE 2.1
DEPTH-AREA REDUCTION FACTORS FOR THE 6-HOUR DURATION RAINFALL
Area, sq. miles
Depth-Area Reduction Factor
(ratio to point rainfall)
0.0 1.000
0.5 0.994
1.0 0.987
2.8 0.975
5.0 0.960
10.0 0.940
16.0 0.922
20.0 0.910
30.0 0.890
40.0 0.870
90.0 0.810
100.0 0.800
Note: Bold values correspond to the 6-hour design storm pattern numbers.
2-6 August 15, 2013